   Avoiding Herbicide Injury in the Landscape - August 27, 2003 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County The array of herbicides (weed killers) available at home and garden centers is mind-boggling. On occasion, I visit these businesses and observe consumer behavior in the pesticide section. I often hear a customer say: "I have no idea what weed it is, I just want it dead". I've also heard a salesperson say: "just buy this, it kills everything". These lines of thinking are simply accidents waiting to happen. It is always best to purchase herbicides from nurseries or garden centers that are staffed by qualified individuals who can recommend the appropriate chemical control when necessary. When considering herbicides, first consider whether you really need one or not. You may be able to control your weed problem by mowing, hoeing, or other cultural practices. If you decide to use a herbicide, select the best one for your situation. Herbicides are normally separated into two general categories: selective and nonselective herbicides. When applied correctly, many herbicides are selective. That is, only certain types of vegetation will be killed. Selective herbicides have the potential to become nonselective (soil sterilants) when deliberately or accidentally applied at high rates. In some cases, the manufacturer intended these chemicals to be used either selectively or nonselectively depending on the weed control desired. Soil sterilants destroy all vegetation to which they are applied. Not all nonselective herbicides are soil sterilants. Chemicals such as glyphosate (Roundup) will kill all vegetation, but do not persist in the soil. The use of herbicides with a short-term residue is ideal in the landscape, since plants may be safely introduced shortly after weeds, brush, turf, etc. have been eliminated. Never apply long lasting, high toxicity soil sterilants under new pavement or sidewalks. Tree roots from nearby species will eventually grow into the site and translocate the chemical throughout the plant. These trees will be severely injured or killed as a direct result of the chemical weed barrier applied. This can also happen to trees growing on neighboring properties. If a neighbor's tree is injured, the person applying the sterilant is responsible for any damage incurred. Strive to use herbicides early in the morning when winds are likely to be calm. When possible, use a coarse droplet spray applied at low pressure. Excessive temperatures can also be avoided in the morning hours. Intense heat can cause phenoxy-type (i.e. 2,4-D) herbicides to volatilize and injure nearby nontarget species. When available, choose herbicide formulations that pose the least risk of drift or other forms of contamination. For example a low volatile ester or amine type formulation of 2,4-D is appropriate when spraying around sensitive, broadleaf ornamentals. Follow this precaution even when plants are dormant. Buds are capable of absorbing phenoxy-type herbicides in sufficient amounts to produce distorted leaves. Although excellent selective grass control herbicides are available, they also have the potential to injure some broadleaf species. They are not totally selective between grassy weeds and desirable broadleaves. These herbicides will have a list of plants adversely affected on the product label. Avoid applying herbicides at their highest labeled rates in soils low in organic matter. Less soil applied herbicide is required for proper weed control in soils low in organic matter. Unfortunately, a smaller margin of tolerance exists for desirable plants growing in these soils. In general, Arizona soils are low in organic matter. There are many other considerations that I could not include in this column. However, you may read more about this topic on the Internet. Oklahoma Cooperative Extension has an excellent publication called Herbicide Injury in the Nursery and Landscape on the web at: http://pearl.agcomm.okstate.edu/hort/greenhouses/f-6704.html. Remember, use herbicides only if you have a specific goal in mind. Be safe and follow label directions. Naming of companies or products is neither meant to imply endorsement by the author nor criticism of similar companies or 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 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/. |