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Species of tree

Juglans nigra, the eastern American black walnut, is a species of deciduous tree in the walnut family, Juglandaceae, 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. 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 both for lumber and food, and many cultivars have been developed for improved quality wood or nuts. Black walnut is susceptible to thousand cankers disease, which provoked a decline of walnut trees in some regions. Black walnut is anecdotally known for being allelopathic, which means that it releases chemicals from its roots and other tissues that may harm other organisms and give the tree a competitive advantage. There is not, however, solid scientific consensus that allelopathic chemicals in black walnut are the primary source of its competitive growth in an area.

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, J. nigra and J. cinerea often grow in the same range as well but they do not hybridize naturally.]] While its primary native region is the Midwest and east central United States, the black walnut was introduced into Europe in 1629 and is also cultivated in Hawaii. Pollination requirements should be considered when planting black walnuts. As is typical of many species in Juglandaceae, Juglans nigra trees tend to be monoecious, i. e produce pollen first and then pistillate flowers or else produce pistillate flowers and then pollen. An early pollen producer should be grouped with other varieties that produce pistillate flowers so all varieties benefit from overlap. Cranz, Thomas, and Neel #1 make a good pollination trio. A similar group for more northern climates would be Sparrow, S127, and Mintle. Sometimes black walnut is planted as part of reclaiming mines. It has gained the Royal Horticultural Society's Award of Garden Merit.

Black walnut nuts are edible and shelled commercially in the United States. About 65% of the annual wild harvest comes from the U. S. state of Missouri, and the largest processing plant is operated by Hammons Products in Stockton, Missouri. NPR affiliate KCUR stated in an article that "Ralph Hammons began the company in 1946 with a nut cracking machine acquired from Tennessee." The Stockton Black Walnut Festival, which has been held annually since 1961, "brings the community together for a 3 day event jam packed with activities including a carnival, tractor pull, nut roll and 2 hour parade," stated Alexa Hodges in a VOX article. The nutmeats provide a robust, distinctive, natural flavor and crunch as a food ingredient. Popular uses include ice cream, bakery goods and confections. Consumers include black walnuts in traditional treats, such as cakes, cookies, fudge, and pies, during the fall holiday season. The nuts' nutritional profile leads to uses in other foods, such as salads, fish, pork, chicken, vegetables and pasta dishes. Tapped in spring, the tree yields a sweet sap that can be drunk or concentrated into syrup or sugar that is not unlike the sap of sugar maple.

The extraction of the kernel from the fruit of the black walnut is difficult. The thick, hard shell is tightly bound by tall ridges to a thick husk. Rolling the nut underfoot on a hard surface such as a driveway is a common method; commercial huskers use a car tire rotating against a metal mesh. Some take a thick plywood board and drill a nut sized hole in it (from one to two inches in diameter) and smash the nut through using a hammer. The nut goes through and the husk remains behind. American pioneers let the nuts dry in the sun, then removed the husks and let the kernels dry—making them less bitter. While the flavor of the Juglans nigra kernel is prized, the difficulty in preparing it may account for the wider popularity and availability of the English walnut.

Black walnut kernels are 5% water, 59% fat, 24% protein, and 10% carbohydrates. In a 100 gram reference amount providing 619 calories, the kernels supply several nutrients in "rich" amounts (20% or more of the Daily Value, DV), including the dietary minerals manganese (186% DV) and phosphorus (73% DV), among others; and B vitamins B6 (45% DV) and pantothenic acid (33% DV). Black walnut kernels are a moderate source of vitamin E (14% DV). Analysis of black walnut fat content showed the most prevalent fatty acids are linoleic acid (33.8%), followed by oleic acid (15.3%), alpha linolenic acid (2.7%), palmitic acid (1.9%), and stearic acid (1.5%). The brownish black dye was used by early American settlers to dye hair. According to Eastern Trees in the Petersen Guide series, black walnuts make a yellowish brown dye, not brownish black. The apparent confusion is easily explained by the fact that the liquid (dye) obtained from the inner husk becomes increasingly darker over time, as the outer skin darkens from light green to black. Extracts of the outer, soft part of the drupe are still used as a natural dye for handicrafts. The tannins present in walnuts act as a mordant, aiding in the dyeing process, and are usable as a dark ink or wood stain.

Walnut shells are often used as an abrasive in sand blasting or other circumstances where a medium hardness grit is required. The hard black walnut shell is also used commercially in abrasive cleaning, a filtering agent in scrubbers in smoke stacks, cleaning jet engines, cosmetics, and oil well drilling and water filtration. which makes it less dense than oak.

Maggots (larvae of Rhagoletis completa and Rhagoletis suavis) 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 (Conotrachelus retentus) 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 (Neonectria galligena). The infection spreads slowly but infected trees eventually die. Important leaf sucking insects include species of aphids and plant lice including (Monellia spp. and Monelliopsis spp. ), 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 (Corythucha juglandis), which causes damage when the adults and nymphs suck the sap from the lower surfaces of walnut leaflets. 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 (Pityophthorus juglandis), 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.

While black walnut is considered allelopathic, meaning it excretes chemicals into its environment that harm competition, research over the past decade [which decade?] 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. 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.

Horses are susceptible to laminitis from exposure to black walnut wood in bedding.

The US national champion black walnut is on a residential property on Sauvie Island, Oregon. It is 8 feet (2 m) diameter at breast height and 112 feet (34 m) tall, with a crown spread of 144 feet (44 m). The tallest black walnut in Europe is located in the Woluwe Park in the city of Sint Pieters Woluwe, Brussels, Belgium. It has a circumference of , height of exactly (measured by laser), and was planted around 1850 (± 10 years). The largest black walnut in Europe is located in the Castle Park in the city of Sereď, Slovakia. It has a circumference of , height of and estimated age of 300 years.