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 members appear to be the most vulnerable to juglone. A few other vegetables will grow very well under a walnut tree should they get enough sunlight and water.
Carrots, onions, beets and parsnips have been observed growing near black walnut trees. Even though these plants aren’t 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 even where they must compete with tree roots for water. Some cultivars are more sensitive than others.
Squash and Melons
Squash and melons tolerate juglone in the ground. They also are far more tolerant of light shade than a number of other plants. Since they have large, sprawling vines, the area around walnut trees might be an excellent place for them to grow. They can be planted beyond the drip line where they can get sun for at least four hours each day and can spread under the canopy of the tree.
Growing sweet corn takes up a lot of room because of the little yield a square foot. Since corn can tolerate juglone in the ground, it can be planted near walnut trees. Since corn requires full sun to be productive, it needs to be planted around the south or southwest side of the tree. Corn grows best when planted in blocks of 16 plants spaced about a foot apart.
Beans, which work nicely as a companion crop for corn, are also immune to juglone. Pole beans or lima beans can be planted at the base of each corn plant. The beans provide nitrogen for the corn, and the the corn offers support for bean vines. Bean stalks and vines could be composted in place after harvest, enhancing the soil’s drainage. Light, well-drained soil will have lower concentrations of juglone.