Many nut trees produce a substance called juglone that is toxic to other plants. Black walnuts and English walnuts grafted onto black walnut rootstock produce enough juglone to be toxic. Other walnuts, butternuts, hickories and pecans do not produce toxic amounts. Tomatoes, potatoes, peppers, eggplant, petunias and other members of the nightshade family seem to be the most susceptible to juglone. Some other vegetables will grow quite well under or near a walnut tree if they get enough water and sunlight.
Carrots, onions, beets and parsnips have been observed growing near black walnut trees. Although these plants are not sensitive to juglone, they require full sun at least six hours a day and moist soil. They will not grow well in the shade of the walnut tree or where they have to compete with tree roots for water. Some cultivars are more sensitive than others.
Squash and Melons
Squash and melons tolerate juglone in the soil. They also are more tolerant of light shade than many other plants. Because they have large, sprawling vines, the area around walnut trees may be an excellent place for them to grow. They can be planted outside the drip line where they will get sunlight for at least four hours a day and can spread under the canopy of the tree.
Growing sweet corn takes up a lot of space because of the small yield per square foot. Since corn can tolerate juglone in the soil, it can be planted near walnut trees. Because corn requires full sun to be productive, it should be planted on the south or southwest side of the tree.
Beans, which work well as a companion crop for corn, also are resistant to juglone. Pole beans or lima beans can be planted at the base of each corn plant. The pole beans provide nitrogen for the corn, and the corn provides support for bean vines. Bean stalks and vines can be composted in place after harvest, improving the soil’s drainage. Light, well-drained soil tends to have lower concentrations of juglone.
Grapes and black raspberries have been observed growing under or near black walnuts. Grapes need at least 6 hours of sun. Raspberries are more tolerant of some shade, but will produce more heavily in 6 or more hours of sun.