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.