Black walnut (scientific name Juglans nigra) produces the chemical juglone, which is toxic to many other plants. Juglone does not travel far, so toxicity is limited to about 50 feet from the outer perimeter of the tree crown. Black walnut shells, bark and wood chips should not be used as mulch or compost. Plants killed by their proximity to black walnut trees or wood products include common garden vegetables, flowers, trees and shrubs.
Vegetables and Annuals
Vegetables that cannot grow within 50 feet of walnut trees include cabbage and other cole crops, peppers, tomatoes, eggplants, asparagus, rhubarb and potatoes. If your yard configuration makes it impossible to keep your vegetable garden more than 50 feet from the outer perimeter of black walnut trees, try raised beds with a layer of weed cloth beneath them to keep out the black walnut roots; this may diminish the impact of juglone.
Tomato seeds can be used as a test for juglone toxicity in mulch or compost. If you suspect a batch of compost or wood chips may contain black walnut materials, place some in a bucket, soak with water, sprinkle viable tomato seeds on the surface and set it in indirect light. Tomato seedlings will not sprout in the presence of juglone.
Annual flowers killed by black walnut toxicity include petunias, nicotiana and potentilla cultivars. Field cover crops like alfalfa and crimson clover cannot be grown near black walnut.
Shrubs and Perennials
According to the Wisconsin Cooperative Extension Master Gardener program, some perennial varieties of daffodil, peony, lily, columbine, and baptisia (false indigo), as well as autumn-blooming crocus and chrysanthemums, are sensitive to juglone. If you plant these near black walnuts, keep a careful watch for signs of distress, at which point you should move the plant.
Lilacs, viburnum, rhododendron, azaleas and all berry bushes and vines may be killed by black walnut toxicity. Hydrangea and privet are also juglone-sensitive.
Apple and pear trees can be killed by black walnut tree proximity, but cherries grow well with walnuts. Other trees affected by walnut toxicity include basswood, birch, black alder, hackberry, all varieties of pine, Norway spruce, silver maples and saucer magnolias.
Butternut trees produce the same toxic substance as black walnut trees. Persian walnuts are often grafted onto back walnut stock, which produces juglone as well. Exercise the same caution in planting around butternuts or Persian walnuts as you would around black walnut trees.