Killer Plants

Some plants -- such as black walnut -- like to squelch the growth of other plants. Here's how to have a garden where all the plants get along.
By: Mary Tebo


Chemical Warfare in the Plant Kingdom

Few organisms seem more passive than plants. They grow almost imperceptibly -- silent, rooted in place. Even so, plants use an arsenal of tools to control their environment. They may compete among themselves by exploiting available resources, with one species so efficiently absorbing water through its roots or hogging light through its crown that no other species can sprout nearby. Or, plants may undermine their neighbors -- or even future residents of the same site -- through an aggressive form of chemical warfare known as "allelopathy."

The word allelopathy is derived from allelon, which means "each other," and "pathos," which means "to suffer." Allelopathic plants make surrounding plants suffer by releasing chemicals in many ways. They can store toxic chemicals in their leaves that are released when the leaves fall off, leaching through the soil to be taken up by other plants. They can release chemicals from their roots that then travel through the soil where other plants absorb them. Some plants can even volatilize these chemicals -- "allelochemicals" -- through the small pores in their leaves, thereby gassing susceptible species nearby.

Who would have suspected such struggles rampage among the quiet green cohabitants of our planet? Gardeners and botanists, of course, whose powers of observation are keen, and who must often find ways to ameliorate or circumvent the wars plants wage against each other.

Figuring it out

As early as 300 B.C., Theophrastus, known as "the father of botany," noticed that chickpeas "exhaust" the soil and appeared to kill weeds. In the year 1, the Roman scholar Pliny wrote of how the walnut tree caused "injury to anything planted in its vicinity."

In the intervening centuries, especially during the 1900s, scientists began to study chemical interactions among plants more closely. The German botanist Hans Molisch coined the term allelopathy in 1937, and an Oklahoma botanist, Elroy Rice, wrote a book on the topic in 1974 that is still considered a cornerstone of the field (a second edition appeared in 1983). Though it's often hard to separate the effects of conventional competition for resources from the effects of chemical interference, scientists have become imcreasingly adept at isolating specific plant chemicals that are toxic to the plants around them.

How allelopathy works

Though scores of plants are known to produce allelochemicals -- Ailanthus (ailanthone), sorghum (sorgoline) -- black walnut (Juglans nigra) is the poster plant for allelopathy.

Scientists were able to isolate the toxic compound in black walnut--juglone--by soaking various parts of the tree in water and then studying those solutions and their effects on other plants. Juglone, like all allelochemicals, is a secondary metabolite, meaning that it is produced as a by-product of chemical reactions that actually keep the plant alive. It has been found in the leaves, bark and wood of the walnut, as well as in the husks of walnut seeds. The highest concentration of juglone, however, is in the tree roots.

Juglone inhibits the growth of certain plants, especially those in the Solanaceae or nightshade family -- as well as azaleas and rhododendrons, mountain laurel -- even privet. Exactly how it does this -- whether by interfering with photosynthesis or water uptake -- is still a question biochemists are exploring. Whatever the mechanism, plants such as tomatoes, potatoes, blackberries and blueberries soon wilt and die if planted within the root zone of walnuts or mulched with the leaves or bark of walnut. Other members of the walnut family -- pecans and hickories -- have also been found to contain juglone and can also produce this effect.

Different strokes for different plants

Many plants, however, easily tolerate juglone; these include vegetables such as melons, beans and carrots and trees such as Eastern redbud and southern catalpa. In low concentrations, juglone even appears to stimulate growth in some species--an anomalous effect observed in other allelopathic associations. For instance, it appears to have this effect on Kentucky bluegrass, a cool-season grass that may also produce allelopathic chemicals. Find out which plants can live with black walnut.

How to cope with allelopathic plants

Dr. Thomas Green, a professor of urban forestry at Western Illinois University, has studied the effects that turfgrass and ornamental trees have on each other. These groups of plants tend to have incompatible needs and compete with each other both above ground and below.

"What we tend to do as horticulturists is pick Plant A that we like and Plant B that we like and put them together," Green said. He points out, though, that some of the associations we come up with have never occurred in natural situations, where associated plant species have often co-existed over eons and have possibly even evolved to tolerate each other's chemical toxins.

Some grasses, and fescues in particular, are allelopathic. In many cases of tree-turfgrass competition, Green has found it best for both the trees and the grasses to simply separate them by heavily mulching an area several feet out from the tree.

"Mulch will eliminate competition of the roots with grass. It also preserves moisture for the fine surface roots of the tree. With mulch we're emulating nature--exactly what happens in the forest." This way, what was taken up by the tree into its leaves is directly recycled. Also, both direct competition and allelopathy between turfgrass and the tree are avoided.

Using allelopathy to your advantage

Often the ways farmers and gardeners have dealt with allelopathy--whether they pegged the phenomenon by name or not--have been simple and practical: separate a plant that inhibits the growth of others, or else use that plant to the garden's advantage.

Rye has allelopathic properties, as do several other cover or "smother" crops--oats, wheat, barley and sorghum. In fact, Rice's Allelopathy cites a 1983 study that showed "populations of common purslane and smooth crabgrass were reduced 70 and 98 percent, respectively, by residues of sorghum."

"Reduce competition--that would be the primary objective," said Dr. Bill Klingeman, a professor of plant sciences at the University of Tennessee, about allelopathic plants. "But the other benefit would be that if they outcompete other plants, then you get a uniform stand. If the plant helps you out, that's an added benefit." Synthesizing allelochemicals for their herbicidal applications, or using certain allelopathic plants in crop rotations or as companion plants, is yet another burgeoning branch of allelopathic research.

Soil Sickness

Allelopathic plants do sometimes pose obstacles that are hard to overcome, however. Soil sickness, a general term for a problem that may well be caused by residues of allelochemicals that persist in the soil after the plant is gone, may make some sites unsuitable for growing other plants.

Rice cites a famous example where foresters tried to establish a birch arboretum on the site of a former black walnut grove at the Vermont Agricultural Experiment Station--a tale summarized by W. J. Gabriel in 1975: "an unusually large number of trees died within the boundaries of the old walnut plantation." Auto-toxicity, where a species chemically prevents germination of its own seeds, is another possible source of soil sickness that may plague apples and roses.

Through research as well as trial-and-error, it is usually possible to find some hardy plant that through chemical connivance or lucky immunity can manage to survive on such sites. Improving the soil by adding organic matter may also help to resolve the problem, as many allelochemicals more easily persist in poorly drained soil such as clay.

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