   Herbicides that Control Summer Weeds - May 23, 2001 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Herbicides can be a very effective tool to control weeds. Whether you agree with the use of herbicides or not, it does not hurt us to understand them. They are used in many commercial and residential landscapes that we visit. Foliar applied, non-selective herbicides control weeds that are green and growing. Different herbicide formulations have advantages and disadvantages depending on the situation. Knowing which type to use will increase your chances of success in controlling the weeds in your given situation. In this column, we will focus on four herbicides that are readily available for use by the homeowner. 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. Product labels provide extensive guidelines to help you reach targeted control. Specific herbicide information follows listing products by trade name with common names italicized in parentheses. All of these products must be used in accordance with the label for desired results. Toxicity information is also included. This is expressed in the form of an LD50 : the lethal dose required of active ingredient to kill 50% of the test animals in mg/kg (parts per million). The larger the LD50, the safer the herbicide is. Also remember to correctly identify the weed before buying and using any herbicide. Reward (diquat) is a contact herbicide. It has best results when used on small annual weeds. Large annuals and perennials will be injured but not killed. Reward can also be purchased for a reasonable price. Spray drift causes minimal damage to non-target weeds. Reward works well in hot or cool weather. However, it has a relatively high mammalian toxicity (LD50=230 mg/kg). Scythe (pelargonic acid) is another contact herbicide that controls small annual weeds. It too will not kill large annuals and perennials. In cold weather, Scythe is not as effective as Reward. Advantages include lower toxicity (LD50>5,000 mg/kg) and very rapid symptom development. Scythe is perceived by many to be an alternative to traditional herbicides. It is similar in chemical makeup to soap and more expensive than Reward. It also has an odor that some people find offensive. Like reward, spray drift will not translocate and kill these plants. Roundup-Pro (glyphosate) is a translocated herbicide which kills annual and perennial weeds. Roundup-Pro has low mammalian toxicity (LD50>5,000 mg/kg) and no residual soil activity. Be patient when using Roundup. Weeds often do not display symptoms of death for seven days or more. Spray drift can severely injury non-target plants, so use care when applying. Recently, Roundup changed its formulation. The new formulation, Roundup-Pro, contains a new adjuvant giving it a lower dermal toxicity, enhanced absorption, and improved resistance to being washed off by rain. With the improved absorption, Roundup-Pro now has a greater potential for injuring conifers. Finale (glufosinate-ammonium) is also a translocated herbicide. It is similar to Roundup-Pro in its mode of action. Finale acts on weeds more rapidly: often within 48 hours. It is less efficient at killing perennial weeds such as field bindweed. Mammalian toxicity is moderate (LD50=3,750 mg/kg). Finale works well on annual weeds that are moderately difficult to control (purslane, foxtail, tumbleweed, etc.). Of all the products listed above, Roundup is probably the most versatile. It kills many of our most difficult-to-control weeds including bermudagrass, johnsongrass, and nutsedge. For bermudagrass and johnsongrass, the most effective time to treat is when they first flower. Nutsedge is most effectively controlled during early summer and as it appears. Remember also that the patent recently ran out on Roundup, so you may purchase other herbicides with the active ingredient glyphosate packaged by other companies. 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 or E-mail us at mgardener@kachina.net 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/. |