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Cornell researchers: no magic in a bottle for tough perennial weeds

Contact: Betsy Lamb, 607 254 8800,

by Mary Woodsen

Cornell researchers: no magic in a bottle for tough perennial weeds

Gardeners who want to avoid conventional herbicides have traditionally had two options for weed control: hand weeding and mulching. Until recently, no least-toxic herbicides have been available that meet their needs. Now new alternative products, some used by organic growers, are lining garden center shelves. Yet few independent evaluations help gardeners choose among them.

That’s why researchers with Cornell University’s New York State Integrated Pest Management (NYS IPM) Program are comparing two brands of alternative herbicides to help supply the advice that gardeners need. Their trials in Geneva and Rochester, New York, are comparing AllDown, a citric acid and garlic formulation, and BurnOut II, made with clove oil and soap. Both brands list vinegar as an inert ingredient.

These products work well on annual weeds—pigweed, sorrel, and lambsquarter, for example. The spray ruptures the cells inside a plant’s leaves and stems. Sunshine does the rest, crisping them like potato chips.

The problem comes in gardens where perennials with strong root systems already stand their ground. Because perennials store extra energy in their roots, they can throw up new shoots when their tops are killed back or yanked off. Even with conventional sprays, perennial weeds may eventually bounce back.

Research by Cornell Cooperative Extension researchers, as well as research in North Carolina, Pennsylvania, and California, has shown that these alternative herbicides don’t measure up to “systemic” herbicides such as Roundup® when it comes to stopping formidable perennial weeds—bindweed, quackgrass, nutsedge, and the like.

The IPM researchers decided to take a different tack. “We’re comparing AllDown and BurnOut II to hand weeding in an established garden where you might prefer not to use a conventional herbicide,” says Betsy Lamb, ornamentals team leader with the NYS IPM Program.

“Whether you weed by hand or spray with an alternative herbicide, the idea is to keep at it until you’ve removed or exhausted all the roots and killed the plant,” Lamb says. “We’re asking if alternative herbicides help save time. And which method is more effective?”

These are the kinds of questions that IPM, which seeks innovative, least-toxic ways for dealing with common pest problems, wants answers for.

The tests take place in two gardens—one plagued by bindweed and nutsedge; the other with quackgrass and dandelions. Annual weeds such as prostrate spurge and pigweed infest both. One morning a week, IPM field technician Debbie Marvin applies AllDown or BurnOut II as needed on some sections of each garden while carefully hand weeding replicate sections, being sure to remove roots.

“I do the AllDown plots just before lunch,” Marvin says. “Garlic and vinegar—it smells just like salad dressing.”

Though the season isn’t over and they haven’t crunched the numbers, Lamb’s IPM team sees a pattern emerging. Both the hand weeded plots and BurnOut II plots look equally as good—or bad, for those who prefer seeing no weeds at all. And Lamb guesses they won’t see much difference in how long it takes for gardeners to achieve the same level of control.

But Lamb speculates that alternative herbicides may be the best choice in cases where weed roots have penetrated landscape fabric—a situation where thorough hand weeding is next to impossible. “I've pulled my share of weeds out of landscape fabric and it’s not much fun,” she says.

As for AllDown—until recently, control in those plots hasn’t been adequate. “We think that maybe the clove oil in BurnOut II, combined with an ingredient commonly found in shampoo, helps it stick to the leaves better than Alldown does,” says Lamb. “This could give it more time to work.”

Marvin points out that BurnOut II has a whitish sheen. “It’s easier to see how well you have covered the weeds,” she says, noting that AllDown takes more care in applying because it can be difficult to see how thorough your coverage has been. “Now that I’m being extremely cautious to drench the weeds, the AllDown plots are looking better.”

The take-home lesson? “There’s no magic in a bottle for gardeners who want alternatives that work on tough perennial weeds and would rather not weed by hand,” Lamb says. “But that can be true of any product.”

And there’s always next year. “We’ll be looking at regrowth,” Lamb says, “to see if there’s an effect over time.”