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Killer Plants Used as an Alternative to Pesticides




Dense clumps of broadleaf plants such as these hostas; lady's mantle and ferns can prevent weed seeds from germinating. Even the most opportunistic weeds cannot grow without sunlight.
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Certain plants that are biologically capable of eliminating other plants are being developed as a low-maintenance, chemical free option for weed control.

Record amounts of commercially available ground covers, grasses and ornamentals have shown an ability to overwhelm weeds. That includes the aptitude for outgrowing or smothering them, or secreting weed-suppressing compounds.

"Obviously, the chemistry of a lot of medicinal plants has been looked at but not many ornamentals," said Leslie Weston, who continues as a consultant after retiring recently from Cornell University, where she was an associate professor of weed management and natural products chemistry.

Besides reducing pesticide use, these plants are establishing themselves well in places once considered difficult, she added.

Some of them also are showing unexpected hardiness, salt tolerance, and insect and deer-browsing resistance. These are all desirable traits for property owners who do not want to use potentially dangerous chemicals in their yards, or who remain skeptical about the effectiveness of organic herbicides, Weston said.

Weed-suppressive plants cannot be expected to clean up entire landscapes but they can be used effectively for spot duty, particularly in problem spots.

Drought- and salt-tolerant plants including moss phlox, dwarf goldenrod and creeping wild thyme are outperforming traditional turf grasses in traffic circles, road medians and areas affected by highway de-icing salts, Weston pointed out. "A number of these plants are functioning well under pressure from challenging environments."

Be careful about planting too much of a good thing, however.

That includes cultivars labeled "vigorous." Some assertive ground covers, including English ivy, peppermint and crown vetch, are invasive and have been banned from use in certain areas.

Ground covers generally are the plants of choice for replacing thirsty turf grasses, for shading soil, slowing erosion and improving the seedy looks of certain landscapes.

However, it is the chemical properties of herbaceous perennials _ their "allelopathic" or suppressive effects--that interest Weston. She specializes in studying the cellular makeup that gives some plants their unique characteristics.

Botanists have long known, for example, that fewer weeds grew in fields where sorghum was planted as a cover crop and then plowed under. What they did not know was why.

Weston and her colleagues eventually determined that when sorghum decomposes it gives off a naturally produced chemical called sorgoleone. This seems to inhibit the photosynthesis of such weeds as crabgrass, barnyard grass and velvetleaf, often more effectively than synthetic herbicides.

Similar weed-inhibiting chemicals are found in other commercially available cultivars, in differing mixtures -- for example, in catmint, pachysandra and ornamental goldenrod, Weston said. These compounds can be released through the leaves, roots or decomposition of the plants themselves.

Scientists are continuing to screen certain plants, setting aside those that seem to be the most plant-suppressive as possible alternatives to pesticides.

"We'll have to figure out how they should be utilized in the landscape," Weston said. "Obviously, trees like black walnuts or butternuts with their widespread root systems will (kill out) things quite a distance away. We may have to establish suppressive plants in dense plantings but even then we can only expect a limited reach."

Turf grass has its critics yet it remains one of the best ground covers in nature. It appeals to the eye, makes a good fire break and tolerates traffic. Some varieties even exhibit naturally occurring weed-suppressive tendencies.

"Fescues are interesting in that regard," Weston said. "We don't know what impacts they have on other grasses but, over time, fescues dominate and other species tend to die out."

She said that she is working with a turf grass breeder to try to develop the weed-suppressive and other positive traits in turf grass, but that this is a job that has only just begun.

Source: The Canadian Press


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June 18, 2019, 8:45 am PDT

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