Allelopathy: Working those bad neighbors

By Lynette L.Walther | Dec 01, 2017
Photo by: Lynette L. Walther Fava, the bean that prefers cool weather, does not like a garden that has had a buckwheat cover crop. The reason is allelopathy, plant chemicals that inhibit the growth of other plants.

Gardeners have long been familiar with the concept of companion planting with varieties of vegetables and sometimes flowers that enhance the growth and production of their neighbors. Combos such as cucumbers and sunflowers or the traditional trio of beans, corn and squash and so on.

Many are not as familiar with the concept of “bad neighbors”: plants that inhibit those grown nearby — combinations such as beans planted near chives, leeks, onions or peppers and broccoli planted near peppers, squash or tomatoes. It is the antithesis of companion planting. In other words, it might be something that at first thought we might want to avoid. It is called allelopathy. But after a closer look at this botanical reaction, we realize we can use this interaction to benefit our gardens for a number of reasons.

According to the Gardening Solutions website of the Institute of Food and Agricultural Sciences at the University of Florida, “Allelopathy is the word that describes the chemical methods one plant uses to benefit or harm other plants growing in the area. These chemicals are sometimes referred to as allelochemicals.”

The site goes on to list some allelopathy examples, such as black walnut, which is notorious for its allelopathic effects on the plants around it. Known to inhibit the growth of many other trees, shrubs and herbaceous plants, black walnut suppresses basswood, birch, pine, hackberry, azaleas and plants in the nightshade family, which includes vegetables such as tomatoes and potatoes. Likewise, eucalyptus and neem trees (not cold-hardy here) have a negative effect on wheat, if it's grown within about 16 feet of the trees. And hackberry trees suppress all sorts of grasses.

With this concept in mind, gardeners can sometimes put that “bad mojo” to work for them. Rye, fescue and wheat can suppress the growth of certain weeds. That adds to its benefit as a cover crop, especially when parts of that cover crop are used as a mulch or cultivated into the soil. Sunflower and buckwheat cover crops provide beneficial residues to reduce weeds in certain bean crops. Likewise Jerusalem artichokes have also been observed to have residual effects on some weed species.

Conversely, studies show that a sun hemp (a fast-growing legume) cover crop could inhibit the growth of lettuce and vegetable seed germination. Even the edible plants in your garden could be having an allelopathic effect on each other. For example, a UF/IFAS publication reports instances of broccoli residue interfering with the growth of other cruciferous vegetables that are planted afterward.

Planting a fall cover crop of forage radish showed weed suppression on the following season's crop. In another study, mustard seed meal derived from white mustard (Sinapis alba) applied as a soil amendment was effective for weed suppression in organic sweet onion, however, crop injury was also significant.

In addition, allelopathic compounds applied before, along with or after synthetic herbicides may increase the overall effect of both materials, in effect reducing application rates of synthetic herbicides. Aqueous extracts of allelopathic plants on crops for weed suppression have also been investigated. In one study, an extract of brassica (Brassica napus), sorghum and sunflower was used on rain-fed wheat to successfully reduce weed pressure. The concept is being explored by organic and other large-scale growers for natural enhancements and weed control issues.

Knowing when scrupulous garden cleanup is warranted and when it is not, knowing which plants can be combined and which ones cannot, could be keys for success in growing certain varieties. The more we understand interactions like allelopathy, the better we can garden. That means we can garden smarter, not harder.

2018 Scholarship applications now open

And there’s this from the Garden Writers Association: Are you or someone you know a college or technical school student interested in pursuing a career in garden writing, photography or blogging?

The GWA Foundation, the 501(c)(3) charitable partner of GWA: The Association for Garden Communicators, continues our commitment to provide financial assistance to deserving students who wish to pursue or further a career in horticultural communications. This year's scholarship awards will range from $500 to $1,000 per student and includes a complimentary full year membership to GWA. Students must be enrolled in either of two categories:

Community Colleges & Technical Schools

​The GWA Foundation Kathleen Fisher Memorial Scholarship is made in recognition of Kathleen Fisher, who was the editor of The American Gardener, the journal of the American Horticultural Society. It is given annually in the amount of $500 to any full-time or part-time post-secondary or graduate student, including technical schools and community colleges, majoring in horticulture, plant science or journalism, with an interest in garden communications.

Colleges & Universities

GWA Foundation general scholarship grants are provided for college-level juniors or seniors, as well as graduate students, enrolled as a full-time student majoring in horticulture, plant science or journalism, with an interest in garden communications, including garden photography. General scholarships are given annually and vary in number (2-9) and amount ($500-$1,000), depending on the earnings of the scholarship endowment. It is beneficial for horticulture or plant science majors to have taken courses in journalism and vice versa, although exceptions will be made.

In recent years, more than $100,000 has been awarded to outstanding students. Grants range from $500 to $1,000 per student at the discretion of the GWA Foundation Board of Directors. Scholarships will be awarded directly to the institutions on behalf of students.

Deadline is Friday, Dec. 15. Apply today. For questions, contact Ashley Hodak Sullivan, GWA Foundation executive director, at asullivan@kellencompany.com or 212-297-2198.

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