|Juglans subsp. var.||Walnut|
Walnuts (genus Juglans) are plants in the family Juglandaceae. They are deciduous trees, 10–40 meters tall (about 30–130 ft), with pinnate leaves 200–900 millimetres long (7–35 in), with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya) but not the hickories (Carya) in the same family.
The 21 species in the genus range across the north temperate Old World from southeast Europe east to Japan, and more widely in the New World from southeast Canada west to California and south to Argentina. The Latin name, Juglans, derives from Jupiter glans, "Jupiter's acorn": figuratively, a nut fit for a god.
The word walnut derives from Old English wealhhnutu, literally "foreign nut".
|Standard Cyclopedia of Horticulture|
Walnut (formerly sometimes written wallnut, but the name has no connection with wall, being rather of Anglo-Saxon derivation signifying "foreign nut," as the product came from the continent). A name applied to Juglans regia and its fruit, to us known mostly as English walnut because the supply yearly reached America through England; also, by extension, to other species of the genus Juglans. The name is sometimes, but provincially, given to hickory-nuts.
The walnuts may be thrown into three horticultural groups: (1) The Persian or English domesticated species, Juglans regia (Fig. 3989), the walnut of commerce and of extensive cultivation in California and other parts of the United States. (2) The North American walnuts, of several species but chiefly known in the black walnut, J. nigra (Fig. 3990). (3) The East Asian walnuts, represented by J. Sieboldiana and allies, promising but yet little grown in this country. To the genus also belongs the butternut, J. cinerea (Fig. 3991), sometimes called white walnut. There is much promise of important cultural races in the species of Juglans, but the markets yet know practically only the nuts of J. regia. See Juglans, Vol. III.
The walnut in California.
The extent of the present Persian or English walnut (J. regia) industry of California amounts to between 45,000 and 50,000 acres, or about 1,250,000 trees. An average crop for the past few years is about 12,000 tons, valued at $3,500,000. The crop for the year 1915 equaled 14,300 tons, valued at approximately $4,250,000 to the growers. The investment in the walnut industry of California represents about $45,000,000.
English walnuts may have first been planted in California by the Mission Fathers. However, it was not until after the coming of the first Americans that this industry attained any commercial importance. The present walnut industry is of comparatively recent origin and owes its establishment to the early efforts of Joseph Sexton, of Santa Barbara, and the late Felix Gillet, of Nevada City. The Santa Barbara Soft Shell seedlings and the several grafted varieties of this type all trace back more or less directly to the efforts of Sexton. The French varieties, such as the Mayette, Franquette, and the like, owe their popularity to the tireless work of the late Gillet in promoting the production of this type of walnut.
Commercial walnut-growing is largely centralized in the following counties mentioned in their order of importance: Orange, Los Angeles, Ventura, Santa Barbara, San Joaquin, and Contra Costa.
This industry is almost everywhere a specialized crop. It is seldom seen as one of two or more general farm crops, but, on the contrary, nuts are the one and only crop produced by many of the orchardists engaged in this industry. Success with this crop depends on the soil and climatic conditions and the availability of irrigation water. A deep rich alluvial loam containing plenty of humus is desirable. Groves planted on the light sandy loams or soils underlaid with a fluctuating water-table or a hardpan within 4 or 5 feet of surface are usually short-lived and unsatisfactory in the end. Although good drainage is imperative to a depth of at least 6 or 8 feet, irrigation water is necessary throughout most of the walnut areas of California for the best production of nuts.
The walnut industry has been most successful throughout the coast regions. In general, the high humidity and frequent fogs, together with a relatively small daily range in temperature, seem favorable to this crop. Walnuts grown inland are subject to sun-scald injury on both the nuts and the trees. The inland regions are subject to a very low humidity, an extreme maximum temperature and a wide daily range. Some of the more recently introduced varieties seem to endure the inland conditions better than the Santa Barbara Soft Shell seedlings. It seems very likely that the inland valleys may yet be devoted to this crop with the proper choice of varieties.
