Since ancient times, scholars have suspected that walnuts have harmful effects on nearby plants. In the 1880s, scientists isolated a compound called juglone from the fruit of walnuts. They demonstrated that injury and sometimes death result when this phytotoxic material interacts with susceptible plants.
In addition to the fruit, juglone has also been found in the leaves, branches, and roots. The actual concentration in each part varies with the season. In spring, juglone is concentrated in the rapidly growing leaves. The amount of juglone in the roots remains relatively high throughout the summer. The concentration of juglone in the hulls of the fruit increases as the crop matures. All species of the walnut family produce juglone. Black walnuts have the highest concentrations. Relatively small amounts are found in butternut, hickory, and pecan. Most toxicity problems are caused by the black walnut.
The sources of juglone in the soil include both living and decaying plant material. Rain droplets leach juglone from the buds, leaves, and twigs. The decomposition of plant debris by soil microorganisms also releases juglone. Living roots exude juglone into the surrounding soil. Vegetables susceptible to juglone include tomatoes, potatoes, peppers, and eggplants. Symptoms include reduced growth, wilting, and possibly death. The presence of large walnut trees near a vegetable garden subjects susceptible plants to double jeopardy. The presence of juglone in the soil, plus the competition for light, water, and nutrients creates an extremely stressful environment. Fortunately, not all vegetables are injured by juglone. Corn, beans, onions, beets, and carrots are tolerant of juglone. If the garden plot receives sufficient sunlight, gardeners should be able to successfully grow these crops with timely applications of water and fertilizer. Gardeners should plant shade tolerant annuals and perennials, such as impatiens, hosta, and ferns, near large walnut trees. (A complete list of plants susceptible and tolerant of juglone is unavailable as little research has been done in the area.)
Gardeners who have large walnut trees near their vegetable gardens should consider alternate sites. The greatest concentration of juglone in the soil exists within the dripline of the trees. Vegetable gardens in this area will undoubtedly experience problems. Plants susceptible to juglone are occasionally damaged well beyond the dripline as the roots of walnuts may extend 2 to 3 times the crown radius (the distance from the trunk to the dripline). Volunteer walnut seedlings which appear in or near the garden should be removed. Walnut leaves and other plant debris, which may accumulate in the garden, should be raked and removed. Sawdust or wood chips derived from walnuts should not be applied as a mulch around susceptible plants.
Black walnuts can create problems for home gardeners. Careful selection of juglone tolerant vegetables and shade tolerant annuals and perennials should help overcome these problems.
This article originally appeared in the May 26, 1995 issue, p. 76.