Growing vegetable gardens near black walnut trees
When establishing vegetable gardens near black walnut trees, select juglone-tolerant vegetables and plant in raised beds with barriers that prevent walnut roots growing into the bed.
The vegetable gardening season will soon be starting. One of the considerations for siting a new vegetable garden is its proximity to black walnut trees (Juglans nigra). As a gardener with numerous black walnut trees in my yard, I have learned over the years there are quite a few plants that don’t coexist happily with black walnut. Black walnut always wins. It exudes a chemical called juglone from its roots into the soil that is toxic to many other plants and kills them, thereby reducing competition for resources. Juglone is also leached into the soil from rainwater coming in contact with fallen walnut leaves, branches and decaying fruits. Plants located beneath the canopy of walnut trees are exposed to the highest concentration of juglone, but the roots extend well beyond the dripline of the tree and may affect susceptible plants at a distance about equal to the height of the tree.
Juglone can be toxic in very low doses. Removing an offending walnut tree from the prospective garden area won’t really take care of the problem because plants sensitive to walnut toxicity can still be affected by decomposing roots of a walnut cut down several years prior just by growing near a remaining chunk of walnut root. It is next to impossible to remove all the root pieces from the soil, and remaining pieces may continue exuding toxins for several years as they decay. Symptoms of plants succumbing to toxicity from black walnuts includes wilting, yellowing of foliage and death. Studies conducted to determine the effects of juglone found that it inhibits plant respiration, resulting in lack of energy for susceptible species to take up water and nutrients.
What can a gardener who is determined to plant a vegetable patch near walnut trees do?
When establishing a garden around a walnut tree, Michigan State University Extension suggests planting species that are tolerant to juglone. Fortunately, there are a number of vegetables that will tolerate juglone, including lima and snap beans, beets, corn, onions, garlic, leeks, parsnip, carrots, cauliflower, soybeans, parsley, Jerusalem artichoke, melons and squash. Avoid planting vegetables that are sensitive to juglone, such as asparagus, cabbage, eggplant, peas, peppers, potatoes, rhubarb and tomatoes.
Raised beds with barriers of wood, stone or concrete in the bottom limits the growth of walnut roots into the bed, but don’t forget to keep the beds free of walnut debris such as leaves, hulls and seedlings. Avoid using walnut wood or bark as mulch around sensitive species.