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For other uses, see Greenhouse (disambiguation)

Greenhouse. In America the word greenhouse is used generically for any glass building in which plants are grown, with the exception of coldframes and hotbeds. Originally and etymologically, however, it means a house in which plants are kept alive or green: in the greenhouse plants are placed for winter protection, and it is not expected that they shall grow. The evolution of the true greenhouse seems to have begun with the idea of a human dwelling-house. At first larger windows were inserted; and later, a glass roof was added. In early times it was thought best to have living-rooms above the greenhouse, that it might not freeze through the roof. Even as late as 1806, Bernard M. Mahon, writing in Philadelphia, felt called upon to combat this idea. The old or original conception of a greenhouse as a place for protecting and storing plants is practically extinct, at least in America (Fig. 1749). In England, the word greenhouse is mostly used for a house or structure in which are kept or grown those plants that do not require a very high temperature.

Other types of plant-houses are the conservatory (which see), in which plants are kept for display; the forcing-house (see Forcing), in which plants are forced to grow at other times than their normal season; the stove or warmhouse; the propagating-pit. Originally the warmest part of the plant-house, that part in which tropical plants were grown, was heated by a stove made of brick, and the house itself came to be called a stove. This use of the word stove to designate the warmest part or room of the range is general in England, but in America we prefer the word warmhouse (and this word is much used in this Cyclopedia). Originally, hothouse was practically equivalent to stove, but this term is little used in this country, and when used it is mostly applied generically in the sense of greenhouse.

It will thus be seen that there is no one word that is properly generic for all glass plant-houses. The word glasshouse has been suggested, and it is often used in this work; but there are other glass houses than those used for plants. It seems best, therefore, to use the word greenhouse for all glass buildings in which plants are grown; and American usage favors this conclusion.

The long, low greenhouse range, of the type we now know in our commercial establishments, probably had a different origin from the high-sided greenhouse. The glasshouse range appears to have developed from the practice of protecting fruits and other plants against a wall. In European countries, particularly in England, it is the practice to train fruits and other plants on stone or brick walls, that they may be protected from inclement weather and receive the greater sun heat that is stored in the masonry. It occurred to Nicholas Facio Duilhier to incline these fruit walls to the horizon so that they would receive the greater part of the incident rays of the sun at right angles. He wrote a book on the subject of "Fruit-Walls Improved," which was published in England in 1699. Facio was a mathematician, and he worked out the principle of the inclined walls from mathematical considerations. Such walls were actually built, but according to the testimony of Stephen Switzer, who wrote in 1724, these walls were not more successful than those which stood perpendicularly. Certain of these walls on the grounds of Belvoir Castle, and over which grapes were growing, received the additional protection of glass sash set in front of the inclined walls and over the vines. In addition to this, flues were constructed behind the wall in which heat might be supplied. The construction of hollow heated walls was not uncommon in that day. The satisfactory results that followed this experiment induced Switzer to design glass-covered walls. The "glasshouse" which he pictured in the "Practical Fruit-Gardener" (1731) represents a greenhouse 3 ½ feet wide in the clear (Fig. 1750). At the back of this house is an inclined heated wall on which the grapes are grown. Three and one-half feet in front of this a framework is erected to receive the sash. There are three tiers of openings or windows along the front, the two lower ones of which are for window-sash, and the upper one is vacant in order to provide for ventilation and to allow space to receive the lower sash when they are lifted up. The whole structure is covered with a roof or coping. Switzer declares that the introduction of these covered sloping walls "led the world" to the "improvement of glassing and forcing grapes, which was never done to that Perfection in any Place as it is upon some of the great Slopes of that elevated and noble Situation of Belvoir Castle." Johnson, in his "History of English Gardening," quotes the remarks of Switzer, and makes the statement that the use of these walls "led to the first erection of a regular forcing structure of which we have an account." The immediate outcome of these covered walls seems to have been the lean-to greenhouse, and from that structure has perhaps developed the double-span glass range of the present day. Long before Switzer's time plants were forced in a crude way, even by the Romans, mostly by being placed in baskets or other movable receptacles, so that they could be placed under cover in inclement weather; but the improvements of Facio and Switzer seem to have been among the earliest attempts in England to make low glass ranges for plants.

It was about the beginning of the nineteenth century that great improvements began to be made in the glasshouse. This new interest was due to the introduction of new plants from strange countries, the improvement of heating apparatus, and the general advance in the art of the building. The ideals that prevailed in the opening of the century may be gleaned from J. Loudon’s “Treatise on Several Improvements in Hot-Houses” in London, 1805. One of the devices recommended by Loudon will interest the reader. It is shown in Fig. 1751. The bellows is used for the purpose of forcing air into the house, so the plants may be supplied with a fresh or nonvitiated atmosphere. “By forcing the air into the house, once a day or so, doubles the quantity of air which the house usually contains" can besecured. The house could be "charged." The tube leading from the bellows is shown at b; it discharges at c. Curtains run on wire, i; the curtain cord is at f.

Greenhouses are now built on the plan of the long low glass range with sides varying from 5 feet 6 inches to 7 feet in height. The tendency in commercial structures is for a height of 7 feet from ground to eaves. The taller glass structures are used for conservatory purposes, housing such table plants as palms, tree- ferns, or the like, or when an architectural feature is desired. The general tendency of the building of glass structures is toward extreme simplicity (Fig. 1547, p. 1256). In the extreme South, lattice-work buildings are sometimes used for the protection of plants, both from light frosts and from the sun (Fig. 1752). The heating now employed in this country is of three different kinds: hot water under very low pressure or in the open-tank system; hot water in practically closed circuits; and steam. Hot water under low pressure is an old-time mode of heating, and is not now popular in this country except for conservatories and private establishments. The heavy cumbersome pipes are not adapted to laying over long distances and under varying conditions. The commercial houses are now heated by means of wrought-iron pipes, which go together with threads. The comparative merits of steam and hot water in these wrought-iron pipes are much discussed. For large establishments, hot water under pressure is now employed to some extent. Much progress has been made in methods of heating in recent years, and either steam or hot water gives good results when competently installed. The merits of one system or the other are very largely those of the individual establishment and apparatus, and the personal choice of the operator (see page 1403; also pages 1400 and 1402).

The simple straight and direct house is now much in favor with the commercial growers of carnations, chrysanthemums, violets, roses, vegetables, and with propagators. Most of the greenhouse construction firms are designing houses most admirably adapted to the growing of these plants. Each firm has a few original forms worked into the detail plans, calculated to appeal to the growers' fancy. Perhaps the ideal structure for carnations, for example, is a single detached house, about 50 feet wide and 500 feet or less in length, with ventilators on each side of the ridge and on each side below the eaves, and the eaves, or the gutters, 6 feet above the grade.

With the refinements of architecture and the growth of satisfaction in home-building, the glasshouse is becoming an integral part of the residence. Sometimes it takes the form of a sun-parlor, and in which certain plants may be kept at least temporarily; sometimes it is a real plant-house added to the residence, a glass or covered garden that carries bloom and verdure through the cold weather and enables the homemaker to span the year. The best results in plant-growing are secured when the structure is separate, with its own heat, its normal exposure, and its own essential set of conditions; but it is worthwhile to add a garden-room to a residence even if the horticultural results are not great. Some of the architectural combinations of glass and other materials are very artistic and interesting.

L. H. B.

Greenhouse construction.

For convenience, this subject may be considered under the following heads; i. e., Location, Plans, Grading, Foundations, Framework, Glazing and painting, Plant-tables, Ventilation, Heating.


Greenhouses which are intended for use in connection with the gardens should be placed, for convenient attendance, within the garden inclosure or along its boundary. A good location for the garden will usually be found the best one for the greenhouse.

A conservatory or greenhouse designed for a private place, where specimen and blooming plants will be kept for the pleasure of the family and entertainment of visitors, should be attached to the dwelling or located as near as possible in a well-kept part of the grounds. A conservatory does not require a full southern exposure. Most decorative plants thrive as well or better and continue in bloom for a longer time if kept in a house having plenty of light, but so located as to receive but little direct sunlight. Large ranges of glass adapted to a variety of purposes are generally kept separate from other buildings. In parks the location should be near a main entrance.

The location of a range of glass for commercial purposes, where the elements of expense and profit are to have the first consideration, is of great importance. The chief items that determine the desirability of a suitable location arc the adaptability and value of the land, cost of fuel delivered, ample and inexpensive water supply and proximity to a market. The top of a bleak hill and the bottom of a valley should both be avoided. Level land, or that having a southerly slope, is the best.


When a site for the proposed greenhouse has been decided upon, full plans should be made before commencing to build. The plans should embrace not only the glass, which is required at once, but should provide for the largest increase which can be anticipated. In this way houses can be erected which are convenient to work and have a good appearance, with small extra cost for building only part at a time. Attention should be given to the special peculiarities of the location, like the exposure to the sun, grade of ground, shape of lot and best location for the heating apparatus. Each compartment should have the proper form of house and exposure to the light adapted to the plants for which it is provided.

It will readily be seen that to locate and plan a range of glass to the best advantage requires skill and experience. In a communication received by the writer from a superintendent of one of the most important botanic gardens in the country, it was remarked that "when the architect prevails, the gardener fails."

It is also true to a greater degree than in almost any other class of buildings that the beginner or amateur who undertakes to plan and construct his own greenhouse is likely to pay well for his experience, and will at least sympathize with the "lawyer who pleaded his own cause and found he had a fool for a client." This is perfectly true, as many know to their cost. To plan a greenhouse satisfactorily, the designer must have a practical knowledge of the requirements. To meet this increasing demand, specialists can be found, known as "horticultural architects," who devote their entire time to this branch of work.


The floor of the greenhouse should be a few inches above the outside grade. As most greenhouses are necessarily built low to accommodate the plants, a small terrace around them adds to the elevation and the good appearance of the structure. It will usually be best to keep the floor of a greenhouse all on one level. When the variation in the grade of the ground is not too great, the floor line should be at the highest point of the grade. In the case of a long house, the floor line is sometimes made the same as the natural grade, but such an arrangement is to be avoided when possible. For locations on a hillside, the different apartments may have different floor-levels, with necessary steps between them.

