Black walnut (Juglans nigra) is considered one of our most valuable native hardwood lumber trees and is often used in large-scale landscapes. In the smaller-scale home landscape, however, the leaves and fruits are considered by some to be a messy nuisance. Furthermore, while many plants can grow well in proximity to a black walnut, the growth of some plant species is thought to be inhibited by this tree. The term “allelopathy” refers to the relationship between plants in which one plant produces a substance that inhibits the growth of sensitive plants nearby. It should be noted that although it is ‘known’ as common knowledge in the gardening community, the allelopathic effect of black walnut has not been studied in depth. Much of what is known is based on one study done many decades ago. This means that any list regarding this topic is incomplete and some of the information may be anecdotal.
Black walnuts produce a chemical called juglone, which occurs naturally in all parts of the tree, especially in the buds, nut hulls, and roots. The leaves and stems contain smaller quantities of juglone, which is leached into the soil after they fall. The highest concentration of juglone occurs in the soil directly under the tree’s canopy, but highly sensitive plants may exhibit toxicity symptoms beyond the canopy drip line. Because decaying roots can release juglone, toxicity may occur for several years after a tree has been removed.
Other trees closely related to black walnut, such as butternut, pecan, shagbark hickory, and English walnut also produce juglone, but at concentrations lower than black walnut. Rarely do these trees affect juglone-sensitive plants.
Most toxicity symptoms arise when juglone-sensitive plants are placed within the walnut’s root zone, an average of 50 to 60 feet from the trunk of a large tree. Plants sensitive to juglone may show signs of decline (wilting, yellow leaves, or stunted growth). Many highly sensitive plants can react negatively to even a small concentration of juglone. There are other factors that may impact the health of plants and those should be considered before black walnut toxicity is assumed to be the problem.