Clean culture, with the use of a winter cover-crop, is the most prevalent type of soil-management practised by the progressive growers. Such cover-crops as melilotus clover, vetch, and rye are often seen. These crops are usually planted immediately after harvest, the latter part of September or October, and should be nearly waist-high at the time they are plowed under, in the latter part of March or April.
Irrigation water is applied by the furrow system in most cases, although occasionally a grove is watered by the basin method, where the land is level or where possibly a sod is grown in the grove throughout the year. From one to five or six applications of water are made in a season, depending on the moisture-holding capacity of the soil and local climatic conditions. Each irrigation should penetrate from 6 to 8 feet from the surface of the ground in order to reach the entire root-system. If the trees are irrigated a week or two before harvest, the shucks will open and remain on the trees, dropping the clean nuts to the ground. In case the trees are drought-stricken at harvest, the shucks are likely to become sunburned, stick to the nuts, and thus cause an increase in harvesting expense.
The larger number of growers do very little systematic pruning of the walnut except to remove the lower limbs which interfere with cultivation. Occasionally, however, a grove is to be seen in which the branches are annually thinned out. Such trees usually bear more nuts on the main limbs near their centers than the unpruned ones.
Companion crops in bearing groves are seldom seen, and in fact young groves, before they reach a bearing age, are sometimes handled with clean cultivation. The interplanting of young walnut groves with lima beans or other hoed vegetable crops, small-fruits, alfalfa, and occasionally apricots and peaches, is a common practice. Certain intercrops, as beans, if properly handled, will commence making returns immediately without detriment to the future walnut grove. Vegetable crops are preferable to tree crops for interplanting. Peaches and especially apricots have an apparent dwarfing effect on the young walnut trees. Their use may be profitable, however, in the end in spite of the injury caused.
The older plantings of walnuts were set too close together. Although 40 to 50 feet apart seemed ample room for development, it is very evident now that a distance of 60 feet is none too much for the larger-growing varieties on the rich loam soils which are best adapted to this crop. It is a matter of common observation to see the outside trees in a grove produce considerably more than the trees in the center. This leads one to believe that perhaps some of the older plantations might produce more walnuts today with fewer trees to the acre.
The older groves are composed entirely of seedlings, most of which are of the Santa Barbara Soft Shell type and trace directly or indirectly to the original trees grown by Sexton at Santa Barbara. It is only within a comparatively recent time, during the last ten to fifteen years, that the walnut has been propagated by budding and grafting in commercial quantities. During this time, a comparatively large number of varieties have been introduced and many of these have been discarded even thus early in the development of the industry. At the present time the following five varieties are being propagated to a greater extent than all other sorts combined: Placentia, Eureka, Franquette, El Monte, and Prolific. In general, the first two mentioned varieties compose nearly 70 per cent of the trees propagated at the present time in southern California nurseries.
The several black walnuts are used as rootstocks. The northern California species (J. Hindsii) is held in the greatest favor at present. This is a strong vigorous tree which will withstand adverse soil conditions much better than the Persian walnut itself. Very few eastern black walnuts (J. nigra) are used for propagation, as they are usually thought to be less vigorous than California species. The southern California black walnut (J. californica) is little used at present, although it was once popular (Fig. 3992). This species starts growth so much earlier in the spring than the Persian walnut that it suckers profusely when used as a rootstock. It is not so rapid-growing as the J. Hindsii. Some of the nurserymen are using the Royal hybrid as rootstock. The Royal hybrid is the name commonly given to a cross between J. nigra and either of the California species. Some observers think the Royal hybrid root more resistant to excessive soil-moisture and general adverse soil conditions than any other rootstock. The Paradox hybrid, which is a cross between J. regia and any of the black species, is an exceedingly vigorous rapid-growing tree. This hybrid can be obtained by planting black walnuts which were produced in the neighborhood of J. regia trees. Such nuts will produce from 50 to 90 per cent hybrid progeny. Although this hybrid makes an excellent root and produces an exceptionally large and vigorous tree, it is rather impractical for general use as it cannot be obtained in wholesale quantities.
