   Natural Herbicides and Invasive Plant Species - May 14, 2003 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County I often get a bur under my saddle about noxious weeds and invasive plants. However, recent research is showing that we may be able to utilize chemical compounds found in invasive plants to control other weeds. This process, called allelopathy, is an interaction between two plants where one plant inhibits the growth or germination of another plant by excreting chemicals toxic to the other plant. Allelopathy has been known to exist in many species including black walnut, sagebrush, sunflowers, and tree of heaven. How many other plants have you seen growing under a tree of heaven? Scientists are working to harness the power of allelopathy and use these naturally occurring chemical compounds to create new herbicides. Spotted knapweed is a noxious weed with allelopathic properties. It is found throughout the west (including some isolate populations in Yavapai County). A Colorado State University professor, Jorge Vivanco, recently isolated and extracted a chemical compound with herbicide qualities from spotted knapweed roots. The compound is called catechin and Vivanco found that it had herbicidal properties as effective as commercially available herbicides when sprayed on some plants. Further research showed that in reduced concentrations the chemical only kills select plants while sparing others. That could allow farmers to protect a crop while killing a weed. Or it could be used as a preventive agent by mixing it with soil before weeds emerge. Other researchers caution that the public may see these compounds as being less harmful to humans. This is not necessarily true. Just like other compounds extracted from other plants, catechin and other naturally occurring compounds can be a danger to non-target organisms (humans, wildlife, pets, insects, crops, soil microbes, etc.). I know that when I have handled spotted knapweed plants, my hands tingled afterwards and now I wear gloves when handling it. As mentioned above, Tree of Heaven (Ailanthus altissima), also displays allelopathic properties. Locally, people know this tree as the one that keeps the Town of Jerome from sliding downhill and merging with Town of Clarkdale. Italian scientists recently confirmed that a chemical compound extracted from the Tree of Heaven (ailanthone) is toxic to many crop plants. They also identified four other poisons in Tree of Heaven roots. These will also be further researched to determine their usefulness as herbicides. Just smell the leaves of a Tree of Heaven sometime. It is very repulsive. Salt cedar is another invasive species of concern in the southwest. It is overtaking many riparian areas including the Verde River. Recently, scientists noticed that salt cedars occurring in the U.S. looked different than those found growing in their native Eurasia. DNA fingerprinting recently confirmed that the most invasive U.S. salt cedars are actually hybrids of two imported salt cedar species (Tamarix chinensis and Tamarix ramosissima). Their native Eurasian ranges do not overlap. However, importation to other locales has allowed them to interbreed and it appears that hybrid vigor factors into their degree of invasiveness. Invasive species are a growing problem worldwide. While we are trying to harness the power of invasive species in new herbicides, billions of dollars are still being spent each year to combat the spread of invasive plants. Meanwhile, natural resources are impacted and native ecosystems degraded. This has prompted the Nature Conservancy and other groups concerned about invasive species to start developing criteria to assess plant's invasive characteristics. The American Nursery and Landscape Association is also beginning to work on invasive species assessment. As gardeners, we must recognize that non-native plants can have far reaching impacts. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |