   Least Toxic Herbicides - June 25, 2003 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Herbicides are widely used to kill weeds in agriculture, commercial and residential landscapes, and home gardens and orchards. While many people accept the risks associated with synthetic herbicide use, increasing numbers are declaring that use of these compounds is not acceptable-especially for use in public places or where children may be exposed. There are many less toxic alternatives to synthetic herbicides: cultivation, pulling, mulching, flaming, solarization, competing vegetation, and least-toxic herbicides. People often use synthetic herbicides because they are easy to apply. Least-toxic herbicides are just as easy to apply, but have are less toxic to people, pets, wildlife, and other non-target organisms. Herbicides alone are not as effective as when used in combination with other weed prevention and control strategies. An integrated approach is the foundation of successful weed management. There are two general categories of herbicides: preemergent and postemergent. The only least-toxic preemergent herbicide I am aware of is corn gluten meal. I have not used it, but the scientific literature indicates that it works. Corn gluten meal is a waste product of corn syrup processing. It has been used in the production of dog food, fish food, and other animal feeds. Corn gluten meal is a fine, yellow powder that suppresses seed germination of many common annual grasses and broadleaf weeds. Apply 40 lbs. CGM/1,000 sq. ft. to the soil surface before weed germination, thoroughly water the area, and leave it to dry. Corn gluten meal also contains about 10% nitrogen, so it will also increase soil fertility. Herbicidal "soaps" are least-toxic postemergent herbicides. These products contain fatty acid salts which break down into carbon dioxide and water. They are most effective on annual weeds because they disrupt the cell membranes only killing the tissue that they contact. One commercially available fatty acid herbicide is called ScytheTM. Scythe contains primarily pelargonic acid: a fatty acid found in seeds and foods. It will not harm plants unless sprayed on them and is most effective when sprayed on a hot day where results can often be seen in minutes. The USDA has been testing vinegar (acetic acid) for use as a least-toxic postemergent herbicide. Grocery store vinegar is 5% acetic acid, but more concentrated solutions are more effective. Acetic acid also disrupts membranes so it only effective against annual weeds. 10, 15, and 20% acetic acid killed 80-100% of the annual weeds tested by USDA. This included foxtail, lambsquarters, and pigweed in their juvenile stages. Vinegar has excellent potential for home landscape use, especially on sidewalks, retaining walls, and patios. The higher concentrations have a stronger vinegar smell, but it readily dissipates in the outdoors. Acetic acid herbicides are currently available alone or in combination with other least-toxic compounds. Least-toxic herbicides are not yet widely available. You may be able to request them to be sold at your favorite nursery or garden center. Research articles about corn gluten meal and a list of distributors are available on the Iowa State University Web Site at: www.gluten.iastate.edu. Acetic acid information and availability is also available from Iowa State University Web Site at: www.weeds.iastate.edu/weednews/vinegar.htm. Scythe is available from local distributors and several Internet suppliers. I hope that some readers research and try some of these least-toxic products. They certainly have their place and offer alternatives to synthetic herbicides. Finally, remember that these products are pesticides and must be handled according to label directions. "Least-toxic" does not mean non-toxic. 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/. |