   Learning About Herbicides - November 8, 2006 Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Herbicides are chemicals designed to kill unwanted plants. They can be a useful tool when used correctly. On the other hand, when herbicides are misused, the results can be harmful to the environment, non-target organisms (such as landscape plants), and human health. By understanding more about herbicides, we can make better informed decisions about whether or not to use them. A weed is any plant that interferes with the management objectives for an area of land at a particular time. Most of us just think of weeds as unwanted plants. Some weeds are nothing more than a nuisance. However, weeds can also invade native ecosystems, affect human and/or animal health, and be incredibly difficult to control. Herbicides should be used in conjunction with mowing, pulling, grazing, and other techniques to create an integrated approach to weed management. Whether or not you use an herbicide to control weeds is your decision. Whatever you decide, you should always correctly identify the weed species before trying to manage it. Identification will help you to identify weed management strategies that are effective and whether or not a given herbicide could be effective. Whether a plant is an annual, a perennial or woody species can determine which herbicide is the best choice for a give situation. You should always read the label of any product before purchasing it. Herbicides contain active ingredients that kill unwanted plants and can be separated into two main categories: pre-emergence and post-emergence. Pre-emergence herbicides prevent plant seedlings from becoming established. Pre-emergent herbicides often work by inhibiting some important cellular function such as cell division, cell wall formation, lipid synthesis, amino acid synthesis, or photosynthesis. Pre-emergence herbicides are commonly used on commercial landscapes where there is lots of bare ground. They require close attention to timing and application rate to be effective. Many homeowners contract professionals to apply pre-emergent herbicides in their landscapes. Post-emergence herbicides kill growing weeds, but most will not prevent weed seeds from germinating. Post-emergence herbicides have similar modes of action to pre-emergence herbicides. Many are designed to target enzyme systems or metabolic processes that exist in plants but not animals. Post-emergence herbicides can be further divided into two categories: selective and non-selective. Selective herbicides affect only certain types of plants (i.e. broadleaves or grasses). Conversely, non-selective herbicides will kill all types of plants. Herbicides have trade names, common names, and chemical names. The following examples use common names. Here common names are used to avoid company endorsements. An example of a selective post-emergence herbicide is 2,4-D. This herbicide is often applied to lawns to kill broadleaf weeds. Another is fluazifop. It kills grasses but not broadleaves (ideal for flower beds). Be sure to read and follow the label directions. Non-selective herbicides, such as glyphosate and glufosinate, enter plants through the foliage. They do not persist in the soil. These herbicides are most effective when plants are actively growing and can be useful for lawn renovations, xeriscape areas, rock gardens, or on weeds that come up in sidewalk cracks. They may require multiple applications for effective weed control. In industrial situations, soil sterilants (i.e. prometon) are used to destroy all vegetation to which they contact and have a residual effect in the soil (up to one year). These herbicides are designed for specialized applications and should be used with extreme caution, if at all, in home landscapes. I have seen partial and entire landscapes killed by these products. This column is only an introduction to herbicides and their uses in home landscapes. If you choose to use herbicides, make sure you read the label completely and follow the instructions explicitly. If you have a question, most products have a toll free phone number listed on the label that you can call for more information. 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/. |