Juglans nigra
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 susceptible to thousand cankers disease, which provoked a decline of walnut trees in some regions. 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 an important tree commercially, as the wood is a deep brown color and easily worked. Walnut seeds 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.
Description
The tree grows to 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 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 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, long and broad. The leaflets have a rounded base and a long pointed 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.
Black walnut is monoecious. The male flowers are in drooping catkins long. These are borne from axillary buds on the previous year's growth. The female flowers are terminal, in clusters of two to five on the current year's growth. During the summer/autumn, a spherical fruit 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
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.
The nuts are food for many rodents and make up to 10% of the diet of eastern fox squirrels. The nuts are also eaten by species of birds. The leaves are browsed by white tailed deer, although they are not a preferred food.
Where the range of the eastern black walnut overlaps that of the Texas black walnut, the two species sometimes interbreed, producing populations with characteristics intermediate between the two species. J. nigra and J. cinerea often grow in the same range as well but they do not hybridize naturally.
The tree's roots often form endomycorrhizal relationships with fungi in the genus Glomus. Some endomycorrhizal relations improve the plant's growth.
Species often associated with J. nigra include yellow-poplar, white ash, black cherry, basswood, American beech, sugar maple, oaks, and hickories. Near the western edge of its range, black walnut may be confined to floodplains, where it grows either with American elm, common hackberry, green ash, and boxelder, or with basswood and red oak on lower slopes and other favorable sites.
Pests
s in the husk are common, though more a nuisance than a serious problem for amateurs, who may simply remove the affected husk as soon as infestation is noticed. The maggots develop entirely within the husk, thus the quality of the nutmeat is not affected. However, infestations of maggots are undesirable because they make the husk difficult to remove and are unsightly. Maggots can be serious for commercial walnut growers, who tend to use chemical treatments to prevent damage to the crop. Some non-chemical controls also exist, such as removing and disposing of infested nuts.The walnut weevil grows to long as an adult. The adult sucks plant juices through a snout. The eggs are laid in fruits in the spring and summer. Many nuts are lost due to damage from the larvae, which burrow through the nut shell.
Black walnut is affected by European canker. The infection spreads slowly but infected trees eventually die.
The walnut caterpillar and fall webworm are two of the most serious pests, they commonly eat the foliage in midsummer and continue into autumn.
Codling moth larvae eat walnut kernels, as well as apple and pear seeds.
Important leaf sucking insects include species of aphids and plant lice including, which suck the juices from leaves and often deposit a sticky substance called "honey-dew" on the leaf surface that may turn black and prevent photosynthesis; and the walnut lace bug, which causes damage when the adults and nymphs suck the sap from the lower surfaces of walnut leaflets.
A disease complex known as thousand cankers disease has been threatening black walnut in several western states. This disease has recently been discovered in Tennessee, and could potentially have devastating effects on the species in the eastern United States. Vectored by the walnut twig beetle, a fungus, Geosmithia morbida, spreads into the wood around the galleries carved by the small beetles. The fungus causes cankers that inhibit the movement of nutrients in black walnut, leading to crown and branch dieback, and ultimately death.
Allelopathy
While black walnut is considered allelopathic, meaning it excretes chemicals into its environment that harm competition, research from 2019 has questioned whether this long-held belief holds up to scientifically rigorous examination. Many publications that have repeated claims of black walnut allelopathy cite a very limited set of dated research literature, which has not held up to scientific scrutiny. Anecdotally, records of walnut toxicity to other plants have been observed as far back as the first century when Pliny the Elder wrote: "The shadow of walnut trees is poison to all plants within its compass."Like other walnuts, the roots, inner bark, nut husks, and leaves contain a nontoxic chemical called hydrojuglone; when exposed to air or soil compounds it is oxidized into juglone that is biologically active and acts as a respiratory inhibitor to some plants. Juglone is poorly soluble in water and does not move far in the soil and will stay most concentrated in the soil directly beneath the tree. Even after a tree is removed the soil where the roots once were will still contain juglone for several years after the tree is removed as more juglone will be released as the roots decay. Well drained and aerated soils will host a healthy community of soil microbes and these microbes will help to break down the juglone.
Symptoms of juglone poisoning include foliar yellowing and wilting. A number of plants are particularly sensitive. Apples, tomatoes, pines, and birch are poisoned by juglone, and as a precaution, should not be planted in proximity to a black walnut.