Do Black Walnuts Poison Other Plants?
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Black walnut leaves, husks, and bark have been rumored to be toxic and not good for use in gardens and compost piles. The start of this rumor comes from the phenomenon of certain types of plants wilting and dying when growing in close proximity to black walnut trees. This is called “black walnut toxicity” or “walnut wilt.’ This phenomenon is not new. In fact, it was noted by Pliny in Roman times.
Scientists have determined that there is a plant chemical called juglone that causes black walnut toxicity. The largest concentration of juglone can be found in the buds, hulls, and roots of black walnut trees, but it’s also found in the leaves, bark and stem tissue.
Juglone is toxic to many other plants. Black walnut toxicity primarily occurs when the tree’s roots exude juglone. Juglone sensitive plants with roots in close proximity to the black walnut roots are affected. Scientifically, juglone inhibits respiration, denying plants the energy they need for plant growth and metabolism. Plant responses range from sudden wilt and death to stunted growth. There are other members of the walnut family, such as Persian walnut, butternut, pecan and hickory, which also contain juglone, but don’t seem to produce it in sufficient quantities to produce toxicity symptoms.
There are a good number of black walnuts grown in this region. Some area gardeners have been concerned about using the leaves and hulls in their compost because of the rumored toxicity. According to an Ohio State University Extension factsheet “walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In soil, breakdown may take up to two months. Black walnut leaves may be composted separately, and the finished compost tested for toxicity by planting tomato seedlings in it.”
However, fresh sawdust, shredded leaves, or wood and bark chips from black walnut should not be used for mulching plants sensitive to juglone. According to the Ohio factsheet, composting the bark for a minimum of six months provides safe mulch even for plants sensitive to juglone.
While rain may leach juglone from leaves and hulls, the juglone is highly reactive and quickly inactivated in the soil. Juglone is also poorly soluble in water and doesn’t move very far in the soil.
Juglone sensitive plants include apple, asparagus, azalea, birch, blueberry, cabbage, ornamental cherry, chrysanthemum, crabapple, eggplant, lilac, linden, saucer magnolia, narcissus (some,) pear, peony, pepper, petunia, pine, potentilla, rhododendron, tomato, and more.
Ohio reports that problems with black walnuts are not limited to plants. Horses may be affected by black walnut chips or sawdust when they’re used for bedding material. Allergic symptoms in both horses and humans may also be produced by close association with walnut trees while their pollen is being shed in the spring.