The nursery propagation of walnuts is usually by crown-grafting in place. The black walnut root is grown one year in the nursery and grafted the second spring just before the leaves start to come out. A short whip-graft is used, tied in place by soft cotton twine or raffia. After tying, the graft and top of the cion are covered thoroughly with hot wax. Some additional protection is usually given to prevent the cion drying out excessively. This is done by covering with a paper bag or by hilling the soil over the union until growth starts. The young trees are staked in the nursery, as they are very supple, due to their rapid growth. The one-year-old trees are preferred by most planters and should be 6 to 10 feet high at this age. As the trees are set in the orchard, they are usually cut back to about 5 feet. Some growers in the inland sections, however, prefer to cut the trees back to 18 inches and then train one sprout from the trunk of the tree. This sprout is pinched back when it reaches a height of 5 feet. The method necessitates staking the trees. At the end of one season's growth such severely headed-back trees may be as large as though they were left 5 feet high in the beginning. The trees cut back to 18 inches grow much more vigorously than trees only moderately pruned.
The harvesting of walnuts is done largely by Mexican families who camp in the groves through the picking-season. A portion of the nuts fall naturally to the ground and the remainder are shaken off by means of hooks attached to long poles. The pickers receive from 80 cents to $1 for 100 pounds for gathering the nuts and placing them in barley sacks. The nuts ripen through a period of a month or six weeks; therefore two or three pickings are made, followed by a gleaning of scattered nuts.
The nuts are washed, dried (Fig. 3993), culled, and sacked on the farm. They are then delivered to a central packing-house to be bleached. This is accomplished by spraying with electrolyzed salt-brine, or dipping in a solution of chloride of lime and sal soda, to which sulfuric acid is added. Either process removes all discoloration from the shells and gives them a bright light tan color, attractive in appearance. There are many ways of handling the nuts after bleaching to hasten their drying. Some packing-houses pass the nuts through a warm air-current in long drums, thence they are elevated to the bins, where they arrive nearly dry enough to sack. Other houses dry the nuts for about twenty-four hours in wire or lath bins. By putting each grower's nuts into several bins as the nuts come from the bleacher, and then drawing from several bins at the time the nuts are sacked, a thoroughly mixed uniform product is packed in each bag. Each bag contains 100 pounds of nuts.
The grading of California walnuts has developed rapidly within the last decade, as compared with sales in the past of seedling nuts ungraded and unbleached, as plain walnuts; the grading has gradually reached a stage where part of the nuts are sold under their variety name and another larger portion is disposed of after being bleached and strictly graded according to size, shape, color of the meat, and quality of same. The California Walnut-Growers' Association has recently introduced a one- and two-pound carton and has standardized the product handled in this package as strictly as breakfast foods and canned goods are graded and packed.
Although the walnut industry has not an established reputation for profitableness which is comparable with citrous fruits of California, it has nevertheless maintained its position as a stable, conservative, permanent crop within this state. The income to the acre for this product will vary widely according to variety, soil, and climatic conditions as well as the personal element of management. Such incomes will fluctuate from $100 to $300 an acre. Whereas the average yield of walnuts for the state is between 800 and 1,000 pounds, the better groves will average from 1,500 to 2,000 pounds to the acre annually.
The future development of this industry seems to be drifting gradually inland, giving way in Orange and Los Angeles counties to citrous culture. The inland valleys were formerly thought to be poorly adapted to walnut-production because of the darkening of the meats by the intense hot sunshine; however, there are several sections which give promise for development along these lines with the proper choice of varieties adapted to these particular environments.
The walnut industry enjoys one of the most notable features of any fruit industry of the country, inasmuch as its product may be successfully stored awaiting disposal for a period of at least twelve months if necessary. This has given the industry a very stable character and has freed this product from the speculative manipulations which are frequently found in connection with the perishable fruit products. It is interesting to note that the importations of walnuts into the United States have gradually increased during the last ten years and within this same period the total production and the prices to the growers of California have also gradually increased. This may be taken as only one of many indications that the walnut is being looked on more and more as a necessary food by the people of this country.
From present indications, this industry is less liable to the dangers of over-production than almost any other agricultural or horticultural crop within the borders of the state.
