Toxicity & Tolerance
The black walnut tree, juglans nigra, often presents a challenge in the garden as it is allelopathic. This means it releases chemicals from roots and other parts of the tree which harm other plants and organisms. This chemical is called juglone and it can damage certain plants up to 80′ away. It is found in all parts of the plant, so it is very important NOT to use black walnut mulch, leaves, branches or hulls in the garden. Even after a black walnut tree has been removed, the presence of juglone will remain in the soil where the roots were for several years to come. Symptoms of poisoning include chlorosis (yellowing foliage), wilting, deformation, and stunted growth eventually leading to death. There is no cure once plants are infected. Identifying where you or your neighbor(s) have any black walnut trees will help you determine which plants will or will not thrive when planting near.