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About 20 species of Juglans are distributed throughout temperate Eurasia and temperate and tropical America from Canada to Argentina and the Caribbean islands. Amongst them, 10 species are tropical or subtropical (although these usually grow at higher altitudes). Walnuts are usually large deciduous trees, with a thick trunk and wide-spreading limbs. New shoots are thick and bear pinnate leaves that leave a distinctive leaf scar on falling. The shoots and leaves are strongly aromatic, especially when young, and with practice this aroma can be helpful in identification. The flowers are unisexual, borne on separate racemes on the same tree, but in most cases cross-pollination is required. Male inflorescences arise from buds on the previous year’s growth, and are long, slender catkin-like racemes composed of many flowers, each subtended by a single entire bract and with three to six sepals and 6–40 stamens. Female inflorescences are terminal on new shoots, and are erect spikes with fewer flowers; the flowers have four sepals, adnate to the ovary but free at the tips, and two styles, stout, with fimbriate stigmatic surfaces. The fruiting spike is erect or pendulous, bearing small clusters of fruits, each being a hard-shelled nut surrounded by a dehiscent or indehiscent husk.
The walnuts are familiar and popular trees in horticulture, but only a handful of species – Juglans regia, J. nigra and possibly J. mandschurica (s.l.) – and their cultivars are encountered frequently. The horticultural diversity of the temperate taxa has been described in an article in the IDS Yearbook 2003 (Grimshaw 2004), which should be consulted for information concerning species and their cultivars not described here, and the very numerous hybrids in this promiscuous genus. The frequency of hybrids makes identification of cultivated Juglans difficult. One hybrid not mentioned in the Yearbook 2003 is that between J. nigra and J. ailantifolia, known by the unpublished name ‘J. ×segreziensis’ from its origin at the Arboretum de Segrez, Saint-Sulpice-de-Favières, France (Picard 2004).
The 10 tropical or subtropical species from Central and South America related to J. nigra remain a temptation to the temperate dendrologist. In the wonderful climate of the University of California Botanical Garden at Berkeley, J. guatemalensis Manning is well established from a collection made by D. Wilamowski in Guatemala in 1992, producing big hairy leaves. An unidentified species also growing at Berkeley was collected in 1997 by Carl Schoenfeld and John Fairey from Nuevo León and may be J. mollis Engelm., which grows in the fog belt of the mountains of eastern Mexico at altitudes of at least 2200 m (Manning 1957). This may be the most hopeful of these species for wider cultivation in our area. As part of his extensive National Plant Collection of Juglans at Upton Wold, Gloucestershire, Ian Bond grows J. boliviana Dode (Bolivia), J. neotropica Diels (Colombia, Ecuador, Peru) (see Plate 15, p. 5) and J. olanchana Standl. & L.O. Williams (Guatemala, Honduras, Mexico, Nicaragua), as potted conservatory specimens, but has not tried any outdoors.
Spring frosts are the biggest threat to the successful growth of Juglans species, and plants should be sited to minimise the risk of such damage. A number of fungal and bacterial diseases can be a problem – especially in commercial situations in North America (Grimshaw 2004) – but the most alarming is Butternut canker (Sirococcus clavigignenti-juglandacearum), which is killing trees of J. cinerea across its distribution; no regeneration occurs, and the species’ range is diminishing (Whittemore & Stone 1997). Butternut canker has been equated with chestnut blight and Dutch elm disease as a scourge of the North American forest (Schlarbaum et al. 1999); fortunately, it does not seem to affect other species significantly. Propagation is by seed (with the caveat that hybrids can be expected from arboretum-grown nuts) or by grafting onto a stock of a related species. Plants should not be confined to pots for longer than is strictly necessary, to avoid contortion of the long, robust tap roots. Juglans are greedy feeders and appreciate rich, moist ground. They thrive best in areas with long hot summers, provided there is sufficient moisture.
The walnuts, of which eight or nine species are in cultivation, are deciduous trees, or occasionally shrubs, with pinnate leaves aromatically scented. Flowers unisexual, both sexes on the same plant; the male flowers very numerous in slender, pendulous catkins, borne towards the ends of the previous year’s shoots; perianth segments adnate to the bracts and bracteoles to form a five- to seven-lobed scale; stamens eight to forty. Female flowers few, in a short spike terminal on a short shoot of the current season; ovary inferior, enclosed in a lobed perianth adnate to the fused bracts and bracteoles; stigma with two spreading branches. Fruit a hard-shelled nut, surrounded by a thin or fleshy husk. The cultivated species are from Europe, N. Asia, and N. America, but two or three species of which little is known are found in S. America. The only other genus of trees with which Juglans is likely to be confused is Carya (the hickories), but among other differences, Juglans is distinguished by the pith of the young shoots being in thin transverse plates, thus dividing the hollow portion of the shoot into a series of chambers, and by the unbranched male catkins. In Carya the pith is continuous, and the male catkins three-branched.
In gardens, Juglans is seldom represented except by the common walnut, grown for its nuts, and by the black walnut, grown for its stately form and noble foliage. The striking group of Asiatic species – J. ailantifolia, cathayensis, etc. – are rare in British gardens. Hopes were once entertained that this group might prove of value for their edible nuts, which they bear, many together, in clusters, but neither they, nor any other species except the common one, is worth growing for the fruit in the British Isles. J. nigra and J. regia both yield a valuable timber.
Walnuts should always, if possible be grown from seeds, and as they bear transplanting badly, should be given permanent places early. The nuts should be sown as soon as ripe, and not allowed to become dry. All the species like a deep loamy soil. The named varieties of common walnut are propagated by grafting on the type. Some of the species are tender in a young state and apt to be cut by late frost, thus rendering them bushy-topped. It is, in consequence, sometimes necessary to tie up a shoot to form a new leader. The walnut flowers have no colour beauty, and are fertilised by wind; hybrids have been obtained from species growing near to each other.