The Purdue Plant and Pest Diagnostic Laboratory

Black Walnut Toxicity

Question: My wife and son planted three tomato plants under our walnut tree this summer. As they grew, then withered, I remembered that walnuts and tomatoes aren't compatible. We did get one tomato, about the size of a walnut. Are tomatoes that grow beneath a walnut tree safe to eat? Will they taste like that black yucky stuff that gets on your hands from walnuts?

Also, my wife now wants me to cut down a second small walnut tree growing amid cedars and bunch of other trees in the back of our wooded lot. She's tried to plant ferns and other shade-loving things beneath it but nothing's ever grown well beneath it. Anyway, when I noted the tomato-walnut tree problem, she immediately deduced that that's the problem with the things she's tried to grow underneath this other walnut tree. I'd rather not cut it down. Can the walnut tree toxin affect these things too?

Answer: Plants adversely affected by being grown near black walnut trees have foliar yellowing, wilting and eventually death. Tomatoes are particularly susceptible. The causal agent is a chemical called juglone, which occurs naturally in all parts of the black walnut.

The largest concentrations of are in the walnut's buds, nut hulls, and roots. However, leaves and stems do contain a smaller quantity. Juglone is only poorly soluble in water and thus does not move very far in the soil or into the plant. The flavor of your tomatoes is not likely to be affected as much as their existence!

Since small amounts of juglone are released by live roots, particularly juglone-sensitive plants may show toxicity symptoms anywhere within the area of root growth of a black walnut tree. However, greater quantities of juglone are generally present in the area immediately under the canopy of a black walnut tree, due to greater root density and the accumulation of juglone from decaying leaves and nut hulls. This distribution of juglone means that some sensitive plants may tolerate the amount of juglone present in the soil near a black walnut tree, but may not survive directly under its canopy. Alternatively, highly sensitive plants may not tolerate even the small concentration of juglone beyond the canopy spread. Because decaying roots still release juglone , toxicity can persist for some years after you remove your tree.

Species survival near or under black walnut trees is further complicated by the fact that the amount of juglone present in the soil depends on soil type, drainage and soil micro-organisms. Competition for light and moisture under the canopy also greatly affects which species survive where. I can grow columbine, often listed as juglone-sensitive, on my glacial outwash under a walnut, but I still don't appreciate being pelted with walnuts in the fall!

There are lists of juglone-sensitive and juglone-tolerant plants in the Purdue Extension publication "Black Walnut Toxicity" (HO-193-W). Ask Purdue Extension office in your county for a copy or find it on-line at http://www.hort.purdue.edu/ext/HO-193.pdf . See the links at the end of the publication for more plant lists. Following is a list of plants you might try growing under walnuts. If you see them decline, move them or lose them!

Landscape plants: arborvitae, autumn olive, red cedar, catalpa, clematis, crabapple, daphne , elm, euonymous , forsythia, hawthorn, hemlock, hickory, honeysuckle, junipers, black locust, Japanese maple, maple (most), oak, pachysandra, pawpaw, persimmon, redbud, rose of sharon , wild rose, sycamore, viburnum (most), Virginia creeper.

Flowers and herbaceous plants: astilbe , bee balm, begonia, bellflower, bergamot, bloodroot, Kentucky bluegrass, Spanish bluebell, Virginia bluebell, bugleweed, chrysanthemum (some), coral bells, cranesbill , crocus, Shasta daisy, daylily, Dutchman's breeches, ferns, wild ginger, glory-of-the-snow, grape-hyacinth, grasses (most), orange hawkweed, herb Robert, hollyhock, hosta (many), hyacinth, Siberian iris, Jack-in-the-pulpit, Jacob's ladder, Jerusalem artichoke, lamb's-ear, leopard's-bane, lungwort, mayapple , merrybells , morning glory, narcissus (some), pansy, peony (some), phlox, poison ivy, pot marigold, polyanthus primrose, snowdrop, Solomon's-seal, spiderwort, spring beauty, Siberian squill , stonecrop, sundrop , sweet Cicely, sweet woodruff, trillium, tulip, violet, Virginia waterleaf, winter aconite, zinnia.

-- Beverly Shaw, Advanced Master Gardener, Purdue University


Information listed is valid only for the state of Indiana.

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by the Purdue University Cooperative Extension Service is implied. Any person using products listed assumes full responsibility for their use in accordance with current direction of the manufacturer. Purdue University is an equal opportunity/equal access institution

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service