JoAnne Skelly: What is allelopathy?

Last week I wrote about companion planting and the research supporting diverse plant associations and the benefits of interplanting. I wanted to explore this topic in more depth. Allelopathy is a scientific term to describe the beneficial or harmful effects one plant can have on another plant through the release of biochemicals (allelochemicals). These chemicals get into the soil and other plants in different ways. They may leach out from various plant parts (leaves, flowers, roots, fruits, stems or even pollen); exude through roots; volatilize as a gas; or come from decomposing plant residues.

Some of the harmful effects of allelochemicals include reduced seed germination and seedling growth. These effects can prevent other plants from growing near the allelopathic plant. The chemicals act almost as a kind of herbicide limiting cell division, nutrient uptake and photosynthesis. They can persist in the soil and reduce the success of future plantings. Many invasive plants are allelopathic, which allows them to thrive, outcompeting native species.

One common example of allelopathic response is walnuts, particularly black walnuts. Very few plants can grow under walnut trees. They turn yellow, wilt and die. Other landscape trees with allelopathic properties are tree-of-heaven, hackberry, American sycamore, cottonwood, black cherry, red oak and black locust. If plants under these trees don’t seem to thrive, it may be due to allelopathy.

Allelopathic damage is less likely to occur in well-drained, well-aerated soils. Soils low in organic matter with little microbial action are more likely to increase the harm from allelopathic plants. Instead of the toxins adhering to organic matter, they are absorbed by plants.

Allelopathic benefit can occur when a particular plant, such as rice, keeps weeds from growing around it. Researchers are looking for ways to synthesize the allelochemicals in order to use them as natural herbicides. Certain plants are selectively allelopathic and do not interfere with desirable plant growth, but do inhibit weeds. These plants include beets, corn, peas, buckwheat, millet, rye, cucumber and lupine. In one study, broccoli production increased by 50 percent when grown with wild mustard. Cover cropping with the residues of allelopathic plants can reduce weeds in some instances.

In a world of increasing population, allelopathy may be a tool to increase food production and food security without the increased use of synthetic herbicides. Allelochemicals may be an alternative way to deal with weeds in agriculture, reducing the impact of synthetic herbicides on the environment and human health.

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.