Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
A tomato seedling planted within the root spread of a black walnut tree may start out its growth with reasonable vigor, but before long, it succumbs to a fatal wilt. The culprit? Juglone, a compound produced by the walnut tree. It is one of many plant-produced chemicals that can harm other plants in a process known as allelopathy.
Allelopathy, the chemical inhibition of the germination or growth of another plant, is a complex phenomenon. Scientists have been studying it since the 1880s, and now feel that it occurs frequently, though only severe interactions, such as between tomatoes and walnuts, are easily noticed. Competition for light or moisture can obscure milder occurrences of allelopathy. Allelopathic chemicals are transferred to other plants in several ways. In arid regions, volatilization of active compounds from leaf surfaces may occur, with the resulting aerosol condensing on other plants. This can be highly effective. In the west, a shrub known as soft chapparral grows with a 1 to 2 yard diameter area of bare dirt around it, as though someone has carefully weeded around each bush. The vegetation has, in fact, been killed or inhibited by volatile chemicals from the chapparral.
In moister areas, water may transport allelochemicals. Juglone, exuded from all parts of the walnut tree, may reach other plants either through root contact in moist soil or when rain leaches the chemical from leaves and branches, dripping on the plants below. Several species of weed seedlings, including common Virginia weeds such as purslane, ragweed, and crabgrass, also exude allelopathic compounds that diffuse through moist soil, to the detriment of vegetable seeds sown nearby. That's one more reason to keep your vegetable garden weed-free.
Besides living plants exuding allelopathic chemicals, the process of decay can release toxic chemicals from plant residues. Vegetables in the crucifer family (broccoli, cabbage, radish, kohlrabi) contain thiocyanate, a chemical that discourages insect feeding. When the foliage and stems are turned into the soil and decay, thiocyanate is released and may inhibit future crops, especially small-seeded ones such as lettuce. Sweet potato residue also has been shown to release chemicals that are toxic, in this case to cowpeas and sweet potatoes themselves.
Other plants exhibit self-poisoning, or autotoxicity. Asparagus bed decline may be attributable to allelopathic chemicals building up in the soil. Other crops can be inhibited as well by asparagus residue, so efforts at intercropping may not work out as hoped. Apple and peach replant disease is a common problem in commercial orchards; home gardeners also may experience problems replacing a peach or apple tree with the same species because of the autotoxic chemicals left by the original tree.
Despite the problems allelopathy can present a gardener, it does have potential for future benefits. Researchers are investigating the possibilities of breeding crops and ornamental plants that are allelopathic to common weeds. Already, we know that sunflowers suppress weeds and that some varieties of cucumbers and oats reduce weed growth. How does the idea of a lawn that resists invasions of crabgrass sound?
Gardeners already take advantage of the allelopathic nature of barley, rye, sorghum, and wheat when they grow these as cover crops, producing green manure and reducing weeds as well. It is commonly believed that these smother crops shade out weeds. Research has shown that these crops produce chemicals toxic to many weeds, probably accounting for much of the observed effect. These same chemicals are one reason gardeners should wait several weeks after incorporating cover crop stubble into the soil before planting, especially in the case of small-seeded plants. Large-seeded plants don't seem to be affected.
Plants Susceptible to Juglone Damage: apple, potato, tomato, azalea, rhododendron, blueberry, red pine, lilac, asparagus, peony, sour cherry, chrysanthemum, pea, mountain laurel, cabbage, okra
Plants Known to Resist Juglone Damage: Virginia creeper, wild grape, fern, ground pine, white clover, mint, violet, aster, yarrow, beet, coleus, begonia, dandelion, red cedar, redbud, quince, black raspberry, Kentucky bluegrass, corn, bean, carrot, zinnia, tall fescue
How can you tell if an allelopathic interaction is causing problems in your garden? The only readily visible symptoms are reduced growth and possibly wilting, neither of which can be positively attributed to allelopathy. A better approach is to avoid allelopathic interactions before they occur. Clear turf away from ornamentals; some lawn grasses, such as fescues, have been implicated in allelopathic interactions. Compost crop residues - any toxins present will be degraded in the compost pile where they won't affect current crops. If composting is impractical, wait several weeks before sowing small seeds in plots where other crops were recently growing. Maintain high organic matter levels in your soil. Toxins adhere to the organic matter rather than being absorbed by desired plants. Furthermore, the organic matter encourages healthy soil microbial populations that can metabolize toxins, returning the soil to a beneficial state. Finally, watch what you plant under a black walnut tree.
(Originally published as "Plant Against Plant," by Ellen M. Silva, Extension Technician, Department of Horticulture, in The Virginia Gardener Newsletter, Volume 8, Number 4.)
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