The chief insect and fungous troubles of the walnut are the walnut aphis, and also the walnut blight or bacteriosis (Pseudomonas juglandis). The aphis may be controlled by means of tobacco sprays; this is rarely done, however, as the damage is only occasional. There is no means of control known at present for the blight or bacteriosis (Fig. 3995). The wide variation among seedling trees in their susceptibility to the disease gives promise of eventual relief through the selection of blight-resistant varieties. Minor losses are due to red-spider, codlin-moth, and melaxuma.
Lewis, C. I., "The Walnut in Oregon," Oregon Agricultural Experiment Station Bulletin No. 43 (1906); "Walnut Special," Better Fruit, Vol. 4, No. 2 (1909). Wickson, E. J., "The English or Persian Walnut," California Fruits, pp. 510-24 (1910). Kraus, E. J., "A Method of Budding the Walnut," Oregon Agricultural Experiment Station Circular No. 16 (1911). Smith, R. E., "Walnut Culture and Walnut Blight," University of California Bulletin No. 231 (1912). Lake, E. R., "The Persian Walnut Industry of the United States," United States Department of Agriculture, Bureau of Plant Industry Bulletin No. 254 (1913). Davidson, W. M., "Walnut Aphis in California," United States Department of Agriculture Bulletin No. 100 (1914). Tylor, A. R., "Spraying for the Control of Walnut Aphis," University of California Circular No. 131 (1915). Fawcett, H. S., "Melaxuma of the Walnut," University of California Experiment Station Bulletin No. 261 (1915). Batchelor, L. D., "Problems in Walnut Breeding," Journal of Heredity, Vol. VII, No. 2 (1916).
Commercial possibilities of walnuts other than Persian in the United States.
Theoretically, the commercial possibilities of a number of walnut species in this country are large and encouraging. The genus Juglans, to which all true walnuts belong, either as native or as planted trees, covers practically every cultivated section of the United States wherever suited to hardwood growth. Apparently some of its representatives may be taken to many new small localities where it is now not found. The nuts of practically all species possess kernels rich in food properties and exceedingly pleasing to the taste. Most of its species are fairly rapid growers and develop into shapely trees. Rightly chosen and well cared for, they may be used most effectively as ornamentals and as nut-producers at the same time. The eastern black walnut, Juglans nigra, is exceedingly valuable for timber purposes, and its planting is now strongly urged by the foresters; to a large extent the various native walnuts and their hybrids make the best stocks now available for the Persian walnut.
Practically, there is little doubt but that each of these species will ultimately be developed for purposes of nut production, although in common with most new industries the progress will be slow and beset with more or less disappointment. It should be borne in mind that these walnuts are wild and uncultivated species and cannot be expected to respond to cultivation with the same readiness as species which have been selected and cultivated for centuries.
The principal problem now before the prospective planter appears to be one of varieties. It is well established that seedlings vary greatly in all essential characteristics and, therefore, are less desirable than are budded or grafted trees of suitable varieties. However, to date, there are fewer than a half-dozen varieties of black walnuts offered by the nurserymen, and so far as generally known, there is none of butternuts or other kinds of walnuts, exclusive of Persian. Therefore, for the present, planters must depend very largely on seedling trees, for which, although they are generally condemned by the leading horticulturists, there are at least four substantial reasons for using, as follows: (1) Budded and grafted trees are as yet offered by the nurserymen only to a very limited extent; (2) the available varieties are new and practically untried; (3) the prices necessarily asked by the nurserymen are beyond the reach of many who would otherwise plant walnut trees; (4) desirable varieties are liable to result from the planting of nuts from choice trees.