All the sod and loam should be removed from the space to be covered by a greenhouse and all the filling necessary made with subsoil. The latter should be laid in thin layers and each wet down and thoroughly tamped. Loam used for filling under a greenhouse is likely to become sour, and will continue to settle for a long time, causing much trouble and annoyance.


Too much care cannot be given to the preparation of good foundations. These are usually of brick, but may be made of stone or concrete. The brick walls take up less room in the house than stone, and are usually less expensive. The foundation walls should be extended down to a point below the frost line, generally 3 or 4 feet deep, and are usually raised about 2 feet above the grade. An inexpensive wall of rubble stone work or of concrete is all that is needed in the ground. The part of the wall showing above grade may be of plain brick, or brick faced with stone, or the entire wall may be built of concrete finished with cement plaster. It is usual to construct the walls of the same material as the surrounding buildings, or with some material that will harmonize with them. Until a few years ago, double boarding was used exclusively for the side walls in greenhouses built by florists, the rafters being carried into the ground about 30 inches. Today, light concrete walls about 4 inches thick are built. The cost of the concrete is almost the same as double boarding but has the advantage of being indestructible.


The construction best adapted for conservatories, park houses and greenhouses, and for private places where the improvements are desired to be permanent in character and attractive in appearance is the combination of iron and wood. In this system, the main frame which supports the weight and strain is of iron, or steel, wood being used in the frames as a setting for the glass and to form a non-conductor of great advantage in the heating of the house. The iron work in this style of construction usually consists of cast-iron sills capping the foundation walls, wrought-iron rafters setting on the sills, about 8 feet apart and running from sill to ridge, forming the side post and rafter in one piece, cast-iron gutters, and angle iron purlins between the rafters, all securely bracketed and bolted together, forming a complete framework of metal, light, strong and durable. The wood used consists of light sash-bars for supporting the glass, sashes for ventilation and doors. This woodwork being entirely supported by the metal frame, and not being used where it will be continually wet, will method of securing the sash-bars in place is very convenient in case of repairs, and renders the structure practically portable. A careful examination of any old greenhouse will show that the parts of the frame which decay first are those pieces of wood which are joined together, for water penetrating the joints soon destroys the wood. This trouble is largely avoided by arranging the frame so that each piece of wood is fastened be found as durable as any other material, and for many reasons better adapted for the requirements of a greenhouse roof. This combination system of metal and wood construction has been extensively adopted by florista, growers of cut-flowers and also the progressive vegetable- growers. In the houses built for the above, the masonry foundation walls are omitted. Posts constructed of wrought-iron are placed in the sides extending from about 30 inches below grade to the height of the eaves. These posts occur at every rafter, to which they are connected with steel or cast-iron fittings. The posts are embedded in concrete below grade, and 4-inch concrete walls built extending from 6 inches below grade up to the underside of the glazing sill.

Gutters are seldom used at the eave line in this type of house. An angle-iron eave-plate is substituted for the gutter so framed as to allow the snow and ice to slide over it. keeping the roof entirely clear from such accumulations which darken a house in winter.

The first cost is somewhat increased over an all- wood construction, but in view of its greater durability and saying in repairs, it will be found in the end, the better investment.

Cast-iron gutters are provided to collect the rainwater from the roof. By exposing the inner side of these gutters to the heat of the house, they are kept free of ice in the winter. Small metal clips fastened with screws are used to connect the wood sash-bars to the cast-iron gutters, angle-iron plates and purlins. This directly to the iron frame instead of to another piece of wood. Joints between wood and iron do not rot the wood, the hitter being preserved by the corrosion of the metal.

The curvilinear form of house (Fig. 1753) is ornamental and particularly well adapted for conservatories, palm-houses and show-houses of all kinds. It is preferred for vineries and fruit-houses, as the form allows the canes to be supported on the line of the roof without a sharp bend at the plate line. The light in a curved house, being admitted at different angles, is better diffused and more natural than when reflected through a long pane of straight glass. The cost of a curved roof is slightly greater in the construction, but the arched frame is stronger and will keep its shape better than a house with straight lines, thus largely compensating for the extra cost. For special purposes and locations, special forms of frames may be used. Good forms of commercial houses are shown in Figs. 1754-56.

The ridge-and-furrow type of house (Fig. 1757) is seldom built now except in cases in which the amount of land available is limited. The superior growing qualities of greenhouses built separately has been thoroughly demonstrated, as has also the increased productiveness of wide houses. A florist now seldom builds a house less than 30 feet wide. It is oftener 40 feet, and houses 55, 65, 75 and 85 feet wide are not uncommon. These wider houses cost less in proportion to build, grow better crops and are more economical as to labor (Fig. 1754).

Greenhouses with curved eaves (Fig. 1756) are being built more and more. This type presents a pleasing appearance and eliminates shade-casting members at the eaves. A combination sill and gutter is substituted for the plain sill on top of masonry wall to which the rafters and bars are secured.

It is commonly admitted that the so-called "sash-bar construction" is not the best or lightest method of construction, but as the absence of most of the framing reduces its cost so that it is the cheapest to build, it remains a popular method of putting up a commercial greenhouse. Circulars showing the various methods adopted by the dealers in greenhouse material can readily be secured by applying to them.

The best wood to use for greenhouse framework and plant-beds is undoubtedly cypress. In purchasing this lumber, care should be taken that only that grown in the states bordering on the Gulf of Mexico be selected. This will be found of a dark red or brown color, quite soft and easily worked. There is an inferior variety of cypress growing farther north, which is light in color, hard and springy, and likely to be shaky. As the latter variety is cheaper than red Gulf cypress it is frequently used by those who do not know the difference, to the serious detriment of the work and the loss of reputation of cypress for such purposes.

In the market there are three grades of cypress lumber, and it is important to know which to choose. The best grade is known as "firsts and seconds," and calls for lumber with a small extent of sap on the edges and occasionally a small sound knot. This is the quality which should be ordered for all the framework of the roof, sash- bars, and so on. In order to make the material entirely free from sap there will be a waste in cutting up this quality of 10 to 20 per cent. The second grade is known to the trade as "selects." This name indicates that it has been graded so that one face of each piece of lumber is of about the same quality as the "firsts and seconds," the other face generally being largely sap. This quality is fit only for outside boarding in greenhouse construction; it has too much sap. The cost is usually about five dollars a thousand less than the best grade. As it looks to the inexperienced eye almost the same as the best grade, too much of it finds its way into greenhouse structures. Such sap lumber usually will not last more than two to five years. Too great care cannot be exercised to avoid its use. The third grade of cypress lumber is termed "cutting up," and is so called because it embraces all the pieces which have imperfections, such as large knots and splits, which bar them from the better grades. This is a good quality to purchase for base-boards and plant tables, for by cutting out the sap and objectionable knots it will be found satisfactory for these purposes. The "cutting up" grade costs about ten dollars less a thousand than the "firsts and seconds." The percentage of waste in cutting up will be somewhat greater than in the other grades. Cheap timber is likely to give unsatisfactory results in greenhouse work.

Cypress lumber which has been in use for gutters, sash-bars, plates, and the like, in greenhouses where high temperatures have been maintained is still, after many years, apparently in as good condition as when first used. Owing to the porous texture of the wood, the paint, when applied, sinks in and does not make so fine a coat as on some other woods, but because of this fact the paint adheres to the wood better and lasts longer.

Glazing and painting.

Ordinary sheet or window glass is in general use for greenhouse glazing. It is better to use only the thickness known to the trade as "double-thick." This weighs from twenty-four to twenty-six ounces a square foot. The thickness known to the trade as "single thick" weighs only about sixteen ounces to the square foot, and is entirely too frail for the purpose. There is very little difference at present in the quality of the imported French or Belgian and the American glass. The weight of most of the glass of American manufacture is about 2 ounces greater a foot than the imported, and therefore it is proportionately stronger. This greater strength is of considerable importance in the additional security which it affords from damage caused by that enemy of the florists, the hail-storm. There is a great difference in the quality of the glass made by different manufacturers in its adaptation to greenhouse use. This difference is caused chiefly by the quality of the material used in the glass, making it more or less opaque, and in the variations in thickness causing lenses which concentrate the sun's rays and burn the foliage of the plants. This last defect in the glass cannot be wholly guarded against, as the product of a factory does not always run the same so that any favorite brand cannot be fully relied upon in this respect. The waves which burn will be found in all the different grades of glass, firsts, seconds and thirds, with little, if any difference, the grading being done chiefly for other defects, such as affect the value of the glass for window purposes. For these reasons, in selecting the glass for a greenhouse, it requires experience to decide what make of glass it will be best to purchase. It will be well to purchase from someone who makes a specialty of furnishing glass for greenhouses or call in the aid of some friend who has had experience in building, and can give intelligent advice.

The second quality of glass is usually selected for the best greenhouse work. The standard widths are from 12 to 16 inches, and lengths vary from 16 to 24 inches.

A favorite size is 16 by 24 inches. This is about as large as it is practicable to use double thick glass, and makes a roof with comparatively few laps.

It is not safe to purchase fourth quality of glass or the so-called "greenhouse glass" frequently offered by window-glass dealers, as both of the grades contain the culls and lights only fit to glaze cheap sash for market- gardeners, and is of doubtful economy even for this purpose. Rough plate and ribbed glass is used on large palm-gardens and conservatories in which the maximum of light is not an essential feature. Where this glass is used larger roof-bars are needed and stock construction has to be materially changed. Recently a few conservatories have been glazed with thick, polished plate- glass, making very handsome roofs, but rather expensive.

To set glass properly in a greenhouse roof, it should be bedded in the best putty on wood sash-bars and lapped at the joints. The bars should be spaced accurately, so that the glass will fit the rabbets with not over A of an inch allowance, and the panes of glass should lap each other not more than from 1/8 to ¼ of an inch. Zinc shoe-nails fasten the glass best, using from four to six to each pane, according to the size of the light. No putty should be used on the outside of the glass. A comparatively new system of glazing has been adopted by some florists in which no putty is used, but the glass is placed directly on the rabbets of the bars and the ends of the panes are butted together and held in place by wood caps fastened to the sash- bars. This system does not make a tight roof, allowing considerable water to enter the house through the joints, nor does it provide any means of escape for the condensed water from the underside of the glass, which is a very serious objection. In ordinary glazing, where each light laps over the one below, the condensed water passes through the joints to the outside, forming a perfect remedy for this trouble. The difference in the cost is very slight, if anything, provided the work is equally well done, as the value of the putty omitted is fully offset by the extra cost of the caps.

