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For the practice of trimming plants, see pruning.

Standard Cyclopedia of Horticulture

Prune (from Prunus), is used in this country to designate a cured dried plum, and also the varieties that are employed for the making of this product. In literature, however, it may be used rather indefinitely for many kinds of plums, particularly those that are firm-fleshed; "dried prunes" is then used for the cured product which in this country is known only as "prunes." The product is now an important article of horticulture and commerce in California and the Pacific Northwest. It is also produced in southern and to some extent in central Europe.

Prunes in California.

There are at least three important characters which distinguish the prune interest of California from that of any other state, viz.: the extent of the industry, the method of curing, and the plum variety chiefly used.

According to figures gathered by George P. Weldon, there were in California 91,470 acres of bearing prune trees in 1915, and 24,774 acres of young trees. This places the prune next to the peach, which is the leading deciduous tree-fruit of California with a total acreage of 144,888. The annual cured prune product of California, during the decade 1905 to 1914, has ranged from 57,000,000 pounds in 1908 to 205,000,000 pounds in 1912, the average annual product being 122,050,000 pounds. The Pacific Coast States produce all the prunes grown in the United States, and, according to the United States Census of 1910, "California reported, in 1909, 85.7 per cent of the total value of dried prunes produced in the United States." The development of this American product has not only reduced importation of European prunes so that they no longer receive distinctive enumeration in the customs reports, but about half the product is annually exported.

There are several reasons why the prune product of California is so overwhelmingly large and is still increasing. Beyond the general suitability of natural conditions for fruit-growing, there is, in the case of plum varieties, the total absence of the curculio, and "black-knot;" the practical freedom from rot-fungi which attack ripening fruits, and a dry condition of soil-surface and air during August and September which favor gathering fruits from the ground and curing in the open air. Curing in evaporators by artificial heat is practically unknown. The process of handling prunes, from the tree to the package, is outlined by an experienced handler, E. N. Richmond, of San Jose, as follows:

"Prunes should never be picked from the tree. They should be allowed thoroughly to ripen and fall to the ground. An orchard should be covered by pickers every seven to ten days—seven days preferably, so as to prevent sunburn of the fruit lying on the ground. The usual form of contract with pickers calls for four pickings, no shaking of the trees until the third picking, and then at grower's discretion.

"The green fruit is hauled to the dipper-shed in picking-boxes and there passed through a light solution of lye. A kettle or tank, holding 200 gallons of water and containing a basket container, is used for this purpose. In many instances the fruit is rinsed by passing from this dip into a vat of clear water and then dumped onto a combination pricking-board and grader, operated by power, which grades the fruit into three grades so that the drying in the field can be uniform. The fruit is then placed on trays 8 by 3 feet and taken to the drying-yard and dried in the sun. The purpose of passing the fruit through the lye-solution and over the pricking-board is that the skin may be slightly cut, thereby hastening evaporation, preventing fermentation and producing a fruit with a clear bright meat. From the dipper-shed to the dry-yard, the fruit is hauled on a one-horse truck especially constructed for this purpose. "The operation of drying requires judgment. Fruit should be allowed to lie in the sun on the trays until about three-quarters dried, and then the trays are stacked in piles, one above the other, leaving air-vents on either end. About twenty trays can be stacked in one pile and the finishing process takes place in this stack. Under normal weather conditions it takes from ten days to two weeks to cure prunes. While the fruit is on the trays in the dry-yard, it should receive at least one turning by hand, shaking the trays or using brooms, so that the fruit secures an equal drying on all sides. It also materially lessens the time of drying and makes a finer grade of fruit. The fruit must not be taken from the trays until it is thoroughly cured.

