I have noticed that once my squash get established, weeds don’t do as well, this explains a lot.
Most gardeners have heard of the “three sisters,” three food plants traditionally grown together by native Americans, and the supposed reasons why the crop combination works: The corn acts as a pole for the climbing beans; the beans fix nitrogen, thereby providing an important plant nutrient; and the squash’s broad leaves shade out weeds. That last bit, about the squash, always sounded fishy to me. My weeds don’t seem to mind a bit of shade—they just grow taller to compensate. As it turns out, squash plants do suppress weeds, but in addition to blocking the sun with their large leaves, they also take on the weeds through an altogether more sophisticated mechanism: allelopathy. Allelopathy is “the chemical warfare between plants,” says Stephen Duke, Ph.D., research leader at the USDA’s Natural Products Utilization Research Unit. Allelopathy occurs when plants produce natural chemicals, known as allelochemicals, that inhibit the growth of competing species. Sure enough, when rain drips off a squash plant’s broad leaves, it takes with it allelochemicals that have been shown to suppress weed growth. Better yet, according to Stephen Gliessman, author of Agroecology: The Ecology of Sustainable Food Systems, these chemicals often inhibit weeds more than they impact crop plants. Some plants exude allelochemicals through their roots, Duke says. In other cases, the chemicals leach from leaf litter that has fallen to the ground or from a dead, decaying plant. No doubt humans noticed long ago that nothing grew well around certain plants, but the phenomenon of allelopathy was not understood until about a century ago. “One of the first examples was with black walnut,” explains Duke. “The actual compound that’s found in the soil around black walnuts is called juglone. It was identified almost 100 years ago, and it’s very toxic to other plants.” Unfortunately, some food plants may have lost some of their natural ability to outcompete weeds. “Most crops have probably had allelopathy bred out of them,” Duke says. “Most of our breeding with crops has been to increase yield, not to make them more resistant to weeds or pests, although we are going back and trying to breed some of those things into them.” As a result, Duke says, “Weeds tend to be more allelopathic than crops [because] we haven’t tinkered with them.” That’s bad news. In addition to taking up space, nutrients, and water in your garden, weeds are chemically suppressing the plants you’re trying to grow. Duke cites nutsedge as a strongly allelopathic weed. Palmer amaranth is another nightmare allelopathic weed. Several varieties of amaranth produce allelochemicals, but Palmer amaranth has made headlines by developing resistance to glyphosate (the active ingredient in Roundup) and other herbicides and, subsequently, giving farmers fits—particularly farmers who grow genetically engineered soybeans and cotton. While allelopathy seemed like an interesting yet irrelevant concept during the decades of “better living through chemistry,” when main-stream agronomists and crop breeders assumed we would rely on herbicides to deal with weeds, attitudes are now changing. For example, scientists have identified a strongly allelopathic variety of rice and bred it with more popular commercial varieties to create varieties that can better compete with weeds.
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