The painting of a greenhouse roof is a very important part of the work. Owing to the extremes of heat, cold, dryness and moisture to which it is exposed, the conditions are decidedly different from ordinary buildings. Three-coat work is the best. The priming coat on the woodwork should be mostly oil, and, as far as possible, the material should be dipped into a tank of paint. Iron and steel framing material should be primed with a metallic paint. The priming coat should be applied before the material is exposed to the weather. The material of the second and finishing coat should be pure linseed oil and white lead. Experience has shown that this material is the best for this work. The color should be white or a light tint of any desired shade may be used, but no heavy color should be adopted which requires coloring matter in place of the lead in the mixing. Each coat should be applied thin and well rubbed out. While the appearance may not be quite so fine when the work is first done, the paint will not peel off, and will last longer and form a better protection for the structure than when it is put on in thick coats. It will also form a good base for repainting, and this should be done in a similar manner. It is economical to repaint a greenhouse every two years, and generally one coat will be sufficient. Neglected unpainted greenhouses soon suffer, and are also very unattractive.


Stages for plants in pots, or raised beds for planting out, usually cover the entire area of a greenhouse except the walks, and their cost constitutes a considerable proportion of the expense. Palms are usually grown in solid beds or in pots or boxes sitting on the ground. Many vegetables are grown in solid beds near the ground-level. Roses and carnations are usually in raised beds. Angle-iron frames supported on adjustable gas-pipe legs, with slate or tile bottoms, form the best plant-tables (Fig. 1758). Wood bottoms which can be readily renewed are frequently substituted, saving a part of the first cost. When the table supports are of wood, care should be taken that they are not fastened against any part of the framework of the house, unless iron brackets are used so as entirely to separate the woodwork.


No greenhouse is complete without a good ventilating apparatus. About one-tenth of the roof should be arranged to open or close for ventilation, although this percentage will vary according to the form of house and the purpose for which it is used. It is not desirable to open all the ventilators in a long house with one set of apparatus, for frequently one end will not need so much ventilation as the other end or may be affected by the wind, forming a current lengthwise of the house. To avoid this, a greenhouse 200 feet long should have three or four sets of apparatus which can be operated separately. In all greenhouses of considerable width it is desirable that ventilation should be provided on both sides of the ridge so that the ventilation can be given on the "leeward" side, which will prevent the wind from blowing directly into the house.


The success of the florist, gardener or amateur in the management of a greenhouse depends largely on the satisfactory working of the heating apparatus. There are two systems of greenhouse heating which, when the apparatus is properly installed, are economical and satisfactory; viz., hot water and steam. The open-tank hot-water heating has more advantage in its adaptation to general use than any other, and is so simple that its management is readily understood by anyone. It is practically automatic and is capable of maintaining an even temperature for ten hours without attention. Low pressure steam-heating is well adapted to large commercial ranges, and to large conservatories in parks and private places where a night attendant can be kept in charge of the fires to turn on and shut off steam from the radiating pipes as the changing outside temperature may require. The heating of greenhouses to the best advantage, under the varying conditions of climate and interior requirements, demands, like the designing of greenhouses, the services of an experienced specialist in horticultural work.

Lord & Burnham Co.

Vegetable forcing-houses.

The evolution of the vegetable forcing-house has been rapid and very pronounced. From the low-built, flue-heated, dark stuffy type of house to the high, well- lighted, steam- or hot-water-heated, well-ventilated house is a change that has come not only in a very short time but which has been as marked as the transition from the ox-cart to the automobile.


In selecting a suitable location for vegetable forcing- houses, one of the most important things to consider is the marketing possibilities. It would be folly to go to the expense of building a forcing-house in which to grow vegetables to make money if they could not be sold at a profit above cost of production. The most desirable markets are those within easy driving distances. If it is necessary to ship the produce to be grown, electric lines will usually be found more economical carriers than steam lines. A grower is nearly always at a disadvantage if he has but one available road to ship over. Competition insures cheaper and better service. Cities with 25,000 to 50,000 population are often more desirable markets than much larger cities. Cities which are the chief shipping-points for southern-grown vegetables are not so good markets for forcing-house products as are the cities which are not Bo accessible from the localities making a business of growing winter vegetables for northern markets.

Another matter of importance to consider in choosing a forcing-house site is the cost of fuel. If natural gas can be secured at a reasonable cost it is a very satisfactory fuel. Coal is used most commonly as a fuel. When calculating the cost of coal, the hauling of it from the nearest shipping-point to the forcing-house should be included. It is expensive to move large quantities of coal a long distance, especially if the road is not good. When a dirt road must be used, it is usually best to do the hauling in late summer rather than in winter.

While any productive soil can be made suitable for forcing purposes, it is easier and cheaper to prepare a sandy soil than a heavy clay soil. Other things being equal, therefore, a location where the soil is a sand or sandy loam is to be preferred to a clay soil.

As large quantities of water are used in the forcing- house, an abundant supply should be known to exist before a site is selected for the houses. It is cheaper to build and easier to operate a forcing-house on level than on sloping land. A level site should, therefore, be selected if possible.

It is also an advantage to have the houses protected on the sides from which the prevailing winds come. Trees, hills or buildings are suitable for this purpose, providing they are not near enough to shade the houses much of the tune. The site should not be far from the dwelling, and the closer it is to the market or shipping-point the better. A location which cannot be satisfactorily drained or which is subject to overflow should of course be avoided.

Types of forcing-houses.

Of the various types of forcing-houses, even-span, three-quarter-span, hillside and lean-to, only two are being built very generally at present. Many New England growers prefer the three-quarter-span while the even-span is most popular in all other sections of the country in which forcing-houses are commonly erected. The three-quarter-span is used on sloping land as much as the hillside type of house, or even more. Good results are secured with either form.

Form of construction.

All-wood houses.—In the all-wood form of construction no iron is used except in the heating-plant. The walls may be all wood, or wood and concrete. The posts may or may not be set in concrete. The all-wood house was by far the most common form of construction only a few years ago and certain growers in various parts of the country still prefer the all-wood houses. Red cedar and cypress are the kinds of wood commonly used for forcing-house erection.

Semi-iron houses.—In the semi-iron form of construction all supporting posts, purlins and braces are made of iron pipes or angle-iron. The walls are usually made of concrete and all interior posts are set in concrete. The semi-iron houses are more expensive to erect than the all-wood houses but they are more durable and most growers think they are cheaper in the end.

All-iron houses.—In the all-iron construction the entire framework is of iron. The various parts are put together in such a way that the houses are very rigid. This form of construction is the most durable of all and will stand a greater weight of snow and more severe winds than the semi-iron or all-wood houses. The first cost of the all-iron houses is from one-third to one-half greater than the semi-iron construction, and this fact alone stands in the way of the general use of this construction. In spite of this objection, a number of large all-iron houses have been erected recently and they seem to be increasing in popularity, especially in the eastern part of the United States.

Trussed houses.—In the trussed form of construction, steel truss-rods are used to take the place of a part or all of the iron or wooden posts and braces used in the other forms of construction. The trussed houses are very convenient to work in and very little shade is cast by the framework. The truss-rods are frequently made to support the heating-pipes. They are also convenient supports for the wires upon which the cucumber and tomato vines are supported. As built in the past, trussed houses have not been strong enough, in all cases, to resist the weight of heavy snows and the force of severe winds. Several such houses have been demolished. If this defect can be eliminated this type of house will be very desirable.

Special features of forcing-house construction.

Width of houses.—The width of the forcing-houses in general use varies from 12 to 150 feet. In the East the tendency is to build houses 40 to 50 feet or more in width. In most parts of the West, the preference is for houses from 12 to 40 feet wide. However, there are individual growers in most sections of the West who prefer houses over 40 feet wide. The narrower houses are cheaper to build and can be kept in repair more cheaply and easily than wide houses. The wider houses, it is thought, can be heated more economically and are better adapted to the growing of warm plants such as cucumbers and tomatoes in winter than the narrower houses.

Length of houses.—The length of forcing-houses varies from 50 to 800 feet. When the gravity system of hot- water heating is used the houses are seldom over 200 feet in length. When either steam or hot water with artificial means of circulating is used, the houses may be of any length up to 1,000 feet. In most forcing centers the length of the houses has not exceeded 500 feet.

Direction of houses.—Lean-to and hillside houses are usually built with a southern exposure. Three-quarter- span houses are generally built to run east and west with the long span to the south. Even-span houses are built to run north and south, east and west and in some cases northeast and southwest. While there is not much difference in the results secured in even-span houses run either direction, there is a better distribution of sunlight throughout the day in houses which are run north and south.

Height of gutter.—Low gutters are almost entirely a thing of the past. Modern houses are usually built with gutters at least 6 feet high and 7-feet gutters are not uncommon. The outside walls of modern houses are very largely of glass construction. Connected houses are commonly built with no dividing partition except in case of extreme width when an occasional glass partition is put in. Some of the advantages of the high gutters combined with glass in the side walls and few or no dividing partitions are: greater convenience in working, better circulation of air and less shading. The former belief that the glass must be close to the plants, for best results has been found to be erroneous.

Pitch of roof.—The roofs of most even-span forcing- houses are built with a pitch of 30° to 35°. Three- quarter-span houses are usually built with the short span of the roof steeper than the long span. Hillside and lean-to houses are sometimes built with considerable less than a 30° pitch to the roof.

Glass and glazing.—Nothing but "A" quality glass is used in modern forcing-houses. Single-strength has been almost entirely replaced by double-strength glass. The standard size is 16- by 24-inch glass. It is usually laid the narrow way, although in sections of the country in which the snowfall is light the glass is frequently laid the 24-inch way. Twenty- by 24-inch glass is used by some growers. Butted glass was popular for a time and is yet with some growers, but lapped glass is most commonly used at the present time. When lapped, the glass is imbedded in putty and secured in place by the use of glazing-points. Butted glass is held in place by grooved strips of wood placed over the edges and fastened to the sash-bars with screws.

