Black walnut contains a chemical called juglone that is toxic to a number of plants, especially tomatoes, peppers, and eggplant. Juglone is found in high concentrations in the roots of walnut trees and in the hulls of the nuts, as well as at a lower level in the leaves, bark, and wood. Most problems arise when a sensitive plant is placed within the root zone of a walnut tree. It's important to realize that the root zone of a mature tree can extend 50, 60, even 80 feet from the trunk of the tree, far beyond the spread of the branches. Around a small tree, the roots can have a spread twice the height of the tree. Plants sensitive to juglone will have yellowed leaves as a result of exposure to the chemical; they will wilt and eventually die. Less-sensitive plants, or those planted where there are only a few walnut roots, may be stunted. The number of plants known to be sensitive to juglone is extensive. Contact your local Extension office for a list specific to your area.
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