August 17, 2000 | By: Aimee D. Heald

Are plants in your garden beginning to wilt? Have you tried unsuccessfully to figure out the cause? You may want to look at the trees growing near the garden. If any of those are black walnut or butternut trees, you probably have found the culprit.

“Affected plants growing near walnuts are exposed to juglone,” John Hartman, plant pathologist for the University of Kentucky College of Agriculture, said. “Juglone is a toxin which can cause plants to yellow, suddenly wilt, then die. It can be found in all parts of the black walnut including leaves, stems, fruit hulls, inner bark and roots. Other walnuts and related trees, such as hickories and pecans, also produce juglone, but in much lower amounts”

The most common victims of wilt caused by walnuts are tomatoes, azaleas and rhododendrons. Although plants located beneath the canopy of the tree are at the highest risk, juglone can affect other garden plants by way of root contact, leakage and decay. Rainwater leaching from leaves and branches also can pass on the toxin.

“Be aware that walnut roots extend well beyond the drip line of the tree,” Hartman added. “You may have affected plants at a distance equal to the height of the tree.”

It doesn’t take much juglone to cause problems, since it is toxic in very low doses.

Wilt associated with walnuts also may affect other trees such as apple, birch, cherry, linden, pear and pine. The toxin also affects sensitive shrubs such as azaleas, mountain laurel, as well as flowers and herbs like chrysanthemums, crocus, forget-me-nots, lily-of-the-valleys, peony and thyme. Juglone exposure will cause yellowing and wilt in fruits and vegetables such as blackberries, blueberries, cabbage, grapes, peas and potatoes. Even alfalfa is susceptible.

“The toxic effects of juglone can be reduced by regularly raking up fallen leaves and fruit from around black walnut trees, especially keeping debris away from sensitive landscape plants,” Hartman suggested.

Hartman also warned not to use walnut debris for mulch unless it has been composted, which detoxifies juglone.

“Maintain high soil organic matter levels so active soil microbial populations can metabolize the toxin,” he added.

Kentucky bluegrass and tall fesue are not affected by walnut toxin.

Perhaps the easiest way to avoid walnut-caused wilt it to plant tolerant species around walnut, or similar trees. If you have a question about what plants are tolerant, contact your local county Extension office.


John Hartman 859-257-5779