Ventilation.—An abundance of ventilation should always be provided, as the health of the plants is governed to a considerable extent by the ventilation given or not given at the proper time. When the narrow ridge-and-furrow type of forcing-house is used, provision is made for ventilators on only one side of the roof. If the houses are 30 or more feet in width, ventilators are usually placed on both sides of the ridge. In some cases, the ventilators are continuous, but owing to the liability of binding when so built most growers use separate ventilator sash. The sash are separated from each other by one or two lines of fixed glass. They are hinged on the ridge or on the header at the lower edge of the sash. When they are hinged on the header and open at the ridge the ventilation is more free, but cold draughts of air and rain or snow are more liable to enter than when the sash are hinged at the ridge and open at the lower edge. Side ventilators are a decided advantage in warm weather. When they are not provided, the air in the houses often becomes stale and oppressive. In such a condition it is unsuitable for normal plant-growth and unpleasant for those who are obliged to inhale it. Easy-working ventilator machinery should be provided for the ventilators both on the sides and roofs.

Healing.—For small forcing-houses, hot water is undoubtedly the most satisfactory method of heating. Some of the advantages of hot water over steam heat are: No night fireman is needed in small forcing-houses as the fire can be left for several hours without attention. Less fuel is required, especially in mild weather. The proper amount of moisture in the air can be maintained more easily. The heating-pipes if kept filled with water will outlast steam-heated pipes. The chief advantage of the steam heat over hot water is that it is cheaper to install. The reason for this is that when gravity is the means of circulating the water, larger pipes are required for properly heating the houses with water than are necessary where steam is the heat used. The gravity system is the principal method used in small hot-water-heated houses. Another advantage of steam over hot water is that the heat can be regulated more easily. When steam sterilization is practised it is an advantage to be able to use the same boilers for this purpose as are used for heating the houses. While a larger part of the large ranges of houses are heated with steam, some of the largest are heated with hot water. A ten-acre range of houses near Toledo, Ohio, is heated with hot water which is pumped through 1 ¼ inch heating-pipes. The houses are 700 feet long and cover a width of over 7OO feet. There is but one heating- plant and it is located at the center of one side of the range. The heating-pipes are close to the ground and are nearly level from one end of the houses to the other. The water is pumped through the entire length of pipes in a very few minutes. The installation of this hot-water heating-plant cost little if any more than a steam heating-plant would have cost and it can be operated more cheaply than a steam plant which would be large enough to heat a range of houses of the same size.

Heating-pipes.—Practically all pipes used for heating purposes at the present time are of wrought-iron. They are threaded and can be united by screwing them into connections made for the purpose. This method of connecting furnishes a tight joint and can be easily put together. Two-inch pipes are used as a rule for hot-water heating with gravity means of circulating. For hot water with forced circulation and for steam, 1 ¼ inch pipes are generally considered the best size to use.

Benches vs. beds. — In nearly all vegetable-forcing centers, except Chicago, raised benches are no longer used except by an occasional grower. The cost of building the benches is so great that most growers think the added cost more than offsets the advantages of the benches over the beds. Cement benches when arranged for sub-irrigation are very satisfactory. They are especially desirable for lettuce and tomatoes. Surface-watered benches are not nearly so satisfactory for these crops as sub-irrigated benches. Ground beds are frequently made with concrete sides but the more recent plan is to have nothing but narrow concrete walks to separate one bed from another.

Service room.—One of the features which goes with an up-to-date vegetable-forcing plant is a conveniently arranged and well-lighted service room. Provision should be made for washing vegetables and for other operations which go with a proper preparation of the vegetables for the market.

Plant-house.—Another important adjunct to a modern forcing-house is a plant-house which is independent of the other part of the range so far as the heating of it is concerned. To grow young plants successfully, especially warm plants such as cucumbers and tomatoes, it is important to be able to provide the proper temperature for each kind of plant. This can be done to best advantage if the plants can be grown in a plant-house built especially for that purpose. C. W. Waid.

Greenhouse glass.

The selection of glass for greenhouses, and the nature of the imperfections which render it undesirable for such use, are questions which have received much attention from horticultural writers, and which have brought forth a variety of answers. Three qualities are essential in all glass to be used in greenhouse construction: first, minimum of obstruction to solar rays; second, strength sufficient to withstand the strain of winds and storms, especially hail; and third, freedom from defects that render it liable to burn plants grown under it.

It is an established fact that plants thrive best under a clear and transparent glass, which lets through the greatest possible percentage of the sun's rays. This includes all the solar rays, calorific or heat rays, and actinic or chemical rays, as well as the colorific or light rays. Clear white glass of the grade known as "single thick" (twelve panes to the inch) lets through from 60 to 70 per cent of the sun's rays; common green glass of the same thickness, 52 to 56 per cent, and "double thick" (eight panes to the inch) common green glass, from 50 to 52 per cent. This percentage is reduced by other colors, dark blue glass letting through but 18 per cent. In connection with the matter of tint, it should be noted that some glass, especially clear white glass purified with arsenic acid, or that in which a large amount of potash is used in proportion to the amount of lime employed in manufacture, becomes dull after long exposure to the weather, the dullness being occasioned by the efflorescence of salts contained in the glass. Before this disintegration has proceeded too far, the crust or efflorescence may be removed with muriatic acid.

The strength of glass depends upon its thickness and upon the thoroughness of the annealing. Glass is annealed by passing through a series of ovens, where it is raised to a high heat and then gradually cooled; whatever toughness and elasticity the finished product may contain is due to this process. The thickness of glass varies, not only with grades (single and double thick), but also more or less within the grades, and even in different parts of the same pane. Single thick glass is too thin for use in greenhouses; in selecting any glass for such a purpose it should be examined pane by pane, and all showing marked variation in thickness, either between panes or in different parts of the pane, rejected. A pane of varying thickness is much more liable to breakage from climatic changes or sudden shocks than one which is uniform in this regard. From the foregoing statements it will be seen that, in general, the ordinary double-thick green glass is best as regards both tint and strength, green glass being less liable to change in tint than white, and the double-thick being the stronger grade. By green glass is meant simply the ordinary sheet glass, the green color of which is noticeable only at the cut edge.

It has long been a common opinion that such visible defects in sheet glass as the so-called "bubbles," "busters," and "stones," produce a focusing of the solar rays passing through them, thus burning the foliage of plants grown under glass containing these defects (Fig. 1759). This view has been held by glass manufacturers and horticulturists alike, and seems not to have been contradicted publicly until 1895 (Bulletin No. 95, Cornell University Agricultural Experiment Station, page 278). In view of the erroneousness of this theory, it is rather remarkable that it should have gained such prevalence. Nearly all bubbles and blisters are thinner in the middle than at the periphery, being thus concave rather than convex lenses, and actually diffusing the rays of light passing through them rather than producing destructive foci. While it is true that sand-stones or Knots in glass may produce foci, these points of focus scarcely ever exist more than a few inches from the surface of the glass; consequently, these defects can do no damage when occurring in roofs several feet distant from the growing plants below.

The only full and complete series of experiments on this subject in this country (conducted at the Cornell University Agricultural Experiment Station, the Physical Laboratory of Cornell University, and a glass factory in Ithaca, New York, but yet unpublished) shows the true cause of the burning by glass to be the variation in thickness of the entire pane, or a portion of it, thus producing a prismatic or lens-like effect (Fig. 1760), which causes a more or less distinct focusing of the sun's rays at distances varying from 5 or 6 feet to 30 feet, or even more, from the glass.

This defect usually occurs along the side or end of the pane and is not visible to the eye, but it may be detected easily by using the micrometer caliper or by testing in the sunlight. It may be found in all kinds of glass, and is caused by the glass-blower while reducing the upper or pipe end of the cylinder from which sheet glass is made, thus facilitating the removal of the cap" or neck end of the cylinder, by which it is attached to the pipe while being blown. The defect, as already stated, is one which may be found in all grades and qualities of sheet glass, of both foreign and domestic manufacture. The fact is well known that differences in the thickness of spectacle lenses, which are imperceptible to the eye, may produce sufficient refraction to vary materially the direction of rays of light passing through such lenses, and it is not difficult to see that the same effect may be produced by similarly imperceptible variations in the thickness of sheet glass. That this is the case has been conclusively shown by the series of experiments mentioned above. These also show that burns on plants caused by defective glass roofs occur in lines and not in isolated spots, burns of the latter description being usually the result of a weakening or deterioration of tissue, due to carelessness in the matter of ventilation, humidity of the atmosphere, water, and temperature of greenhouses, rather than to defects in the glass.

If, therefore, it is not possible to secure glass of uniform thickness with certainty, it may be found cheaper and often fully as satisfactory to purchase the lower or common grades of double-thick glass, using in the roof only those panes which show, after testing in the sunlight for foci, an entire lack of the prismatic character which makes them dangerous to plants grown under them. J. C. Blair.

Greenhouse heating.

In all sections in which the temperature drops below the freezing point, it is necessary to provide some artificial means for heating greenhouses. Nearly all modern structures are warmed either by steam or hot water, although hot-air flues are occasionally used. While hot water is preferred for small ranges of glass, as it can be depended upon to furnish an even degree of heat when left for a number of hours, steam is very generally used for extensive plants, as the cost of piping the houses is much less than when hot water is used. Steam boilers require more attention than hot-water heaters, but when there is more than 10,000 or 12,000 square feet of glass, it is best to have a night fireman and watchman, and the extra expense can be made up by the saving in the cost of fuel, as it will be possible to use a lower grade of coal. Under these conditions the cost of running a steam plant will be as low as with hot water, but in small houses, where hard coal is used, and the fires receive no attention for six to eight hours during the night, hot-water heaters will be cheapest to operate, and will be most satisfactory. Some of the up-to-date ranges of the largest size make use of hot water and are able to secure a perfect circulation by the use of steam or electric pumps, which also make it possible to reduce the size of the piping, and as a higher temperature is maintained in the water, the amount of radiation required and consequently the cost of piping the houses is reduced practically to that in steam systems. Similar results can be secured in closed systems where some method of placing the water under pressure is used. See, also, under Forcing-houses, p. 1402. As the various flowers and vegetables grown under glass require different temperatures, the piping of greenhouses has to be varied accordingly. Thus, although it may vary from 3° to 5° for different varieties of the same species, our common plants require the following night temperatures: violets and lettuce, 45° to 55°; radishes and carnations, 50° to 55°; roses and tomatoes, 60° to 63°; cucumbers and stove plants, 70°.


