Juglone is a chemical found in black walnut extract that can be harmful to plants as they grow in the surrounding area. Taken from the hulls of black walnuts, walnut extract can prevent sensitive plants from breathing and rob them of the energy they need to survive. Many plants are tolerant of black walnut extract, but several do die from it.
Plants take in carbon dioxide and turn it back into oxygen. The juglone present in walnut extract has been shown to inhibit this respiration in some plants.
Symptoms of juglone toxicity include leaf yellowing, wilting, and eventual death, according to the Purdue University Cooperative. Plants are classified as being either sensitive or insensitive to walnut toxicity.
According to the Morton Arboretum, plants sensitive to black walnut toxicity include asparagus, cabbage, eggplant, pepper, potato, tomato, columbine, hydrangea and lilac. Shrubs that can't tolerate juglone include blueberry, honeysuckle and rhododendron.
Plants that are tolerant of juglone exposure include daylilies, peppermint, bee balm, daffodils and phlox. Sweet corn, beans, carrots, soybeans and parsnips can also safely be grown near black walnut trees, according to the Ontario Ministry of Agriculture. However, raised beds with a plastic lining should be used to prevent walnut tree roots from getting into the garden and potentially harming susceptible plants.
If the soil has proper drainage, the plants near a black walnut tree will not be as affected by juglone, because there will be less of it stored in the soil. Even plants that are susceptible to black walnut extract may grow normally, as long as the root system of the tree does not come near the growing plants.