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Some plants are natural weed whackers

Hate weeding? Grow weed suppressors that will do the job for you - without chemicals.

By Dean Fosdick, Associated Press
Published October 6, 2007

Goodbye, weeds
Information about low-maintenance landscaping and a list of 130 plant species, detailing their particular cultures and weed-killing capabilities, is available on this Cornell University Web site: www.gardening.cornell.edu. Click on "Allstar Groundcovers."

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It has long been known that some plants are biologically capable of eliminating other plants. Now that ability is spurring their development as a low-maintenance, chemical-free option for weed control.

Scores of commercially available ground covers, grasses and ornamentals have shown an aptitude for overwhelming weeds. That includes being able to outgrow or smother them, or to secrete 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 resistance to insects and deer. These are all desirable traits for property owners who don't want to use 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 areas.

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

It pays to understand your yard's makeup if you want natural weed suppressors to work effectively, she said.

"What are its limitations? Is it mostly in full sun or full shade? Acid (soils) or no? Carefully selecting plants to match those surroundings will minimize your chances of failure," Weston said. "You're also better off utilizing a few (plants) of a certain species but establishing them in mass plantings. That seems to contribute to their success."

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.

Dense clumps of broadleaf plants such as hostas, lady's mantle and ferns also can prevent weed seeds from germinating. Even the most opportunistic weeds can't grow without sunlight.

But it's 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 didn't 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.

Much of Weston's graduate-school research centered on the chemical composition of crabgrass, which also displays allelopathic properties.

"We had noticed it suppressed legume growth," she said.

"But we still don't understand much about the biochemical pathways or what makes them act in certain ways."

[Last modified October 4, 2007, 18:33:38]