For small ranges, whether steam or hot water is used for heating, the best boilers are those constructed of cast-iron as thev will be found more durable than those in which wrought-iron or steel is used. By using either vertical or horizontal sections, it is possible to build up boilers of considerable size, but, especially if to be used for steam heating, it will be preferable to use wrought-iron or steel boilers if they have a capacity of more than 2,000 square feet of radiation. Except for those of extremely large size, the ordinary tubular boilers will be found adapted both for steam and hot- water heating, although when used for hot water they will be more effectual if the entire shell is filled with tubes, as there is no occasion for leaving a steam space at the top of the boiler. Such boilers are of low cost, economical and durable.

There are also on the market several forms of wrought tubular boilers which are giving good results for heating greenhouses with hot water. For ranges of the largest size, where forced draft is used, water-tube boilers are extremely powerful and very satisfactory. When installing a heating-plant, it will be safest to use two or more boilers rather than one large one of the same capacity, as when there is only a single boiler serious losses may result if repairs to the boiler become necessary in extremely cold weather, which might be lessened or entirely prevented when there are two or more boilers in the battery, and it is possible to cut out the one which has become damaged. Especially in mild weather during the spring and fall, the firing will be more economical when it is possible to use a boiler just large enough to heat the houses, rather than one which is several times larger than is necessary' at that time, as would be the case when only one boiler is used. The durability of the boiler and the economy of heating will be greatly increased when the heating capacity is considerably larger than is really necessary, as when the firing is forced in extremely cold weather it will not only result in a loss both in fuel and labor, but will shorten the life of the boiler.

The size of hot water boilers is usually expressed in terms of radiation, or the number of square feet of heating surface it can supply economically. In a given boiler there is a fixed ratio between the size of the grate and the area of the fire surface of the boiler, but this will depend very largely upon its construction and efficiency of the fire surface, as well as upon the size of the boiler. In the case of small hot-water boilers the ratio between the grate and fire surface is often as small as 1 to 15, while it may be as much as 1 to 35 in larger ones, and even more when the boilers have frequent attention and hard coal is used. One reason for using a relatively large grate in small boilers is because it makes it possible to leave the fire for eight or ten hours without care or attention, while for large boilers and where a night fireman is employed, the ratio between the grate and fire surface may be much greater.

The capacity of steam boilers is usually rated in horse-power, and it is considered that for each horsepower a boiler will heat 100 square feet of radiation; an average of 15 square feet of fire surface is considered equal to one horse-power, it being customary to estimate that 10 or 12 feet in a large boiler will equal one horse-power, while in a very small one as much as 18 feet would be required. Thus, in medium-sized boilers an area of 10 square feet of grate will answer for 250 square feet of fire surface and this will be sufficient for about 1,700 square feet of radiating surface when steam is used; and as 75 to 100 per cent more radiation will be required when hot water is used, a boiler of the above size will answer for 2,800 to 3,400 square feet of hot-water radiation. In the case of small boilers that will not have attention at night, it is usually advisable to reduce the above estimates about 25 per cent, and when a boiler is required for 1,000 square feet of radiation, we should select one that is rated at 1,250 square feet.

Home-made coil boilers axe sometimes constructed for hot-water heating since the cost will generally be considerably less than for tubular boilers. As a rule, however, they will be found less durable and lacking in efficiency as compared with the better class of greenhouse boilers now on the market. For making such boilers, 2-inch wrought-iron pipe in lengths of 4 to 6 feet is used. Formerly 1-inch pipe was used for coil boilers but it is comparatively thin, and, especially where the threads were exposed it was quickly eaten through so that it proved far from being as durable as the larger sizes of pipe. There was also more trouble from the boiling over of the water than when larger pipes were used and if the boilers are constructed of 1-inch pipe it is necessary either to have an elevated expansion tank or run it as a closed system. In making a coil boiler, the pipes are cut of the desired length and the ends are connected either by return bends or by manifolds so as to form a number of vertical coils, each containing from six to ten pipes. The upper ends of the manifolds are joined at the front end of the heater and connected with the main flow-pipe; while the lower ends of the rear manifolds are joined to the returns. As a rule, the grate is of the same width as the coils and from one- half to two-thirds as long.

Although a box coil is much cheaper than a cast- iron heater, when we have added the cost of the grate, doors and other fittings, and of bricking it in, the amount saved will not oe large, and its use will often be found less economical, especially as the coil boilers are, as a rule, not more than one-half as lasting as cast- iron boilers, most of which are complete in themselves and require no brickwork or trimmings.

Hot-water piping.

When hot water first came into use for the heating of greenhouses, 4-inch cast-iron pipes were used, but, as the joints were packed with oakum, cement or iron filings, they frequently gave trouble by leaking and it was much more difficult to make changes or repairs than in the present systems for which small, wrought- iron pipes with screw joints are used. Owing to the large amount of water in the cast-iron pipes, the circulation was necessarily quite sluggish and it was not easy to secure the high temperature in the water that can be obtained with smaller pipes. Another objection to the use of these large pipes is that it is not possible to carry the flows overhead, while with smaller pipes one may not only have the flows but some or all of the return- pipes above the level of the benches. By elevating the pipes above the level of the boiler, the rapidity of the circulation and the temperature of the water in the pipes can be considerably increased.

In case a number of houses are to be supplied from one boiler, or if the heater is at some distance from the coils, it is better to start from the boiler with one large flow-pipe, or with two pipes leading from different Bides of the boiler, rather than carry an independent pipe to each house. When there are several houses to be heated, it is customary to have them side by side and one large flow-pipe can then be run across the nearest end of the houses from the boiler. If the houses run north and south, the boiler may be located at one corner or in the middle of the north end of the range, and either a work- or storeroom, or some other form of a head house, should be constructed in which the main heating-pipes can be carried, as well as to protect the north end of the houses and facilitate getting from one to another. Sometimes greenhouses run east and west, in which case there should either be a head house at the east end of the range, or if the houses are more than 200 feet in length it may be run through the center of the houses.

The size of the main feed-pipe as well as of the branch pipes should be in proportion to the amount of radiation they supply. In determining the amount that can be handled by pipes of different sizes, it is always advisable to use somewhat larger supply-pipes when all of the radiation, both flow and return, are under the benches, than when all of the flow-pipes, at least, are overhead. A similar allowance should be made when the boiler is partly above the level of the returns, as compared with systems in which the coils are a number of feet above the top of the boiler, since in the latter case a much smaller supply-pipe will suffice. In a general way, the following sizes can be used as supply-pipes:

Size of pipe Square feet of radiation.

1 ½ inch 75 to 100

2 inch 150 to 200

2 ½ inch 250 to 350

3 inch 400 to 300

3 ½ inch 600 to 800

4 inch 1,000 to 1,200

5 inch 1,500 to 2,000

6 inch 2,500 to 3,500

The main supply-pipe or pipes should, if possible, rise vertically from the heater to a point somewhat higher than the highest point in the system, and then as it runs out through the houses should be given a slight fall, say 1 inch in 20 feet, so that there will be no opportunity for the pocketing of air in the pipe. While a slight downward slope will unquestionably give better results than the uphill arrangement which is sometimes used, the difference will be comparatively slight and, if the circumstances make it preferable to run the flow-pipes uphill, satisfactory results will be obtained provided they are considerably elevated above the boiler. Especially, if the flow-pipes run uphill, it will be advisable to have them of good size.

When taking off the supply for each of the houses, one large pipe of a size sufficient to provide the amount needed may be used, or from two to five smaller pipes may lead from the main flow-pipe into each of the houses. For houses up to 250 feet in length, it will generally be found desirable to run 2 1/2 -inch flow-pipes through the house, but for longer houses 3-inch flow- pipes should be used. Just how many flow-pipes will be needed will depend not only upon the length of the house, but upon the number of return-pipes to be supplied. Thus, while a 2 1/2 inch flow will supply two 2-inch returns in a house 250 feet long, the number of returns which it will feed in shorter houses will be nearly in inverse proportion to the length of the return coils. When the amount of radiation to be supplied does not exceed 250 to 350 square feet, one 2 1/2-inch flow-pipe in a greenhouse will be sufficient and this should preferably be placed from 1 to 3 feet below the ridge. For slightly larger houses, two flow-pipes may be located on the wall plates. If as many as five pipes are necessary, the fourth and fifth pipe may be suspended from the roof under the middle of the sash-bars. In the case of houses so large that more than five 2 1/2 -inch flow- pipes are required, 3-inch flows should be used.

The length of the coils and their height above the boiler will determine the size of the pipe which should be used for the returns, since a smaller size will answer in short coils and in those that are considerably elevated than for long coils which are but little, if any, above the level of the boiler. For the construction of coils 75 feet or more in length, 2-inch pipe should be used, and it will generally be found preferable to a smaller-sized return-pipe when they are only 50 feet in length, especially if the flows are under the benches or when the coils are below the top of the boiler. For short coils, pipes as small as 1 ¼ th inch may be used where they are somewhat elevated but for ordinary commercial greenhouses it will be better to use 2-inch pipe for the returns, although 1 ½ inch pipe might answer in houses up to 75 feet in length, as, while small pipe furnishes the most effective radiation to the square foot, the increased friction impedes the circulation.

In narrow houses, the return-pipes may be placed upon the side walls, but as the width increases it will be generally advisable to have from one-third to one- half of the returns either under the benches or in the walks when beds are used. From the fact that running the pipes overhead will not only improve the circulation but will prevent cold draughts of air upon the plants, it is often desirable when but one overhead flow-pipe is used to bring back one return upon each of the purlins. When the end of the house is much exposed, it is an excellent plan to drop down one feed - pipe from the end of the main, or two when there is a door in the end of the house, and supply coils running in either direction to the corner of the house and thence along the walls toward the end where the heater is located. Particularly when the pipes are but little, if any, above the top of the boiler, the circulation will be improved by carrying the return-pipes as high as possible, but of course care should be taken when they are under the benches not to have them so high that they will dry out the soil.

