Issue: October 9, 2004

Walnut tree allelopathy

What can I grow under a walnut tree? I was told that the walnut tree kills other plants.

Carol S.

It is true that the walnut tree produces allelopathic chemicals that inhibit the growth of other plants. The black walnut that grows in much of New Mexico is the most prolific producer of this chemical, but the English walnut and Carpathian walnut that grow as far north as central New Mexico produce the chemical in lesser amounts. The walnut tree and other plants produce allelopathic chemicals to discourage other plants competing for water and nutrients.

As an example of "turnabout is fair play" in the plant world, some grasses that cannot compete with trees for light will produce chemicals to inhibit root growth in trees. This explains why some trees planted into a lawn are slow to establish and begin growth.

All plants are not equally affected by juglone, the chemical produced by the walnut tree. Perhaps the most well known plant sensitive to the toxicity of the walnut tree is the tomato plant, but many others are also affected. Relatives of the tomato plant, such as eggplants, chiles, potatoes and petunias, are sensitive. In addition, some ornamental plants are also sensitive. These include potentillas, lilacs, columbines, lilies, peonies, mugo pines, and various privet shrubs. Blackberries, cabbage, and rhubarb should also be kept at least 50 feet from the dripline of a black walnut (may perhaps be grown closer to other walnuts).

Many plants can be grown under a walnut tree. Those that are not sensitive are various beans, corn, beets, onions, and raspberries. Ornamentals tolerant of walnut trees include forsythia, hawthorn, oaks, wild rose, daylilies, iris, phlox, Shasta daisy, and Virginia creeper.

The plants listed as sensitive or tolerant were gleaned from information by Cooperative Extension Services in the East and Midwest where walnut trees are more common than in New Mexico. There were some contradictions among these sources that may be due to differences in soil texture or other local factors. For that reason, you may wish to try some of those listed as intolerant to see if they will grow under your conditions. The web sites I used are those from Purdue University, Ohio State University, and West Virginia University Cooperative Extension Service sites.

You can determine which plants will survive in the soil near the walnut tree by trial and error. Plant those plants you wish to grow in the soil under the walnut tree and watch for symptoms such as yellowing, stunted growth, and death of the plant. Plants grown closest to the tree's roots are those most likely to exhibit symptoms. Remember that damaging the tree's roots when gardening under the tree will injure the tree. For that reason, under the walnut tree (or any tree for that matter) it is best to cover the ground with a thin layer of gravel and carefully place flagstone or pavers to provide walking surface and level places for planters. The closer to the tree you want to garden, the more important it is to protect the tree's roots. If you don't desire to keep the tree, you can remove it but the chemicals will persist in the soil for several years, inhibiting growth of flowers and vegetables.

Plants sensitive to the jugalone may be grown near or under the walnut tree if they are grown in containers. If you grow containerized plants (in pots or planter boxes above grade), the greatest concern is finding proper lighting for each plant. Although some of the allelopathic chemical will wash from leaves, relatively little will find its way into the soil in pots. Remove any leaves or nuts that fall into the pot.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!