"At the packing-house the fruit is carefully separated into the different grades, varying from thirty to forty prunes to the pound, up to prunes running smaller than 120 to the pound. The grades as to weight and size are obtained by passing the fruit over a large grader which consists of a series of screens of different sizes, commencing with the smaller size and increasing to just a trifle larger size every 3 or 4 feet. There are from eight to nine different screens, the largest fruit passing over the end. As the fruit comes from the grader, it is carefully tested by weighing and counting, and taken to the proper bin. From there it is taken as required for packing to the processor or cleanser. The fruit in the field has been subjected to considerable dust and dirt as well as insect life. The processor or cleanser conveys the fruit through a long vat of boiling-hot water, thoroughly washing and cleansing it. From the processor the fruit is dumped on a long shaker which further assists in the cleansing process, so that by the time the fruit is put into the boxes it is in a most sanitary condition. Prunes are packed in packages varying from one pound to fifty-five pounds according to the requirements of the trade for which they are intended."

The third distinctive feature of California prune-growing is the predominance of the Prune d'Agen—the variety which has been chiefly used in the commercial prune industry of France from early times. This variety (which is discussed in the article on Plum) has the "prune character" developed to a degree which no other plum variety has thus far attained. It has a sweetness often reaching above 50 per cent of fruit-sugars in the cured fruit, but this is not its distinctive character. The really distinctive characters are: (1) the high aromatic flavor in the cured fruit; (2) the dense fine texture of the flesh, which gives this variety unequaled tenderness and mouthing quality, both as a confection and as a slightly cooked fruit; and (3) the smallness, thinness and smoothness of the pit, which the tongue gratefully accepts. The chief objection to the variety is that, when grown without thinning and the tree is allowed to carry too much bearing wood, the fruit will be small. For this reason there has been a demand for the last fifty years for a prune retaining all the characters of the Prune d'Agen and adding greater size. Although continued effort has been made to find such a prune elsewhere in the world and to originate such a one in this state, this end has not yet been reached. All rivals of the predominant variety are, when dried, either flat or acid in flavor, coarse and stringy in flesh and large and rough in pit. It is quite probable that California growers are repeating the experience of the early French growers who have given us the Prune d'Agen as the result of their prolonged selection. Leonard Coates, of Morgan Hill, has emphasized the fact that there are variations toward better size among established trees of the true characters of the Prune d'Agen and propagation from such variations is being pursued. In view of long experience of disappointment in importation and origination, this seems at present the most promising avenue toward gaining size without losing other characters. The varieties which have been brought to notice as substitutes for the French prune have been planted only on a small acreage, have sold well for size and style and may continue to be profitable, although they can never satisfy habitual prune-eaters, for the reasons stated.

The culture of the prune tree as pursued in California has already been outlined in the article on the Plum.

Prunes in the Pacific Northwest.

Prune-culture in the Pacific Northwest has had a very checkered career. The early pomologist took much interest in plums and prunes, because of the magnificence of the products secured, and the ease with which they were grown. This interest increased up to the early nineties, when the prune reached a boom period. Thousands of acres were planted in a few years. They were planted on all soils and exposures and a great many varieties were tried. Toward the latter part of the nineties, there was a tremendous production of the fruit, mostly of a questionable value. Few men knew how to evaporate prunes properly. Much of the product decayed in transit, while other portions were evaporated so hard as to be practically inedible. There was little or no market for the dried tart prune; consequently, there was but one inevitable result: namely, a collapse, and in a few years thousands of acres of trees were taken out. The United States Census shows that there were nearly a million less prune trees in the state of Oregon in 1910 than were growing in 1900. About 1905 the industry began to pick up. Those growers who had good locations and proper varieties, and who had mastered the process of evaporation, began to find a market. This market has steadily improved, until in the last seven or eight years the prune has proved to be a very profitable crop, either shipped green, or evaporated. The increase in acreage in the past four years has been very large, and the industry now seems to be thoroughly established. The United States Census for 1910, giving the number of plum and prune trees, shows the following figures:

There are two distinct areas in which the prunes of the Northwest are produced. In western Oregon and Washington, prunes are grown entirely for evaporation, the conditions there being strong loamy soils and abundant rainfall. East of the mountains the prunes are grown very largely in the irrigated valleys, although some of the dry-farming areas are producing a splendid fruit. The product at present, however, is largely centralized in such valleys as the Boise and Payette valleys of Idaho, the Grande Ronde and Freewater districts of Oregon, and the Walla Walla and Yakima valleys of Washington. In these districts the prunes are rarely evaporated, but are shipped out in the fresh condition to eastern markets, where they are generally known as plums.

