What to try
Here are some attractive weed-suppressive perennials rated good to excellent by Cornell University researchers, either for their sun-denying canopies or their naturally occurring volatile chemistry. Most are suggested for use in USDA Hardiness Zones 4 through 8.
¢ Shady spots: Thyme, a grasslike plant with red foliage and purple summertime flowers; coral bells, a clump-forming plant with deep-red foliage and delicate pink flowers; bigroot cranesbill, a sun-to-shade varietal with foliage that turns crimson in autumn and produces creamy white or pink flowers in summer.
¢ Areas tending toward full sun: Dwarf goldenrod, an upright plant that produces yellow blooms in late summer; catmint, another upright, this one offering blue flowers in summer; lady's mantle, a clump-forming cultivar, with large round leaves that hold droplets, and which produces bright yellow flowers.
¢ Water-deprived landscapes: Heath aster, which grows in a dense clump thick with flowers in early fall; hardy ice plant, a mat-forming or trailing succulent producing light yellow flowers in mid to late spring; creeping baby's breath, a mat-forming plant with white to purple flowers in the spring.
New Market, Va. It's long been known that some plants are biologically capable of eliminating other plants. Now that 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 the ability to outgrow or smother them, or 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 insect and deer-browsing resistance. These are all desirable traits for property owners who don't 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."
Finding a match
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.
How they work
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."
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 impact (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's only just begun.