Juglans nigra, the eastern American black walnut, is a species of deciduous tree in the walnut family, Juglandaceae, native to central and eastern North America, growing mostly in riparian zones. Black walnut is an important tree commercially, as the wood is a deep brown color and easily worked. Walnut seeds (nuts) are cultivated for their distinctive and desirable taste. Walnut trees are grown for lumber and food, and processors have found additional markets for even the tough outer hulls by finely grinding them for use in products such as abrasive cleansers. Many cultivars have been developed for improved quality wood or nuts. In 2017, the United States Department of Agriculture valued U.S. walnut timber at $530 billion. A significant amount is grown in Missouri. Black walnut is allelopathic, releasing chemicals from its roots and other tissues that may harm other organisms and give the tree a competitive advantage, but there is no scientific consensus that this is a primary competitive factor. Black walnut is susceptible to thousand cankers disease, which provoked a decline of walnut trees in some regions.
Description The tree grows to 30–40 m (100–130 ft) tall. Under forest competition, it develops a tall and straight trunk. When grown in an open area it has a short trunk and broad crown. The bark is typically grey-black and deeply furrowed into thin ridges that gives the bark a diamond-shaped pattern. The pith of the twigs is chambered and light brown. The buds are pale silky and covered in downy hairs. The terminal buds are ovate, and 8 mm (5⁄16 in) long, and slightly longer than broad, the lateral buds are smaller and superposed. The leaves are pinnately compound and alternately arranged on the stem. They are 30–60 cm (1–2 ft) long, typically even-pinnate but there is heavy variation among leaves. The stems have 15–23 leaflets, when terminal leaf is included, with the largest leaflets located in the center, 7–10 cm (2+3⁄4–4 in) long and 2–3 cm (3⁄4–1+1⁄4 in) broad. The leaflets have a rounded base and a long pointed (acuminate) tip as well as having a serrated edge. The leaves are overall dark green in color and are typically hairy on the underside. The leaf scar has three prominent bundle scars and has a notch on the side that points toward the tip of the branch (distal side). Black walnut is monoecious. The male (staminate) flowers are in drooping catkins 8–10 cm (3+1⁄4–4 in) long. These are borne from axillary buds on the previous year's growth. The female (pistillate) flowers are terminal, in clusters of two to five on the current year's growth. During the summer/autumn, a spherical fruit (nut) ripens. It comprises a brownish-green, semifleshy husk and a brown, corrugated nut. The whole fruit, including the husk, falls in October; the seed is relatively small and very hard. Most parts of the tree including leaves, stems, and fruit husks have a very characteristic pungent or spicy odor. This odor is lacking in the nut itself.
Similar species J. cinerea (the butternut) is similar in leaf shape and range. The fruits are quite different, as black walnut fruits are spherical and butternuts are more oval-oblong. When a fruit is not available, two species can be differentiated based on the leaf scars: the butternut's has a flat upper edge and with a velvety ridge above that flat part, but black walnut's is indented with no hairy ridge.
Distribution and habitat The species is native to North America. It grows mostly in riparian zones, from southern Ontario, west to southeast South Dakota, south to Georgia, northern Florida and southwest to central Texas. Wild trees in the upper Ottawa Valley may be an isolated native population or may have derived from planted trees. The range of the black walnut is strongly correlated with low maximum vapour-pressure deficit.
Ecology Black walnut is primarily a pioneer species similar to red and silver maple and black cherry. Because of this, black walnut is a common weed tree found along roadsides, fields, and forest edges in the eastern US. It will grow in closed forests, but is classified as shade intolerant; this means it requires full sun for optimal growth and nut production. Black walnut's native range extends across much of the eastern US. It is absent from the coastal plain south of North Carolina as well as the Mississippi Valley, and does not occur in the northern tier of the eastern US, where the frost-free season is too short for the nuts to develop. Its western range extends all the way to the eastern Great Plains, beyond which climate conditions become too dry for it. Black walnut is one of the most abundant trees in the eastern US, particularly the Northeast, and its numbers are increasing due to epidemics that have affected other tree species, including emerald ash borer, chestnut blight, butternut canker, wooly hemlock adelgid, dogwood anthracnose, Dutch elm disease, and spongy moth infestations. Widespread clear-cutting of oaks due to spongy moth damage in the 1970s–1980s particularly aided in the tree's spread. The aggressive competitive strategy of black walnut such as its fast growth, alleopathic chemicals, and rodent-dispersed seeds, have also contributed.
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