Walnut trees provide grace, shade and delicious nuts in a home garden, but they make poor neighbors for some other kinds of plantings, especially vegetables. All walnut trees contain allopathic chemicals, juglones, that inhibit the growth of many popular summer vegetables anywhere close to the tree. Of all walnut varieties, black walnut trees contain the highest percentage of juglones. With some planning, it is still possible to grow some summer vegetables in the vicinity of a walnut tree.
Walnut trees were prized by the ancient Romans, who described the trees as "royal." Their wood was used for furniture, their nuts eaten and a peculiar characteristic of their leaves and roots used as a vermifuge. From early times, gardeners were aware of the walnut trees' self-protective qualities, although the exact compounds (juglones) were not identified until centuries later. Juglones, allopathic compounds contained in all parts of walnut trees but most intensely in the roots, keep walnut trees free of pests and weeds by making soil touched by the trees chemically inhospitable to a variety of other plants.
County Extension online lists provide information on plants and plant families sensitive to juglone. Symptoms of sensitivity range from stunting to withering to failure to bear fruit. Of particular concern to the vegetable gardener are plants in the nightshade family-- including tomatoes, peppers, eggplants and potatoes--that are especially sensitive to juglone. Experts recommend that, when planning a vegetable garden, these mainstays be located 60 to 80 feet from mature walnut trees.
Although most Extension offices stress that the list is not complete, vegetables that appear to perform best in the area of walnut trees are members of the bean and squash families. Snap beans and lima beans grow well, as do summer and winter squash vines, cucumbers and melons.
Large Garden Space
For a large vegetable garden, arranging plants according to juglone tolerance is less of a challenge than for a small-plot garden. Putting sensitive plants 60 to 80 feet from mature walnut trees places them outside the "drip-line" (literally, the outer edge of the leaf span from which rainwater can drip from a tree) and beyond the root span, which in mature walnut trees can be larger than their canopy. Providing a physical barrier between walnut roots and ground crops can be tricky because damaged roots can produce a damaged tree. If possible, distance is the best juglone barrier.
Small Garden Space
For small-plot vegetable gardens, the best juglone barrier for vegetables is the use of containers. Large pots and fresh soil, not from the tree area, can accommodate determinate tomato plants, pepper plants and eggplants. (As suburban and urban gardeners discover that their ambition exceeds their energy, the gardening industry has responded with an increasing variety of garden containers, including no-dig potato bags.)
While walnut trees and summer vegetables can coexist, taking some extra measures gives vegetables their best chance. These measures include removing all walnut tree trash (leaves, branches, fruit) frequently and thoroughly; mulching trash separately from garden compost; and providing adequate fertilizers and water for vegetables on a consistent basis to prevent hungry plant roots from straying toward juglone-contaminated soil. Keeping records of plant varieties that seem to have the highest juglone tolerance can help to ensure repeated vegetable garden success.