   Least-Toxic Weed Management - August 15, 2007 Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County Summer rains may have initiated weed growth in your landscape. Whether you agree with the use of herbicides or not, they can be an effective tool to manage unwanted vegetation in your landscape or garden. Foliar applied, non-selective herbicides will control weeds that are green and growing. Some non-selective herbicides are more environmentally friendly than others. These are often called “least-toxic” products. Once you correctly identify your weeds, then you can choose a product that fits your objectives and philosophy. Foliar applied herbicides fall into two general categories: contact and translocated (systemic). Contact herbicides kill only green tissue contacted by the spray. Translocated herbicides move within the plant from point of application to other plant parts. The least-toxic herbicides that I am aware of are all contact-type products. This makes them largely ineffective on perennials such as bermudagrass, but very effective when used on annuals – especially if applied prior to seed maturity. Remember that all pesticides must be used in accordance with the label for desired results. Scythe is a least-toxic herbicide that controls small annual weeds. It will not kill large annuals and perennials. In cold weather, Scythe is not as effective as conventional alternatives such as diquat. Advantages include lower toxicity (LD50>5,000 mg/kg) and very rapid symptom development. The active ingredient (pelargonic acid) is similar in chemical makeup to soap and has an odor that some people may find offensive. Scythe can be used for large areas of annual weeds or to spot-treat individual plants as long as the spray mix does not contact foliage or green bark. It is also labeled for use on unwanted suckers from woody plants and excessive cane growth on berry brambles. It is not easy to locate for purchase, but I did find some internet venders that carried it. BurnOut was originally introduced with acetic acid (strong vinegar) as the only active ingredient. The more recent introduction of BurnOut II still has acetic acid, but now contains clove oil and sodium lauryl sulfate as the primary active ingredients. According to the product label, BurnOut II will be most effective on annual weeds, but can be repeatedly applied to control some perennials. It is more widely available and come in both ready-to-use and concentrated formulations. On my recent organic agriculture tour in Michigan, I heard from researchers that were finding clove and other essential oils to be effective at weed control. The perennial grasses (fescue, quackgrass, and Kentucky bluegrass) were not completely killed, but vigor was greatly reduced. I think that bermudagrass and johnsongrass may prove too vigorous to be effectively managed by BurnOut II. The approach to least-toxic weed control is no different than other any other pest control strategy. You need to integrate as many control methods as possible to reach the greatest level of management. Herbicides should be used in conjunction with other strategies such as prevention (exercise discrimination before importing plants and landscape products that could contain weeds or other pests), mulches, hand digging, cultivation, flaming, mowing, grazing, and hot water/steam treatment. If you find that least-toxic methods are not providing the desired level of weed control, then you may decide to apply a conventional herbicide to get the toughest weeds under control. The most common of the conventional alternatives is glyphosate (sold as Roundup, Touchdown, Ranger, and others). Glyphosate is a translocated herbicide and effective at controlling bermudagrass, johnsongrass, and other tough perennials when applied according to label directions. Once the toughest weeds are under control, then you can revert back to least-toxic methods. Remember that environmentally responsible pest management methods seek to minimize harmful effects to non-target organisms (people, pets, wildlife, beneficial and benign insects, desirable plants, etc.) while also protecting the non-living elements of the environment. When you have these principles in mind, then you can make well-informed choices about weed management. Naming of products is neither meant to imply endorsement by the University of Arizona nor criticism of similar products not mentioned. 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 cottonwoodmg@yahoo.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://cals.arizona.edu/yavapai/anr/hort/byg/. |