Definite steps are now being taken toward the development of these species and already some distinct progress has been made; but as yet it is very doubtful whether commercial planting of trees of any species of Juglans, other than J. regia, for purposes of nut production alone, is to be recommended. In general, it is unwise to attempt the growing of trees for the dual-purpose of timber and nut production, as for the former, the trees should be set close together in order to induce the development of long trunks with a minimum of top, while for the latter purpose, they should be given orchard space between in order to allow for the development of low heads, large tops, and a maximum fruiting surface. But in view of the uncertainty of outcome with any of the present available varieties and the value of black walnut timber, it is possible that trees of this species might wisely be planted at one-half or one-quarter the usual orchard distances apart, with the idea of allowing them to become forest trees, if for any reason the nuts should not justify their retention for orchard purposes. It is very doubtful whether any other species of walnut could be recommended for such use, as with the possible exception of the two forms of hybrids common in California, Paradox and Royal, no other species of walnut is now being seriously considered for forest-planting and except in rare instances, neither of these Californian forms produces nuts of value in commercial quantities.
Without doubt, the most promising place for walnut planting at the present time is about the home grounds, both in the city and in the country, and along the fence-rows everywhere. Very often walnuts yielding both beauty and product could as well be planted as trees of other species capable of affording beauty and shade only. In the country, few grounds are so crowded, that there is not room for a few walnut trees, which could be procured at small initial cost and which could be developed into useful trees at practically no further expense. If one-quarter of the American farmers were to plant even two walnut trees about their premises, it is difficult to estimate what would be the aggregate increased value to such farms by the end of a quarter-century, but certainly it would be very appreciable.
Among the species of walnuts not usually under cultivation, but which give promise of commercial possibilities, some are discussed in the following:
The American black walnut, Juglans nigra.
As a producer of marketable nuts, this species now gives greater promise than does any other secondary species of Juglans. Its natural range extends from middle New England to north Florida, in the east, and from Minnesota to Texas on the west. Although best suited to deep fertile loams, moist yet well drained, it readily adapts itself to conditions less favorable. It attains its best development in the basin drained by the Ohio River but is common at practically all altitudes in the eastern states up to about 1,400 feet where it is superseded by the butternut. The tree is a symmetrical and fairly rapid grower; usually moderately productive and very useful both in the landscape and as a forest tree. The nuts usually are thick-shelled, and it is seldom that the kernels can be separated from the cracked shell in perfect halves. A few varieties, the kernels of which crack out more or less perfectly, are now listed by the nurserymen. The two best known are the Thomas, introduced from Pennsylvania in the early eighties, and the Stabler from Maryland in 1915.
The butternut, white or long walnut, J. cinerea.
In many respects, this species is similar to the preceding. Its northern range is somewhat more extensive than is that of the black walnut and its southern and western limits are less by about 300 miles. The tree is shorter-lived, not as symmetrical in form, nor as capable of adapting itself to unfavorable conditions, and the timber is of inferior value to that of black walnut. The nuts have thicker and rougher shells and are more difficult to crack but the kernels are more readily separated from the broken shells in perfect halves than are those of the former species. By many, the kernels of the butternut are much preferred to those of any other nut.
The Japanese walnut, J. Sieboldiana.
This species and its variety cordiformis, described in Vol. III, page 1723, as there explained do not breed true to type but revert to each other or to intermediate forms, and not infrequently to a type, the nuts of which are often practically indistinguishable from those of the butternut. These forms are now fairly common throughout much of the eastern and southern United States. The trees are dwarfish in habit, broadly spreading, ornamental, precocious, and usually prolific. A few varieties have been recognized and propagated to a limited extent, but so far as can be ascertained, none is now listed by the nurserymen. However, enough good strains may now be selected to cover practically every section of the United States, with the possible exception of the dry Southwest. The nuts vary in size and form, but typically are broadly rounded at the base, conical, and smaller than are those of J. nigra. When struck with a hammer, they tend to open at the suture, thereby breaking both half-kernels into quarters. Frequently, nuts of the cordiformis type open automatically at the apex, and with the aid of a knife-blade, the half-shells may be separated entirely and the whole kernel removed without breaking. In color, texture, and flavor of kernel, the Japanese walnuts are very similar to those of the butternut, J. cinerea.
Miscellaneous species of Juglans.
