DIGGING DEEPER: Are Black Walnut Trees Toxic to Other Plants?

Black walnut (Juglans nigra) is a common tree in our area that is valued for its many qualities – edible hard mast (nuts) for wildlife and humans, leaves as food for over 100 species of moth and butterfly larvae, attracting songbirds to the landscape, valuable wood for furniture and cabinet making, and high tannin content for dying. While black walnuts are prized for many attributes, they also cause gardeners some challenges when planting nearby. Black walnut produces an allelopathic chemical called juglone in all parts of the tree with higher concentrations in the roots, buds, and nut hulls. “Allelopathy” refers to the chemical inhibition of a plant/organism to another, due to the release into the environment of substances acting as germination, growth, survival, or reproduction inhibitors. Allelon which means “of each other”, and pathos which means “to suffer.” Black walnut trees use juglone as a defense mechanism - hindering insect and animal herbivory while also preventing the competitive growth of neighboring plants. What an evolved super-power!!

The roots, branches and leaves of the black walnut tree release juglone into the soil, discouraging the growth of other plants nearby and thereby reserving more of the area's natural resources for the tree itself. For many plants, this toxin leads to yellowing leaves, leaf drop, wilting and eventual death. It's very common to see a bare patch around the base of a black walnut tree.