If you've ever tried to landscape beneath a black walnut tree, you may already know that the results can be frustrating. The roots of black walnut trees produce a substance called juglone, which is toxic to many plants. Plants that are sensitive to juglone will typically wilt and die in a few months. Even plants placed beneath a different species of walnut may experience difficulty, due to the fact that tree specialists often graft other walnut varieties such as Persian walnuts onto black walnut roots.
Select plants for landscaping or gardening under a walnut tree that are resistant to juglone. Some examples of plants observed growing beneath walnut trees include trees such as Japanese maples, Southern catalpa, eastern redbud, Canadian hemlock, vines including clematis, shrubs such as rose of Sharon, Virginia creeper and creeping forsythia and annuals including marigolds and zinnias.
Avoid using plants that require full sun. Although garden plants such as squash and beans are resistant to juglone, they require at least six hours of sun to produce abundant vegetables. A walnut tree will shade these vegetables.
Dig holes for transplants between tree roots with a garden trowel so that the plant does not disturb the roots. Do not start plants from seed. Seeds require light, soil and nutrients that the roots of trees may steal. Add a handful of compost to each individual planting hole. Place the transplants in the planting holes and cover with soil. Check the transplant daily and water to keep the soil around it as moist as a wrung out sponge.
Mulch around the plants with a 2-inch layer of wood chips to help choke out weeds. Never apply mulch is a 3- to 4-inch layer. About 90 percent of a walnut tree's roots are in the surface of the soil. Adding a thicker layer of mulch can suffocate the roots.
Clean up falling leaves and nuts from the walnut tree to help reduce the amount of juglone in the soil.