A species from northeastern China (J. mandshurica), the nuts of which are intermediate in form between those of J. cinerea and J. Sieboldiana, was introduced into the United States some years ago but is not yet sufficiently well tested to make possible a definite report. It should be hardy and therefore of value in the northern states. Aside from those already included, there are a number of species of Juglans which are more or less common in parts of the United States, but all are of minor importance, so far as nut production is concerned, and apparently of use only in sections where the better species are un-adapted and as stocks for superior varieties. Among such are included J. californica, J. Hindsii, J. major, and J. rupestris. For full accounts of these species, see Vol. III, pages 1721 to 1724.
The various walnut species so freely interpollinate, when grown in close proximity to each other, that when pure strains are desired it is not safe to plant the nuts where there is danger of such pollination having taken place. The familiar Paradox and Royal of California, crosses of J. regia with any species of black, and of any California black with the eastern black, respectively, are typical examples of such natural hybridity. In the East, there are numerous crosses of J. regia with other species, viz., J. intermedia (J. regia x J. nigra); J. quadrangulata (J. regia x J. cinerea); and one between J. regia and J. Sieboldiana, which apparently has not yet been described. Frequently, individual trees of these forms are sturdy growers and make valuable stocks for other species, as already noted, but usually they are practically nonproductive and of little value to the orchardist.
The two most commercially important species are J. regia for timber and nuts, and J. nigra for timber. Both species have similar cultivation requirements and are widely grown in temperate zones.
Walnuts are light-demanding species that benefit from protection from wind. Walnuts are also very hardy against drought.
Interplanting walnut plantations with a nitrogen fixing plant such as Elaeagnus × ebbingei or Elaeagnus umbellata, and various Alnus species results in a 30% increase in tree height and girth (Hemery 2001).
When grown for nuts care must be taken to select cultivars that are compatible for pollination purposes; although some cultivars are marketed as "self fertile" they will generally fruit better with a different pollination partner. There are many different cultivars available for growers, offering different growth habit, flowering and leafing, kernel flavour and shell thickness. A key trait for more northerly latitudes of North America and Europe is phenology, with ‘late flushing’ being particularly important to avoid frost damage in Spring. Some cultivars have been developed for novel ‘hedge’ production systems developed in Europe and would not suit more traditional orchard systems.
The leaves and blossoms of the walnut tree normally appear in spring. The male cylindrical catkins of the Walnut tree are developed from leafless shoots from the past year, they are about 10 cm in length and have a large number of little flowers. Female flowers appear in a cluster at the peak of the current year’s leafy shoots..
Nuts and Kernels
The nut kernels of all the species are edible, but the walnuts most commonly available in shops are from the Persian walnut, the only species which has a large nut and thin shell; black walnut species are more difficult to crack and remove from the shell. One horticultural form selected for thin nut shells and hardiness in temperate zones is sometimes known as the 'Carpathian' walnut.
Of the more than 30 varieties of J. regia grown there, Chandler and Hartley account for over half of total production.
Pests and diseases
In California commercial production, the Hinds' black walnut (J. hindsii) and the hybrid between J. hindsii and J. regia, Juglans x Paradox, are widely used as rootstocks for J. regia cultivars because of their resistance to Phytophthora and to a very limited degree, the oak root fungus. However, trees grafted on these rootstocks often succumb to black line.
The genus Juglans is divided into four sections:
- Sect. Juglans. Leaves large (20–45 cm) with 5–9 broad leaflets, hairless, margins entire. Wood hard. Southeast Europe to central Asia.
- Sect. Rhysocaryon. Leaves large (20–50 cm) with 11–23 slender leaflets, finely pubescent, margins serrated. Wood hard. North America, South America.
