RESEARCH SAYS PLANTS MAKE CHEMICALS THAT HELP, KILL OTHERS

                         Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: THURSDAY, February 13, 1992                   TAG: 9202120070
SECTION: CURRENT                    PAGE: NRV-14   EDITION: NEW RIVER VALLEY 
SOURCE: Joe Hunnings
DATELINE:                                 LENGTH: Medium
RESEARCH SAYS PLANTS MAKE CHEMICALS THAT HELP, KILL OTHERS
Chemical warfare has been practiced by the plant kingdom for longer than it has been practiced by man.
Canada thistle, yellow nutsedge and quackgrass do hand-to-hand combat with crops to rob them of water, nutrients and sunlight, and they also produce toxic chemicals that inhibit the other plants' growth.
Allelopathy is the term used to describe the toxic effect of one plant on another.
Toxic chemicals can be released by the roots or leaves of living or dead plants. In some cases, they may be produced by micro-organisms that break down the organic matter.
The most common occurrence of this characteristic is with the black walnut (Juglans nigra). Since the time of ancient Rome, gardeners have observed that walnut trees are toxic to many other plants.
Although this toxicity is not fully understood, it is known that a toxic substance called juglone is exuded from the root of black walnut trees.
Not all plants are susceptible to damage. In fact, some seem to have improved vigor when grown near a black walnut.
Studies show that roots of the tree must have contact with, or be very close to, susceptible plants to cause injury.
This may include a much larger area, however, than most people expect, as the root area of a mature black walnut may be five or six times the size of the tree's crown.
Even after a walnut tree is removed, the toxic substance will remain in the soil for about a year.
Although the leaves, husks, and bark contain juglone, mulches of these tree parts are reported not to cause the toxic response that occurs with actual root contact.
Plants that are harmed by black walnut include tomatoes, potatoes, blackberries, blueberries, rhododendron, azaleas and mountain laurel.
Plants that do well, or better, under black walnuts include Kentucky bluegrass, snap beans, sweet corn, red cedars, daffodils and maples.
Because soil under black walnuts is alkaline (the pH may be 0.7 points higher than the soil beyond the roots), one researcher suggests that this may influence which plants perform well.
Other researchers are looking at allelopathy as a possible way to control weeds.
For example, certain selections of rye, wheat and barley significantly reduce weed growth, in part, by the release of toxic root substances. Therefore, they offer partial control of certain annual weeds when grown as cover crops in fruit orchards.
Although research is in early stages, published reports indicate a potential for use that someday may be worthwhile for consideration by the home orchardist.
The use of allelopathic plants will not eliminate the need for herbicides, but it is exciting to know that research is being conducted that may provide another alternative to mechanical or synthetic chemical control of weeds.

Joe Hunnings is the Virginia Cooperative Extension Service agent for agriculture in Christiansburg. If you have questions call the Montgomery County extension office at 382-5790.

by Bhavesh Jinadra by CNB