There is considerable controversy, especially in the western section, as to the better locations for prunes. Some growers prefer the bottom lands—either the sandy loams along the rivers, or the stronger clay soils. The contention is that these lower elevations produce larger prunes and a greater yield. Another set of growers, however, stoutly maintain that the rolling hills are the only places for prunes, and while their plums are smaller, nevertheless they are heavier and sweeter, and their orchards are more reliable. East of the mountains, the prunes are generally planted in the silt loams.

Since all plum trees blossom in early spring, they are very subject to loss from frosts and cold rains. To offset the loss from frosts, the southern and eastern exposures should be avoided, as these are undesirable since the thawing out on such exposures is very rapid, supposed to lead to a breakdown of the tissues.

When planted on the lighter loams, the peach root is preferred, but when on the stronger loams, plum roots are better. As yet, not enough investigation has been conducted to determine what species of plum roots are the most desirable for the various locations. On the lighter soils, or higher elevations, the trees are planted from 18 to 20 feet apart, but when grown on the stronger loams, from 20 to 22 feet should be allowed.

Some growers think that on extremely rich soils, 25 feet is a more desirable distance.

The tillage given prunes is very similar to that for other deciduous fruits grown in the Northwest. In all young orchards, the tillage should be very thorough in the early spring. With trees not in bearing, tillage should cease by the middle of July. In many of the bearing orchards, where the tillage has been very thorough in the early spring months, sufficient vigor of tree and size of fruit is often obtained so that tillage may be discontinued by the middle of July or the first of August, but in many of the orchards it will be necessary to continue the tillage up to about the time of harvest, which comes later, varying from the first of August to the middle of September.

Formerly, the trees were all headed from 30 to 40 inches in height. In more recent years, however, many growers are heading from 20 to 24 inches and producing very satisfactory trees. The same general principles that apply to the pruning of apple trees, also apply to the prune. Care should be taken to have the main scaffold limbs spaced as far apart as possible. Strong heading back is necessary the first few years. With many orchards, summer pruning can be conducted advantageously, the pruning being done largely in June and consisting of a cutting back of the terminals to the point where it is desirable to force out new laterals. Occasionally a little thinning out of the laterals is practised. When the trees reach their heavy bearing, which is about the seventh year, it is desirable to give them moderate pruning annually, the aim being to keep the trees well supplied with strong one- and two-year-old wood, as the large plums are found almost invariably on the vigorous wood. When orchards have been allowed to run down somewhat, it is often found desirable to thin out the spurs with hand shears, and in this way reinvigorate the remaining spurs. When trees are very much run down, the most satisfactory treatment will probably be to dehorn them, forcing out a new vigorous top which, in three to four years, will produce commercial crops of fruit. Very little hand-thinning is done with plums and prunes in the Pacific Northwest. The Italian prunes generally thin themselves. Some varieties of plums, however, must have hand-thinning.

Very little work has been done as yet with manures or fertilizers. Commercial fertilizers, where tried, have never given striking results in the older orchards. The growers are finding that, in mature orchards, a stable compost is very desirable. When it is impossible to secure such material, vetch or rye planted the latter part of August or early September, and plowed under in the early spring, is very beneficial. Care has to be taken, however, not to use excessive amounts of nitrogen, as this element tends to make the trees unproductive, and generally makes the skin of the prune so heavy that it is difficult to evaporate.

The prune industry in the Pacific Northwest is not old enough as yet to demonstrate how long an orchard will remain profitable. However, there are several orchards in Oregon forty years old, that are still very productive. On the other hand, there are orchards twenty years of age that have passed their usefulness. The trees in this latter class, however, have been neglected. Where good soil is obtained, and proper care given, it is safe to say that the orchards will be productive at least fifty years.

There are a number of insects which are troublesome to the prune. The San Jose scale attacks the tree, but is very easily controlled with the lime-sulfur spray. The borers—both the peach-root (Sanninoidea opalescens) and the shot-hole (Xyleborus dispar)—are very bad.

