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.
Black walnut leaves, bark, and wood chips should not be used as landscape mulch or composted on garden plants.
Although the breakdown of juglone can take months in the soil, when black walnut leaves are composted, they tend to degrade within 2 to 4 weeks, depending on their exposure to water, air, and soil organisms. Still, it’s recommended that you compost these leaves separately and not use the finished compost on extremely sensitive plants.