Black walnut is a tree that is common in many areas throughout the United States. Black walnuts are a problem when close to gardens and landscape areas. The black walnut tree has a level of toxicity that may stunt the growth of plants. Some plant species, when in close proximity, may die.
Black walnut trees produce a substance called juglone, which is toxic to other plants. Juglone inhibits the respiration of plants, says Perdue University Cooperative Extension, causing stunted growth. Damage by juglone depends on how resistant the plant is to juglone poisoning.
Symptoms of black walnut depend on the affected plant species. Juglone toxicity usually causes wilting and drooping of leaves, then leaf drop. Woody plants will have tender growth that is weak, making the entire plant look sick, says the University of Minnesota Extension.
How Plants Are Affected
Leaves that drop into a garden will transfer juglone into the garden. Tree roots that grow into a garden will poison healthy plant roots. Gardens and landscaping plants are best placed far away from the tree. Cement or another physical barrier buried underground will prevent roots from extending into the garden. Any leaves or walnut debris require removal from the garden to prevent poisoning.
When a black walnut tree is present, it is a good idea to grow plants that are resistant to juglone. Lima beans, snap peas, onion and squash are some of the vegetables resistant to juglone. Fruits such as black raspberry and cherry are resistant as well. Landscape plants such as hemlock and maple trees are resistant, as well as flowers such as bell flower, bloodroot and Kentucky bluegrass.
Juglone in Soil
Juglone will build up in soil over time. A soil test is required for areas that are under or near black walnut before planting. Many extension services will provide this service for a charge. If juglone is present in the soil due to bad drainage or soil type, cutting down the tree will not prevent poisoning.