Young trees are often severely attacked with aphis. Other insects which are more or less troublesome at times are the leaf syneta (Syneta albida), the Indian meal moth (Plodia interpunctella), the rose-leaf hopper (Empoa rosae), and the tipulid (Ctenophora angusti- pennis). Of the other diseases, the mushroom root-rot (Armillaria mellea) is very serious, especially when the trees have been planted on newly cleared land. Brown-rot (Sclerotina fructigena) is the worst pest of the fruit and is becoming more serious. Other diseases that have to be contended with are crown-gall, rust, and bacterial canker. The latter three diseases, however, are not nearly so serious as the first two mentioned.

Of the varieties of prunes that are grown in Oregon, the Italian (Fellenberg) comprises about seven-eighths of the planting, and the percentage in favor of the Italian is constantly increasing. For evaporation, it is the only one worth consideration in the Northwest. For shipping purposes, however, numerous plantings have been made of the Tragedy, and also of the Hungarian. Other varieties that are grown to a limited extent are the French, locally called the Petite, or Prune d'Agen, the Pacific, Willamette, Clairac Mammoth, Columbia, Tennant, Silver, and Sugar. There is, of course, to be found scattered over the Northwest a miscellaneous list of soft plums that are grown largely as local fruit. Since the Italian prune is benefited by growing with other varieties, there will probably always be a scattering of other kinds planted in our orchards. Many growers report that, wherever the Italians are near other varieties, a more satisfactory set of fruit is obtained on the Italian. In the evaporated fruit districts, the Petite will be the pollinator. The greatest drawback of this prune is its small size. It, however, dries heavier than the Italian and, size for size, sells somewhat higher.

When prunes are to be shipped in their green state for eating fresh in the eastern markets, it is customary to pick the fruit while it is still very hard and green. The plums, however, will have developed to a very large extent their true color before the packing is undertaken. The fruits are graded carefully and packed in five-pound baskets, four baskets being placed in a crate. While this crate virtually holds about twenty pounds, the weight of the fruit will range from eighteen to thirty pounds, according to varieties, size, and the general condition of the fruit. These four-basket carriers are the typical ones used for plums, apricots, and vinifera grapes constantly seen in the eastern markets. The fruit, when properly refrigerated, has not only been shipped all over the United States, but successful shipments have also been made to Europe, Mexico, and Alaska. When the fruit is to be evaporated, it is first allowed to ripen on the trees and should not be gathered until it drops naturally to the ground (Fig. 3197), or will drop with very little shaking. The fruit is picked from the ground in bushel boxes, the pickers going through the orchard every few days to gather it up. The yield varies tremendously, from 1,000 to 8,000 pounds to the acre. As soon as the fruit is gathered, it should be hauled to the evaporators and evaporated quickly in order to avoid brown-rot, which often spreads rapidly in the containers.

There are two main types of evaporators used for drying fruit.—steam and hot air. The steam driers are used only where a very large output is obtained. It is customary for most of the orchardists to dry their own fruit. Since the orchards on the whole are rather small, an inexpensive building is used for the process, and the hot-air type of building is erected. These hot-air driers are of two distinct types, the tunnel, and the stack. There are many forms of tunnel driers. These tunnels consist of groups of long nearly horizontal pipeways, built over a fire pit. They vary in length from 25 to 50 feet. Each tunnel in itself may be complete, or they may all be connected. The tendency in the past has been to have the tunnels too long. In the newer buildings, however, are tunnels from 16 to 25 feet in length. The capacity of the drier can be increased more satisfactorily by increasing the number of tunnels rather than by increasing the length of the tunnels. The heat pit is found directly below the tunnels and, as a rule, brick arch furnaces, or iron stoves, such as are commonly known as the hop stoves, are employed. In order to distribute the heat more uniformly, it is generally conducted from the furnaces by long pipes ranging from 9 to 15 inches in diameter, decreasing the farther they get away from the source of heat.