- J. australis Griseb. (J. boliviana Dode) — Argentine Walnut
- J. boliviana (C. DC.) Dode — Bolivian walnut, Peruvian walnut
- J. brasiliensis Dode — Brazilian Walnut
- J. californica S.Wats. — California Black Walnut
- J. hindsii (Jepson) R.E.Smith — Hinds' Black Walnut
- J. hirsuta Manning — Nuevo Leon Walnut
- J. jamaicensis C.DC. (J. insularis Griseb.) — West Indies Walnut
- J. major (Torrey) Heller (J. arizonica Dode, J. elaeopyron Dode, J. torreyi Dode) — Arizona Black Walnut
- J. major var. glabrata Manning
- Juglans microcarpa Berlandier (J. rupestris Engelm.) — Texas Walnut or Little Black Walnut
- J. microcarpa var. microcarpa
- J. microcarpa var. stewartii (Johnston) Manning
- J. mollis Engelm. — Mexican Walnut
- J. neotropica Diels (J. honorei Dode) — Andean Walnut, Cedro Negro , Cedro Nogal , Nogal , Nogal Bogotano
- J. nigra L. — Eastern Black Walnut
- J. olanchana Standl. & L.O.Williams — Cedro Negro, Nogal , Walnut
- J. peruviana Dode — Peruvian Walnut
- J. soratensis Manning
- J. steyermarkii Manning — Guatemalan Walnut
- J. venezuelensis Manning — Venezuela Walnut
- Sect. Cardiocaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Fruits borne in racemes of up to 20. Nuts have thick shells. Northeast Asia.
- Sect. Trachycaryon. Leaves very large (40–90 cm) with 11–19 broad leaflets, softly downy, margins serrated. Wood soft. Fruits borne in clusters of 2-3. Nuts have a thick, rough shell bearing distinct, sharp ridges. Eastern North America.
- J. cinerea L. — Butternut
The best-known member of the genus is the Persian walnut (J. regia, literally "royal walnut"), native from the Balkans in southeast Europe, southwest & central Asia to the Himalaya and southwest China. This species is widely cultivated for its delicious nuts. According to the UN Food and Agriculture Organization (FAO), Shahmirzad orchard in Iran is the largest in the world (700-750 ha). In Kyrgyzstan alone there are 230,700 ha of walnut-fruit forest, where J. regia is the dominant overstory tree (Hemery and Popov 1998). In non-European English-speaking nations, the nut of the J. regia is often called the "English walnut"; in Great Britain, the "common walnut." Tuiserkan is the second city in Iran that is famous for the best quality of its walnut.
The Black Walnut (J. nigra) is a common species in its native eastern North America, and is also widely cultivated elsewhere. The nuts are edible, but have a smaller kernel and an extremely tough shell, and they are not widely grown for nut production. The wood is particularly valuable.
The Butternut (J. cinerea) is also native to eastern North America, where it is currently endangered by an introduced disease, butternut canker, caused by the fungus Sirococcus clavigignenti. Its leaves are 40–60 cm long, the fruits are oval, the shell has very tall ridges, and the kernel is especially high in fat.
The Hinds' Black Walnut (J. hindsii) is native to northern California, where it has been widely used commercially as a rootstock for J. regia trees. Hinds' black walnut shells do not have the deep grooves that are characteristic of the black walnut (J. nigra).
The Japanese Walnut (J. ailantifolia) is similar to Butternut, distinguished by the larger leaves up to 90 cm long, and round (not oval) nuts. The variety cordiformis, often called the heartnut has heart-shaped nuts; the common name of this variety is the source of the sectional name Cardiocaryon.
- Juglans x bixbyi Rehd. — J. ailantifolia x J. cinerea
- Juglans x intermedia Carr. — J. nigra x J. regia
- Juglans x notha Rehd. — J. ailantifolia x J. regia
- Juglans x quadrangulata (Carr.) Rehd. — J. cinerea x J. regia
- Juglans x sinensis (D. C.) Rehd. — J. mandschurica x J. regia
- Juglans x paradox Burbank — J. hindsii x J. regia
- Juglans x royal Burbank — J. hindsii x J. nigra
- ↑ http://fruitandnuttrees.com/walnut-tree-j-regia-j-nigra Fruit and Nut Trees
- ↑ http://www.padil.gov.au/viewPestDiagnosticImages.aspx?id=601
- ↑ Aradhya, M. K., D. Potter, F. Gao, C. J. Simon: "Molecular phylogeny of Juglans (Juglandaceae): a biogeographic perspective",Tree Genetics & Genomes(2007)3:363-378
- Standard Cyclopedia of Horticulture, by L. H. Bailey, MacMillan Co., 1963