The returns may be arranged in horizontal coils under the benches, or in vertical coils on the walls, or on the sides and supports of the beds and benches. The pipes in the coils may be connected at their ends either by means of manifolds, or by tees and close nipples, but in either case provision should be made for expansion of the pipe which with vertical wall coils may be done by running them partly across the ends of the houses and the same means may be used in horizontal coils, or the headers at the lower ends of the coils may be connected with the ends of the pipes by means of nipples and right and left ells. Whenever possible, there should be at least two returns supplied by each of the flow-pipes and the number may be increased until the capacity of the flow is reached. In determining just how many returns may be supplied by a given flow- pipe, one should always make allowance for the radiation furnished by the flow-pipe itself and, as the friction will be greater in a large number of short returns than for the same radiation with long returns, this should be considered in adjusting the ratio between the flow- and return-pipes.

Even greater attention should be given to the grading of the small return-pipes than to the larger flow-pipes, as the danger from pocketing of the air will be increased. For the smaller sizes, it will be advisable to give them a slope of at least 1 inch in 15 feet; but, if carefully graded and securely supported at intervals of 10 feet, good results can be obtained with 2-inch pipe with a fall of 1 inch in 20 feet; and if no more than 1 inch in 30 feet is available even this light fall will generally suffice to rid the pipes of air. This is really the main object for which the pipes are sloped, as the circulation would be fully as good, or better, if they are run on a level from the highest point in the system, provided the air did not pocket.

By having the highest point in the system near the boiler and attaching the expansion-tank at that point, one secures a downhill arrangement of the pipes which not only gives a better circulation than when the flow-pipes run uphill, but it does away entirely with air- valves which must be provided when the flow-pipe runs uphill and which often give trouble.

The method of piping which has been advocated, i.e. running one or more pipes in each house to the farther end and there connecting them with the returns, will give a more even temperature than can be secured in any other way. Formerly, it was the custom to connect the supply-pipes with the coils at the end of the house nearest the boiler. In some cases, one-half of the pipes in the coils served as flows to feed an equal number of return-pipes, or all of the pipes in the coil were connected at the farther end of the house with a main return-pipe, of the same size as the feed-pipe, which was brought back underneath the coil, or all of the coils in the house were connected into one main return. When the latter arrangement is used, the heating of the house is less uniform than with an overhead flow-pipe, the farther end of the house being cooler than the one near the heater.

Unless the heating system is connected directly with the water-supply system, which is used as an expansion- tank, a special tank must be provided and connected with the highest part of the flow-pipe or with one of the returns near the heater. While it would answer if this tank is located at some point but slightly above the heating system, it is always desirable to have it somewhat elevated, as this will raise the boiling-point of the water in the system and hence increase its efficiency, as well as lessening the danger of its boiling over. The pipe connecting the expansion-tank with the heating-pipes should not be less than ¾ inch and this should be increased to 1 ½ to 2 inches in large systems. The size of the expansion-tank should be sufficient to equal the amount which the water in the system will increase in volume when it is raised from a temperature of 40° to 200°, with a margin of perhaps 50 per cent. By connecting the expansion-tank with the highest part of the system, one not only does away with the necessity of using air-valves but also lessens the tendency of the water to boil over.

When there are several houses in the range connected with one system, it is always a good practice to have a valve upon the supply-pipe leading to each house, with other valves upon at least one-half of the coils. It will thus be possible to reduce the radiation in each house or to cut it out entirely if desired.

Hot water under pressure.

Especially in large ranges it is now becoming customary to place the water under pressure, thus making it possible to raise the temperature at which it will boil, and in this way the circulation can be improved, and instead of the water in the returns having an average temperature of 150°, it can be maintained several degrees above the ordinary boiling-point of water. The principal objection to this plan is that the water in the boiler being hotter, the gases of combustion are not cooled down to the same extent as when the water is at 160° or less. This results in lessening the economy of coal-consumption, placing it upon about the same plane as when steam is used. On the other hand, this system has the merit of reducing the amount of radiation required in the heating - system, and in this way lessening the cost of piping the greenhouse fully twenty-five per cent.

Various methods of placing the water in the heating- system under pressure have been employed. Among them is to use a safety-valve and a vacuum-valve, either upon the expansion-tank, or if this is not closed, upon the expansion-pipe within the tank. The safety- valve allows either the air or the water, as the case may be, to pass out of the system when the pressure desired is reached, while the vacuum-valve permits the air or water to re-enter the system when the pressure drops.

What is known as the "mercury generator" or "circulator" also serves the same purpose. In these a column of mercury prevents the escape of the water in the system until the pressure has reached the point desired, when it allows a portion of the water to escape and, later on, to re-enter the system when the pressure decreases. It will be seen that this acts in exactly the same way as the safety-valve and vacuum-valve described above. By raising the boiling-point of water and improving the circulation, it not only makes it possible to use smaller pipes both for flows and returns, but the amount of radiation required will be considerably reduced. In fact, although it is not advisable to carry it to that extent, it is possible to reduce the amount of radiation practically to that required for steam-heating.

This system is of value particularly in sections of the country in which the usual winter temperature is well above zero but where the mercury drops 10° to 15° for a short period each winter. By piping the houses so that the desired temperature can be obtained for the houses in ordinary weather by using an open system, it will then be possible by using a "circulator" to maintain the same temperature in the houses even though the mercury drops 15° or 20° lower. This will make a considerable difference in the cost of piping the houses and the efficiency of the system so far as coal is concerned will be affected only during the few days when the use of the "circulator is necessary.

The use of a closed system is also helpful when, owing to local conditions, it is necessary to place the boiler upon or slightly below the level of the walks in the houses. While much can be done to secure a circulation by using overhead flows and keeping the returns as high as possible, the circulation can be still further improved if it is run as a closed system. Still another method of increasing the rapidity of the circulation and the efficiency of the heating-system is to place either upon the main flow- or return-pipe a pump, worked by steam or electricity, by which it will be possible greatly to accelerate the circulation of the water, so that such matters as the relative elevation of the boiler and heating- pipes will need but little consideration and it will be possible to decrease to a considerable extent the size and number of the heating-pipes.

Estimating hot-water radiation.

Owing to the great variations in temperature and the differences in the construction of greenhouses, and also in their exposures, it is impossible to give any explicit rules regarding the amount of radiation that will be required under all conditions; but experience has shown that in well-built houses any desired temperature can be secured. Knowing the minimum outside temperature and the temperature to be maintained within the house, it is necessary only to install a heating-plant with a radiating surface having a certain definite ratio to the amount of exposed glass and wall surface. It is, of course, understood that there must be a proper adjustment between the size of the boiler and the radiating surface and that the system is so arranged as to (five good results. Thus, when a temperature of 40° is desired in sections in which the mercury does not drop below zero, it will be possible to maintain it when 1 square foot of radiating surface is provided for each 5 square feet of glass; if 45° is required there should be 1 foot of radiation for 4 ½ feet of glass. Under the same conditions, 50°, 55°, 60°, 65° and 70° can be obtained, respectively, by using 1 square foot of radiating surface for each 4, 3½, 3,2½ , and 2 square feet of glass. When the outside temperatures are slightly under or above zero, there should be a proportionate increase or decrease in the amount of pipe used; and, if the houses are poorly constructed or an exposed location, it will be desirable to provide a still further increase in the amount of radiating surface. Under the very best conditions, the temperatures mentioned can be obtained with a slightly smaller amount of radiation, but the greatest economy so far as coal-consumption and labor are concerned will be secured when the amount of radiation recommended is used.

In determining the amount of exposed glass surface, the number of square feet in the roofs, ends and sides of the houses should be added, and to this it will be well to add one-fifth of the exposed wooden, concrete or brick wall surfaces. If the amount thus obtained is divided by the number which expresses the ratio between the area of glass and the amount of radiation which will be required, it will give the number of square feet of heating-pipe which must be installed. The unit of measurement of wrought pipe is its interior diameter, while its radiating surface is determined by its outside circumference, and, although it will vary slightly according to the thickness of the pipe, it is customary to estimate that 1-inch pipe will afford about .344 square feet of radiating surface to the linear foot, while 1 ¼ -, 1 ½ -, 2-, 2 ½ -, and 3-inch pipe will furnish respectively .434, .497, .621, .759 and .916 square feet of radiation for each foot in length of pipe. Tne following example will perhaps aid in determining the amount of radiating surface and its arrangement in a greenhouse. If a house is 32 feet in width and 200 feet in length, with 30 inches of glass in each side wall and with one end only of exposed glass, and a concrete wall 3 feet high on two sides and one end, there will be about 9,000 square feet of glass. To heat this to 50° in zero weather it will be necessary to use one-fourth as much radiating surface, or 2,250 square feet. In a house of this length it will be possible to supply this amount of radiation by means of five 2 ½ -inch flow-pipes, and the remaining radiation will be provided by means of ten 2-inch returns which will allow two for each of the flow-pipes. These figures are intended to apply when an open system is used but, if a "generator" is attached, not to exceed four flows and eight returns will be required.

The use of long, straight runs of pipe will give the best results and, whenever possible, ells and tees should be avoided, but if they must be employed special hot-water fittings should be secured.

In conservatories with high side walls it is desirable to place the flow-pipes at the plate and the returns on the walls or under the tables. Figs. 1764-1766 illustrate the lay-out of pipes in carnation-, rose-, and violet-houses.

Heating by flues.

When fuel is cheap, and when either a low temperature is desired in the house, or the outside temperature does not drop much below the freezing point, hot-air flues may be used but, while the cost of constructing them is small, the danger of fire is so great that they are often found to be far from economical. A brick furnace is built at one end of the house and from this a 10- or 12-inch flue is constructed to carry the smoke and hot gases through the house to the chimney which may be either at the farther end of the house or directly over the furnace, the flue, in the latter case, making a complete circuit of the house. When the houses to be heated are more than 60 feet long, it is advisable to have a furnace in each end, with the flue from each extending only to the center of the house and returning to the end from which it started. For the first 30 feet the lining of the flue, at least, should be of fire-brick, but beyond that the flue may be constructed of sewer- pipe.

