Make use of plants that don't get along

By Gary Raham
Nature Writer and Illustrator

Back to Gardening Articles List

If you use the garden to escape from bickering kids, be warned that plants don't always get along, either.

It may seem peculiar to think of plants fighting with each other. After all, they have no obvious weapons and have to anchor themselves to one spot, but plants don't always suffer each other's company graciously. Various plants can emit an assortment of noxious chemicals from their roots, leaves or other tissues that transmit the message "give me space" to would-be herbaceous trespassers.

The study of such plant warfare is called allelopathy (literally, "to suffer each other") and the allelochemicals produced by plants show great promise as natural herbicides. The astute gardener can also use a knowledge of allelopathy to keep plant squabbling at a minimum in yards and gardens.

Although botanist Hans Molisch coined the term allelopathy in 1937, careful plant watchers have known for a long time that some plants give other plants grief. The Roman, Pliny, in 1 A.D. wrote that the walnut tree caused "injury to anything planted in its vicinity." The black walnut tree, in fact, has become a classic example of green antagonism. It produces a substance called juglone--exuded especially from its roots but also found in leaves, bark, wood and seed husks--that wilts tomatoes, potatoes, blueberries and blackberries that dare to germinate within their root zone. Juglone may persist for years in the soil causing a "soil sickness" that inhibits the growth of certain plants.

Surprisingly, while some plants wilt in the presence of juglone, melons, beans and carrots tolerate the substance and Kentucky bluegrass even thrives.

The interaction between plants also varies depending on environmental conditions. Stress brought on by drought may increase sensitivity to allelochemicals.

In the Rocky Mountain west, sagebrush such as Artemisia tridentata flexes its allelopathic muscles against a species of tobacco (Nicotiana attenuata) that shares its habitat. Artemesia releases a compound called methyl jasmonate (MeJA) that inhibits the germination of N. attenuata seeds. Other tobacco species without a history of territorial conflict are less sensitive to the substance.

Let nature work

Janet Raloff reported in the March 17, 2007, issue of Science News that scientists are pursuing allelochemical research in the hopes of developing more environmentally friendly and effective herbicides. A Spanish researcher, Francisco A. Macias, and his colleagues are "identifying and boosting weed-fighting activity in plants ranging from wheat and rice to lawn grasses and mustards."

Leslie A. Weston at Cornell has been looking at fine-fescue grasses. It turns out that the roots of fescues exude copious amounts of m-tyrosine --a variation on a common amino acid--that weeds snatch up as a useful chemical, but find they can't use. Weed roots become stunted and deformed and the plant dies.

David Gealy with the USDA has been studying allelochemicals in rice, wheat, barley and corn. While some hybrid rice strains suppress weeds well in certain planting regimens, they aren't always the same strains that have the best characteristics for maintaining upright stalks during heavy storms or producing seed heads that survive the milling process/

Plants with potent allelochemicals may be able to act as "bodyguards" for wimpier species. Sorghum and some mustards fall into this category. Sorghum produces an oily substance through its root hairs called sorgoleone that acts like a controlled-release herbicide, only entering the soil gradually, as needed. Sorghum may serve as a good rotational crop for wheat. Researcher Stephen O. Duke also thinks that rice plants contain "most of the genetics" necessary to produce sorgoleone with a bit of genetic tinkering.

Mustards, including cabbages and the rapes that produce canola oil, make glucosinolates in their tissues. Once crushed, these compounds become isothiocyanates --potent allelochemicals that degrade into even more effective ones. Some mustards also produce nitrogen to help fertilize the soil. Matthew J. Morra at the University of Idaho in Moscow envisions some of these mustards as rotational crops that can become "green manures" for other crops.

Nearly all commercial herbicides, in addition to being expensive and causing environmental "friendly fire" issues, poison weeds by inhibiting photosynthesis. Many allelochemicals attack unwanted neighbors through three or more distinct mechanisms. This makes it harder for weeds to develop resistance--another plus that brings a gleam to the eyes of herbicide researchers.

Maintain peace

The field of allelopathy is so new that, at least in the short term, the average gardener will have to rely on personal observation in many cases to determine which plants do well together - the same skills useful in deciding whom to invite to a party. In general, these tips may be useful:

• Separate contentious plants.
• Clean up debris like fallen leaves and fruit from known allelochemical bullies like black walnut trees. (A relevant web site is www.ces.ncsu.edu/fletcher/staff/rbir/blackwalnutcompanions.html.)
• Compost plant debris to degrade toxins.
• Maintain high levels of organic matter in the soil. This fosters a healthy microbial population capable of detoxifying noxious chemicals more quickly. Also allelochemicals tend to hang around longer in poorly drained soils like clay.
• Consider fescues and other grasses that police their own weed invaders when planting a new lawn.
• Lastly, keep an eye on those scientists who take the time to learn nature's secrets about getting along with the neighbors. Millions of years of time-tested experience have produced some feisty plants with lessons about communal living we all should heed.