   Weeds and Herbicides - April 5, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Enough with the mundane topics, let's deal with something contentious: herbicides. These compounds are designed to kill unwanted plants and can be a useful tool when used correctly. Some people are very adamant that herbicides are poisons that should never be used. However, in my experience, some of these folks often understand very little about herbicides and their modes of action. On the other hand, when herbicides are misused, the results can be harmful to the environment and human health. By understanding more about herbicides, we can make better informed decisions about whether or not to use them. Herbicides are most often used to control weeds. A technical weed definition is: any plant that interferes with the management objectives for an area of land at a particular time. A simpler definition is: an unwanted plant. Weeds often are very prolific, invasive, competitive, harmful, destructive, and/or difficult to control. Herbicides can be separated into two main categories: pre-emergence and post-emergence. Pre-emergence herbicides prevent plant seedlings from becoming established. These products work in many different ways. They usually work by inhibiting some important cellular function. This can be cell division, cell wall formation, lipid synthesis, amino acid synthesis, or photosynthesis. Before applying a pre-emergence herbicide, read the label and consider the following characteristics: soil persistence, photo degradation, and volatilization. Soil persistence be influenced by the application rate, the speed at which soil microbes can degrade the active ingredient, the chemical life of the active ingredient, and irrigation method/rate. Light degrades some active ingredients and these must be incorporated into the soil. Other active ingredients will volatilize (go into the atmosphere) and also need to be incorporated. Explicitly following the label instructions will provide the desired weed control and prevent unwanted damage to non-target plants. Post-emergence herbicides have similar modes of action to pre-emergence herbicides. Many are designed to target enzyme systems or metabolic processes that exists 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-p-butyl. 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 glufonisate, 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. 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. These are just a few examples of herbicides and their uses in home landscapes. There are many other products and much more to know about herbicides. In any case, always read and follow label directions (have I said that already?). By the way, there is an event happening this Saturday in Payson. It's the Waterwise Gardening and Xeriscape Festival. It starts at 9:00 AM and goes until 2:00 PM Saturday, April 8, 2000 at Green Valley Park. Author Jim Knopf will be presenting two slide show talks (9:00 AM and 10:30 AM). The infamous Terry Mikel will be discussing soils, the US Forest Service will cover firescaping, landscapers will be installing a small xeriscape garden and water feature, and there will be fun activities for kids. The University of Arizona Cooperative Extension has publications and information on herbicides and xeriscaping. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113, visit our web site |