Piping for steam.

Except that it is possible to use smaller flow- and return-pipes, the arrangement of the piping for steam- heating is not very unlike that described for hot water. Unless the houses are more than 30 feet wide and 150 feet in length, only one flow-pipe need be used and that can be carried from 2 to 4 feet below the ridge. In wider and longer houses, it is generally advisable to put in two or more flows. One of these flows can be carried on each wall-plate and in extremely wide houses others may be under the ridge and purlins.

For determining the size of steam mains, a good rule to use is to take one-tenth the square root of the radiation to be supplied and consider this to be the diameter in inches of the main required. Thus for supplying 400 square feet of radiating surface we would take one- tenth the square root of 400 (i/400-1-10 = 2), which will give 2 inches as the diameter of the main required. As the amount of radiation increases, a slight reduction can be made in the size of the mains and 2 ½ -, 3-, 3 ½ -, and 4-inch supply-pipes will answer respectively for 700, 1,000, 1,400 and 1,900 square feet of radiation. This is intended to apply with low-pressure steam, and as the steam-pressure is increased above five pounds a slight decrease in the size of the mains would be permissible.

The size of the pipes to be used for the coils will also depend upon the length of the house. For ordinary lengths 1 ¼ -inch pipe will be desirable, but, when they are more than 250 feet in length, 1 ½ -inch pipe may be used with low pressure steam and, in those much less than 100 feet, 1-inch pipe will answer. The location and arrangements of the coils will necessarily be determined by the openings in the walls and whether beds or raised benches are used. One of the simplest and most satisfactory ways of piping a greenhouse of moderate size, say from 20 to 30 feet in width and up to 150 feet in length, is to run the flow-pipe, which would be either 2- or 2 ½ -inch, overhead and bring back the coils on the walls, or, if raised benches are used and crops for which bottom heat will be helpful are to be grown, from one-third to one-half of the return- pipes may be distributed under the benches and the remainder may be on the walls. The return-coils should of course be given a slight slope toward the boiler, care being taken that no opportunity is afforded for the air to pocket and prevent the free flow of the water from the condensed steam back toward the boiler. A fall of 1 inch in 10 feet will suffice, and even less will answer if care is taken in grading and supporting the pipes.

In order to prevent the water from backing up in the coils, it is desirable that they should be at least 18 or 20 inches above the level of the water in the boiler, while 3 or 4 feet would be even better and will be necessary in large ranges. Unless this can be secured it will not be possible to return the water of condensation to the boiler by gravity and either a steam trap or pump should be provided for the purpose. By means of these, the water can be carried to a water feed-tank from which it can be fed into the boilers.

There should be an automatic air-valve at the end of each coil and, in order to regulate the amount of steam, a shut-off valve should be placed upon both flow- and return-pipes leading to each house. Unless there are several coils in each house, one or more of which could be cut off by means of valves, it will always be well to have valves upon a number of the pipes in the coils so that all but one or two can be cut off if desired. To prevent the water from being forced out from the boiler when the steam is turned into the houses, there should be a check valve in the main return-pipe near the boiler.

The amount of radiation which will be required to secure a given temperature will vary to some extent with the amount of pressure carried in the boiler, or in the coils, when a reducing-valve is used, but as a rule, this is not much more than five pounds and often it is even less. It will be best to provide a sufficient amount of radiation to furnish the temperature desired in ordinary cold weather without carrying any pressure and then, by raising the pressure to five to ten pounds, secure the heat required during the cold waves.

In determining the amount of radiation for a steam- heated house, for zero weather, it will answer if one considers that 1 square foot of pipe will heat 9 square feet of glass when 40° are desired, and will suffice for 7, 5 and 3 where 50°, 60°, and 70°, respectively, are, required. Fig. 1767 illustrates the piping required for heating a rose-house with steam. L. R. Taft.

Greenhouse management

Persons usually learn to grow plants under glass by rule of thumb. Such practical knowledge is always essential, but better and quicker results are secured if underlying truths or principles are learned at the same time. Even if no better results in plant-growing were to be attained, the learning of principles could never do harm, and it adds immensely to the intellectual satisfaction in the work. There is no American writing that essays to expound the principles of greenhouse management, although there are manuals giving direct advice for the growing of different classes of plants. There are two kinds of principles to apprehend in greenhouse management,—those relating to the management of the plains themselves, and those dealing primarily with the management of the house.

The first principle to be apprehended in the growing of plants under glass is this. Each plant has its own season of bloom. Every good gardener knows the times and seasons of his plants as he knows his alphabet, without knowing that he knows. Yet there are many failures because of lack of this knowledge, particularly among amateurs. The housewife is always asking how to make her wax-plant bloom, without knowing that it would bloom if she would let it alone in winter and let it grow in spring and summer. What we try to accomplish by means of fertilizers, forcing and other special practices may often be accomplished almost without effort if we know the natural season of the plant. Nearly all greenhouse plants are grown on this principle. We give them conditions as nearly normal to them as possible. We endeavor to accommodate our conditions to the plant, not our plant to the conditions. Some plants may be forced to bloom in abnormal seasons, as roses, carnations, lilies (see Forcing). But these forcing plants are few compared with the whole number of greenhouse species. The season of normal activity is the key to the whole problem of growing plants under glass; yet many a young man has served an apprenticeship, or has taken a course in an agricultural college, without learning this principle.

The second principle is like unto the first: Most plants demand a particular season of inactivity or rest. It is not rest in the sense of recuperation, but it is the habit or nature of the plant. For ages, most plants have been forced to cease their activities because of cold or dry. These habits are so fixed that they must be recognized when the plants are grown under glass. Some plants have no such definite seasons, and will grow more or less continuously, but these are the exceptions. Others may rest at almost any time of the year; but most plants have a definite season, and this season must be learned. In general, experience is the only guide as to whether a plant needs rest; but bulbs and tubers and thick rhizomes always signify that the plant was obliged, in its native haunts, to carry itself over an unpropitious season, and that a rest is very necessary, if not absolutely essential, under domestication. Instinctively; we let bulbous plants rest. They usually rest in our winter and bloom in our spring and summer, but some of them—of which some of the Cape bulbs, as nerines, are examples—rest in our summer and bloom in autumn.

The third principle from the plant side is this: The greater part of the growth should be made before the plant is expected to bloom. It is natural for a plant first to grow: then it blooms and makes its fruit. In the greater number of cases, these two great functions do not proceed simultaneously, at least not to their full degree. This principle is admirably illustrated in woody plants. The gardener always impresses on the apprentice the necessity of securing "well-ripened wood" of azaleas, camellias, and the like, if he would have good flowers. That is, the plant should have completed one cycle of its life before it begins another. From immature and sappy wood only poor bloom may be expected. This is true to a degree even in herbaceous plants. The vegetative stage or cycle may be made shorter or longer by smaller or larger pots, but the stage of rapid growth must be well passed before the best bloom is wanted. Fertilizer applied then will go to the production of flowers; but before that time it will make largely for the production of leaf and wood. The stronger and better the plant in its vegetative stage, according to its size, the more satisfactory it should be in its blooming stage.

Closely like the last principle is the experience that checking growth, so long as the plant remains healthy, induces fruitfulness or floriferousness. If the gardener continues to shift his plants into larger pots, he should not expect the best results in bloom. He shifts from pot to pot until the plant reaches the desired size; then he allows the roots to be confined, and the plant is set into bloom. Over-potting is a serious evil. When the blooming habit is once begun, he may apply liquid manure or other fertilizer if the plant needs it. The rose- grower or the cucumber-grower wants a shallow bench, that the plants may not run too much to vine.

A carnation-grower writes that there is "little difference in the yearly average as to quality or quantity of flowers, but plants grown on shallow benches come into flower more quickly in the fall. Those grown in solid beds produce an abundance of flowers later in the season. The preference of commercial carnation- growers is for raised benches so that there may be more blooms early in the fall and at the Christmas holidays."

The natural habitat of the plant is significant to the cultivator; it gives a suggestion of the treatment under which the plant will be likely to thrive. Unconsciously the plant-grower strives to imitate what he conceives to be the conditions, as to temperature, moisture and sunlight, under which the species grows in the wild. We have our tropical, temperate and cool houses. Yet, it must be remembered that the mere geography of a plant's native place does not always indicate what the precise nature of that place is. The plant in question may grow in some unusual site or exposure in its native wilds. In a general way, we expect that a plant coming from the Amazon needs a hothouse; but the details of altitude, exposure, moisture and sunlight must be learned by experience. Again, it is to be said that plants do not always grow where they would, but where they must. Many plants that inhabit swamps thrive well on dry lands.

Yet, the habitat and the zone give the hint: with this beginning, the grower may work out the proper treatment. Examples are many in which cultivators have slavishly followed the suggestion given by a plant's nativity, only to meet with partial failure. Because the dipladenia is Brazilian, it is usually supposed that it needs a hothouse, but it gives best results in- a coolhouse. Persons often make a similar mistake in growing the pepino warm, because it is Central and South American. Ixia is commonly regarded in the North as only a glasshouse subject because it is a Cape bulb, yet it thrives in the open in parts of New England, when well covered in winter.

The best method of propagation is to be determined for each species; but, as a rule, quicker results and stockier plants are secured from cuttings than from seeds. Of necessity, most greenhouse plants are grown from cuttings. In most cases, the best material for cuttings is the nearly ripe wood. In woody plants, as camellias and others, the cutting material often may be completely woody. In herbaceous plants, the proper material is stems which have begun to harden. Now and then better results are secured from seeds, even with perennials, as in grevillea and Impatiens sultani.

Coming, now, to some of the principles that underlie the proper management of the house, it may be said, first of all, that the grower should attempt to imitate a natural day. There should be the full complement of continuous sunlight; there should be periodicity in temperature. From the lowest temperature before dawn, there should be a gradual rise to midday or later. As a rule, the night temperature should be 10 to 15° F. below the maximum day temperature in the shade. A high night temperature makes the plants soft and tends to bring them to maturity too early. It makes weak stems and flabby flowers. The temperature should change gradually: violent fluctuations are inimical, particularly to plants grown at a high temperature.