The stack drier is arranged to contain trays which are placed one over the other, the bottom of the stack being open. A single stack consists of three or four small vertical compartments generally open to each other. The fruit is first placed in the top compartment and after slightly drying is removed and placed in a lower compartment. The stack driers turn out a very good product, but require a maximum amount of labor. The buildings should be very well ventilated; these ventilators should be of an adjustable nature so that they can be opened and closed quickly. Cold air intakes are also provided. These should be in below the vent pipes so as to furnish fresh air rather than to assist in rapidly sucking out the warm air. There is a very close relation between ventilation, air-circulation, and the humidity of the atmosphere. Such relationship, unfortunately, has not been well studied by the larger number of those operating the evaporators.

The temperature is gradually increased during the drying process, starting in the neighborhood of 125° to 135° and finishing at 160° to 180°. It requires about thirty-six hours on the average to dry prunes well, the time depending on the building, ripeness of the fruit, and atmospheric conditions. The fruit will generally produce about twenty pounds of dried fruit to a bushel of fresh. Before the prunes are placed over the heat, it is customary to wash and grade the fruit. The more modern buildings now have automatic machinery which does all of this labor in one process. The prunes, after being sorted, are dipped into boiling lye. This is generally at the strength of one pound of lye to thirty to fifty gallons of water. This use of lye is adopted solely for the purpose of checking the skin of the fruit so that the gases can escape more readily and the prune be more easily dried. In most cases, as good results could be secured by boiling water. There are probably cases, however, when the skin of the fruit is so thick that it is difficult to secure as quick and satisfactory results without the use of lye. As soon as the prunes have been dipped into the lye, they are quickly dipped into clean water. A chemical analysis of the rinsing waters has shown that they are generally acid rather than alkali and it is very doubtful whether lye remains on the fruit any length of time or, if it does, it is not sufficient ever in any way to be injurious to the health. As soon as the prunes have been thoroughly dried, they are taken from the driers and stored in large bins and allowed to sweat. They are then ready for the processing. The processing is largely a steaming operation. This is generally done in central buildings owned or controlled chiefly by the buyer or packers. The prunes are submitted to the steam for a very short time. This steaming cleans and sterilizes the fruits and adds luster to the products. They can be so handled as to add considerable weight. When this is done, however, it is unscrupulous on the part of the packer and will sooner or later lead him into trouble. The processing of the prunes also softens them so that they can be packed more easily. Formerly some bleaching was done, but very little is now practised in the Pacific Northwest.

The finished product is subject to considerable loss from fermentation, mold, and the attack of mites. It will be necessary for considerable scientific investigation to be made before the problems connected with these losses will be entirely understood. As soon as the prunes have been processed, they are taken in the hot, warm condition and packed in boxes. These boxes range in capacity from ten to fifty pounds. The bottom of the box is faced. Uniform, well-proportioned prunes are flattened with the fingers. This makes a very attractive top for the box when it is reversed. Lace paper and lithographs are used on the better packs. In selling prunes, they are bought entirely according to weight, but based on the number of prunes to the pound: such as 30-40's, 40-50's, and so on, indicating the number of prunes to the pound. The table on page 2816 illustrates the method used in basing the prices for any given size of prune in the Pacific States.

The figures below the words "bulk basis," such as 30 to 35, 70 to 75, mean the number of prunes in a pound of fruit. The figures to the right of the words, "bulk basis," such as 2, 2 1/8, refer to the so-called base price paid for prunes. The base price is, in this case, figured on the sizes running from 75 to 80 prunes to the pound. Note that the figures to the right of 75 to 80 are the same as the figures to the right of the words "bulk basis."

As an illustration of the way the table works, take the first figure to the right of the words "bulk basis," which is 2. That means then, that for prunes running from 75 to 80 to the pound, the buyer will pay 2 cents a pound. Should the prunes be so large, however, as to run 30 to 35 to the pound, note that the figure to the right of this number is 4 1/4. Should they run, for example, 55 to 60 to the pound, note that the figure opposite is 3.

Should the base price at any time be more than 5 cents, one could easily enlarge this table by adding 2 1/4 cents to the base price for prunes running from 30 to 35 to the pound, and decrease the price 1/4 cent for each smaller size in proportion to the size of the prunes.

The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

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