Also called the American walnut, the black walnut appears naturally in the eastern and central U.S. Its European relative, Juglans regia, is believed to date back to ancient Persia. For centuries the walnut’s medicinal and poisonous properties were well known, gaining a mention by Pliny of the Roman Empire era.
The walnut’s place in Roman myth was as a food for the gods. Today the black walnut continues to occupy a primacy of many uses, with certain important cautions.
Black Walnut Tree Characteristics
The black walnut is a beautiful tree that can grow to 100 feet tall at maturity. Its thick outer covering is a dark brown heavily ridged bark. Its “fruit” is a large puzzle-piece shaped nut encased in a solid husk within a hard, thick sticky shell. The nuts emerge on the tree separately or as pairs.
Walnuts are high in oil content and will spoil easily if not carefully kept. They are best stored shelled in a cool dry place, preferably refrigerated.
The black walnut’s richly green leaf extends to an overall 12 to 24 inches and is made up of 15 to 23 leaflets. Each single leaflet can be 2 to 3 inches long.
Since early American settling, the wood of the black walnut has been prized for fine furniture and gunstocks. Like many popular overused resources, its numbers today have dwindled seriously. As a result, its fine wood has been stretched in use in the form of wood veneers, rather than solid wood construction.
The black walnut’s oil is dark brown and will stain skin and surfaces. Therefore, wearing gloves is recommended when handling the nuts or portions of the tree.
The Nut of the Black Walnut
It is the black walnut’s English cousin that is most commonly sold for cracking and eating. The black walnut, more bitter, is nonetheless a popular ingredient in desserts, pestos and dips. For a delicious example, see Hammons Products Company’s recipe for Black Walnut Fancy Cake, touted as a Missouri State Fair winner (iced with Black Walnut Butter Cream Frosting).
Commercial Uses of the Black Walnut
An abrasive made from the black walnut shell is used to polish a variety of surfaces, including wood, fiberglass, stone and plastics. It can be recycled for multiple polishing applications.
Black walnut shell is used in the petroleum industry for filtration and other applications.
The ground shell is even used in cosmetics, such as skin exfoliator, soap, and tooth polish.
Black walnut nut hulls are used to create a dark brown dye, that can also be made at home. Basketmakers.org has a black walnut dye recipe.
Medicinal Use of the Black Walnut
Black walnut bark and nut hulls are used as ingredients in naturopathic remedies. Native Americans knew its efficacy as a treatment for many human maladies. Today it is promoted for treating a variety of skin conditions (e.g. ringworm, athlete’s foot, jock itch, psoriasis, eczema, wounds), constipation, and internal parasites. It has been recommended as a gargle to soothe sore throat.
Cautions and Contraindications:
Black Walnut is Poison for Horses, Dogs, and Plants
Black walnut is a contraindication for pregnancy in humans and animals. It is also poisonous to dogs and horses. In stables it has been deadly when included in wood shavings bedding in as little presence as five percent. Horses exposed to it have come down with acute laminitis (a fever in the feet that destroys the laminae hoof line, causing permanent lameness), swelling, gastroenteritis and breathing distress (anaphylactic shock). Its effects have been known to result in death.
Because of the acute toxicity of black walnut for horses, their bedding should be obtained only from known sources where there can be assurance that no black walnut tree shavings or nut hulls are included in it. For the same reason, horses should not be pastured where black walnut trees are growing, and certainly should not be fed the nuts.
The adverse effects of black walnut will manifest quickly, usually within 12 to 24 hours of the horse’s exposure. It has such an acute effect that farms have had to not only cut down black walnut trees in paddocks and pastures, but also completely remove the stumps, roots, and all soil exposed to any part of the trees before being able to allow horses on the land.
Similarly, dog owners should be aware of the dangers of exposing their dogs to kennel bedding that could contain walnut bark, wood, or hulls and should not allow their dogs to eat or chew the nuts or wood of the black walnut.
Black walnut’s poisonous toxin is called juglone, which is present in nearly all parts of the tree, including the roots. This property is known to adversely affect other plants in proximity to the black walnut. The negative effect can range from killing other plants and trees to causing them to do poorly.
Plants especially susceptible include tomatoes, blackberries, peonies, chrysanthemums, potatoes, asparagus, alfalfa and lilacs. Plants poisoned by the black walnut will exhibit wilting, stunting and death, caused mainly by their root systems uptaking the juglone given off by the black walnut's root system.
For more information:
Kansas State Research and Extension, "Walnut Wilt" fact sheet
Purdue University Vet. School's fact sheet no. 45. Black Walnut