In greenhouse cultivation, every plant is to receive individual care. In the field, the crop is the unit: there we deal with plants in the aggregate. In the greenhouse, each plant is to be saved and to receive special care: upon this success depends. There should be no vacant places on the greenhouse bench; room is too valuable. All this means that every care should be taken so to arrange the house that every plant will have a chance to develop to its utmost perfection. Patient hand labor pays with greenhouse plants. The work cannot be done by tools or by proxy. Therefore, the gardener becomes skilful.

Every caution should be taken to prevent the plants from becoming diseased or from being attacked by insects. The greater part of insect and fungous troubles in the greenhouse is the result of carelessness or of mistakes in the growing of the plants. Determine what diseases or pests are likely to attack any plant; discover under what conditions these diseases or pests are likely to thrive; then see that those conditions do not arise. Keep the house sweet and clean. Destroy the affected parts whenever practicable. Then if trouble come, apply the fungicide or the insecticide. Remember that the very protection which is given the plants, in the way of equable conditions, also protects their enemies: therefore, it is better to count on not having the difficulties than on curing them. If uncontrollable diseases or pests have been troublesome, make a complete change of soil or stock before the next season, if practicable. At least once every year there is an opportunity to rid the place of pests. Nematodes may be frozen out. Many gardeners carry their troubles year by year by trying to fight them, when they might succeed by trying to avoid them.

Of course, the greenhouse man must provide himself with the best insecticides and fungicides, and with good apparatus. The efficiency of these materials and appliances has greatly improved in recent years, and most of the old pests may now be controlled.

The higher the temperature and the more rapid the growth, the greater the care necessary to insure good results. Plants grown under such conditions are soft and juicy. They are easily injured by every untoward circumstance, particularly by drafts of cold air. Let a draft of cold air fall on cucumbers or rapid-growing roses, and mildew will result in spite of bordeaux mixture and brimstone.

In dark weather, grow the plants "slow." If given too much heat or too much water, they become soft and flabby, and fall prey to mildew, green-fly and other disorders. A stocky plant is always desirable, but particularly in the dull weather and short days of midwinter: at that time, extra precautions should be taken in the management of the house.

Watering plants under glass requires more judgment than any other single operation. Apply water when the plants need it, is a gardener's rule, but it is difficult to follow because one may not know when they need it. Yet if the gardener will put the emphasis on the word need he will at least be cautioned: novices often apply the advice as if it read: Apply water when the plants will stand it. Water thoroughly at each application. Mere dribbling may do more harm than good. Many persons water too frequently but not enough. Remember that in benches evaporation takes place from both top and bottom; and in pots it takes place from all sides. Water on a rising temperature. This advice is specially applicable to warmhouse stuff. Watering is a cooling process. The foliage should not go into the night wet, particularly if the plant is soft-growing or is a warmhouse subject. Water sparingly or not at att when evaporation is slight, as in dull weather.

In all greenhouse work, see that the soil is thoroughly comminuted and that it contains much sand or fiber. The amount of soil is small: see that it is all usable. In the garden, roots may wander if good soil is not at hand: in pots they cannot. The excessive watering in greenhouses tends to pack the soil, particularly if the water is applied from a hose. The earth tends to run together or to puddle. Therefore, it should contain little silt or clay. The practice of adding sand and leaf- mold to greenhouse soil is thus explained.

Ventilation is practised for the purpose of reducing temperature and of lessening atmospheric moisture. Theoretically, it is employed also for the purpose of introducing chemically fresh air, but with the opening and shutting of doors, and unavoidable leaks in the house, it is not necessary to give much thought to the introduction of mere fresh air. Ventilating reduces the temperature by letting out warm air and letting in cool air. The air should be admitted in small quantities and at the greatest distance from the plants in order to avoid the ill effects of drafts on the plants. Many small openings are better than a few very large ones. Ventilate on a rising temperature.

Most plants require shading in the summer under glass. Shading is of use in mitigating the heat as well as in tempering the light. A shaded house has more uniform conditions of temperature and moisture. If plants are grown soft and in partial shade, they are likely to be injured if exposed to bright sunlight. Sun- scalding is most common in spring, since the plants are not yet inured to bright sunshine and strong sun heat. The burning of plants is due to waves (not bubbles) in the glass. Other things being' equal, the larger the house the easier is the management of it. It is less subject to fluctuations of temperature and moisture. Greenhouses built against residences are specially liable to violent fluctuations; the body of air is small and responds to all external changes. L.H.B.

The Royal Greenhouses of Laeken. A masterpiece of 19th-century greenhouse architecture
A greenhouse in Saint Paul, Minnesota.
Nymphaea at the botanical Garden in Braunschweig, Germany

A greenhouse (also called a glasshouse or hothouse) is a building where plants are cultivated.



Main article: solar greenhouse (technical)

A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming solar radiation from the sun warms plants, soil, and other things inside the building. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall. These structures range in size from small sheds to very large buildings.

The glass used for a greenhouse works as a selective transmission medium for different spectral frequencies, and its effect is to trap energy within the greenhouse, which heats both the plants and the ground inside it. This warms the air near the ground, and this air is prevented from rising and flowing away. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature drops considerably. This principle is the basis of the autovent automatic cooling system. Greenhouses thus work by trapping electromagnetic radiation and preventing convection. Miniature greenhouses are known as a cold frame.


Greenhouse effects are often used for growing flowers, vegetables, fruits, and tobacco plants. Bumblebees are the pollinators of choice for most greenhouse pollination, although other types of bees have been used, as well as artificial pollination.

Mowing young tobacco in greenhouse of half million plants (Hemingway, South Carolina)

Besides tobacco, many vegetables and flowers are grown in greenhouses in late winter and early spring, then transplanted outside as the weather warms. Started plants are usually available for gardeners in farmers' markets at transplanting time.

The closed environment of a greenhouse has its own unique requirements, compared with outdoor production. Pests and diseases, and extremes of heat and humidity, have to be controlled, and irrigation is necessary to provide water. Significant inputs of heat and light may be required, particularly with winter production of warm-weather vegetables. Special greenhouse varieties of certain crops, like tomatoes, are generally used for commercial production.

Greenhouses are increasingly important in the food supply of high latitude countries. The largest greenhouse complex in the world is in Willcox, Arizona, USA where 262 acres of tomatoes and cucumbers are entirely grown under glass.

Greenhouses protect crops from too much heat or cold, shield plants from dust storms and blizzards, and help to keep out pests. Light and temperature control allows greenhouses to turn unarable land into arable land. Greenhouses can feed starving nations where crops can't survive in the harsh deserts and arctic wastes. Hydroponics can be used in greenhouses as well to make the most use of the interior space.

Biologist John Todd invented a greenhouse that turns sewage into water, through the natural processes of bacteria, plants, and animals.

Backyard hobby greenhouse use has increased dramatically in the United States in the past decade. Companies such as Rion, Solexx and Juliana have introduced entire lines of backyard greenhouses for use by the hobby gardener. Major retail establishments as well as small niche players sell hobby greenhouses primarily over the internet. Backyard hobby greenhouse use is still more popular in Europe and England, however.


19th Century Orangerie in Weilburg, Germany
Victorian conservatory, Kew Gardens
A modern glasshouse in RHS Wisley

The idea of growing plants in environmentally controlled areas has existed since Roman times. Doctors for the Roman emperor Tiberius prescribed him a cucumber daily. The Roman gardeners used artificial methods (similar to the greenhouse system) of growing to have it available for his table every day of the year. Cucumbers were planted in wheeled carts which were put in the sun daily, then taken inside to keep them warm at night. The cucumbers were stored under frames or in cucumber houses glazed with either oiled cloth known as "specularia" or with sheets of mica. (Pliny the Elder and Columella).

The first modern greenhouses were built in Italy in the sixteenth century to house the exotic plants that explorers brought back from the tropics. They were originally called giardini botanici (botanical gardens). The concept of greenhouses soon spread to the Netherlands and then England, along with the plants. Some of these early attempts required enormous amounts of work to close up at night or to winterize. There were serious problems with providing adequate and balanced heat in these early greenhouses.

Jules Charles, a French botanist, is often credited with building the first practical modern greenhouse in Leiden, Holland to grow medicinal tropical plants.

Originally on the estates of the rich, with the growth of the science of botany greenhouses spread to the universities. The British some times called their greenhouses conservatories, since they conserved the plants. The French called their first greenhouses orangeries, since they were used to protect orange trees from freezing. As pineapples became popular pineries, or pineapple pits, were built. Experimentation with the design of greenhouses continued during the Seventeenth Century in Europe as technology produced better glass and construction techniques improved. The greenhouse at the Palace of Versailles was an example of their size and elaborateness; it was more than 500 feet long, 42 feet wide, and 45 feet high.

In the nineteenth Century the largest greenhouses were built. The conservatory at Kew Gardens in England is a prime example of the Victorian greenhouse. Although intended for both horticultural and non-horticultural exhibition these included London's Crystal Palace, the New York Crystal Palace and Munich’s Glaspalast. Joseph Paxton, who had experimented with glass and iron in the creation of large greenhouses as the head gardener at Chatsworth, in Derbyshire, working for the Duke of Devonshire, designed and built the first, London's Crystal Palace. A major architectural achievement in monumental greenhouse building were the Royal Greenhouses of Laeken (1874-1895) for King Leopold II of Belgium.

In Japan, the first greenhouse was built in 1880 by Samuel Cocking, a British merchant who exported herbs.

In the Twentieth Century the geodesic dome was added to the many types of greenhouses.


  • Woods, May (1988)Glass houses: history of greenhouses, orangeries and conservatories Aurum Press, London, ISBN 0-906053-85-4 ;
  • Cunningham, Anne S. (2000) Crystal palaces : garden conservatories of the United States Princeton Architectural Press, New York, ISBN 1-56898-242-9 ;
  • Vleeschouwer, Olivier de (2001) Greenhouses and conservatories Flammarion, Paris, ISBN 2-08-010585-X ;
  • Lemmon, Kenneth (1963) The covered garden Dufour, Philadelphia;
  • Muijzenberg, Erwin W B van den (1980) A history of greenhouses Institute for Agricultural Engineering, Wageningen, Netherlands;
  • Enoshima Jinja Shrine Botanical Garden

See also

External links


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