These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.
Walnut Toxicity to Plants
October 28, 1999
Black walnut trees can be attractive trees in the landscape. Walnuts are also popular with people, squirrels, and wildlife as a food source. As you may or may not know, however, the black walnut can also be toxic to certain plants growing in the vicinity.
Having a walnut tree in your yard certainly does not mean the landscape will be barren. But if problems have occurred for tomatoes or other sensitive plants near a walnut, the tree is in fact the likely source of the problem.
Black walnuts and butternut, both Juglans species, contain a chemical known as juglone. Juglone occurs in all parts of the plant, but is most concentrated in the buds, nut hulls, and roots. Lesser amounts are found in the leaves and stems. Juglone does not move much in the soil. Greatest quantities are found in the area immediately under the walnut tree, where roots are concentrated and decaying nut hulls and leaves accumulate.
Problems occur for sensitive plants primarily when these plants are growing in the root zone area of the black walnut. This is known as allelopathy, or one plant producing a substance that affects growth of another. Affected plants may vary in symptoms from the toxicity. Symptoms range from stunting, yellowing, partial to total wilting, to complete death.
Apparently the problem occurs when roots of susceptible species contact the tissue of the Juglans species, rather than the juglone being released into the soil. Even if the tree is removed, as long as the roots remain in the soil problems could continue for sensitive plants.
Tomatoes and potatoes are two of the popular plants sensitive to juglone. Other species known to be affected include alfalfa, some apple varieties, rhododendron, white pine, white birch, eggplant, pepper, lilac, cotoneaster, and privet, among others.
Some plants are known not be to affected by juglone. Among the plants on this list are Kentucky bluegrass, forsythia, most maples, pachysandra, most viburnums, ferns, daffodils, daylily, winged euonymus, snap beans, corn, and onions.
The best advice when gardening near black walnuts or butternuts is to use caution. If possible, locate gardens or landscape beds away from the root zone of the tree. Well-drained soils apparently present fewer toxicity problems than poorly drained soils. Avoid mulching garden areas with walnut leaves or nut husks. Root barriers to prevent walnut roots from advancing into garden areas may be helpful.