Black walnut trees can be great assets but do not always make good neighbors. Handsome shade trees of furniture-quality hardwood, black walnuts contain a self-protective chemical called juglone, which makes it difficult for a number of other plants and trees to grow nearby. Juglone produces illness in animals and allergic reactions in some humans as well. With management, however, a black walnut tree can greatly enhance your landscape.
Black Walnut Toxicity
All parts of black walnut trees contain some level of juglone, 5-hydroxy-l, 4-naphthalenedione, as do other members of the walnut (juglans) family: English walnut, pecan, butternut, and hickory. Quantities vary from variety to variety, with black walnut containing the highest levels. Most is found in branches, leaves, nut-husks and the roots, seldom affecting the edibility of nuts produced. Black walnut nuts are prized for their highly aromatic and distinctive taste; their rarity has to do with difficulty in harvesting, not their edibility.
Toxicity to Other Plants
Extreme sensitivity to the toxic affects of juglone characterizes all members of the nightshade family, which includes tomatoes, potatoes, eggplant and peppers. A number of other plant families show sensitivity: stunted growth, little or no fruiting, withered yellow leaves and early death. Apparently, juglone interferes with the oxygen/carbon-dioxide exchange essential to plant survival. Some plants show little or no sensitivity: among vegetables, cucumbers, squash, melons, corn and some grains. Most county extension programs have lists of plants that are sensitive or resistant to juglone.
Toxicity to Animals
The Merck Veterinary Manual advises that horses can be affected by black walnut juglone, especially if it is used as part of stall bedding. Symptoms include hoof problems, lassitude, digestive issues and elevated pulse and temperature. As little as 20 percent of black walnut sawdust or shavings incorporated into bedding can cause equine illness. Cows are not similarly affected. Dogs can manifest severe digestive problems if permitted to chew on black walnut tree refuse.
Toxicity to Humans
Opinions vary on how strongly juglone affects humans. Prolonged exposure to any potential irritant (raking plant refuse, handling lumber or sawdust or harvesting and handling nut hulls) dictate protecting eyes, lungs and skin from contamination. Symptoms of excessive exposure are generally described as "allergic." Children should be cautioned not to play with twigs, leaves or other refuse from black walnut trees.
Management of Toxicity
Problems with black walnut trees can generally be averted by giving the trees the space they attempt chemically to keep for themselves. Vegetable gardens and plantings of sensitive flowers or shrubs should be located considerably beyond the tree's drip line (roots can extend half again beyond the radius of the branch canopy, or drip line). Tree refuse (branches, bark scraps, leaves and nut hulls) should be raked out of garden beds frequently and refuse should be composted separately from other yard trash. Sawdust from cutting branches or trees should be collected and disposed of.