Cover Story
Controlling
Crabgrass and Goosegrass on Warm-Season Golf Courses By Jay McCurdy, Ph.D., Assistant Professor, Mississippi State University
R
egardless of budget and expectations, preemergence herbicides are an important line of defense against most spring- and summeremerging weeds. Yet, I am astounded by the number of calls, emails and texts I get every summer concerning control of crabgrass and goosegrass. My response is almost always, “What preemergence herbicide did you use?” and frequently the answer is, “None.” Preemergence herbicides target the rapid root or shoot growth that occurs immediately after a seed germinates. To be effective, they must be applied prior to seedling germination. They must also be present in the soil at a lethal concentration, which correctly suggests that many of these herbicides have relatively long soil half-lives, enabling them to remain effective over an extended period of time. In fact, rowcrop farmers call many similar products “residual” herbicides, because 10 • Mississippi Turfgrass • Winter 2018
they provide lasting preventative control of many troublesome weeds. Due to microbial activity, sunlight and moisture movement, each preemergence herbicide degrades uniquely. Efficacy may also be reduced by the herbicide binding to organic matter and clay particles. Because these factors are somewhat predictable, companies often recommend repeat applications (sometimes called “split applications”). Typical reapplication dates range from 6 to 12 weeks after the initial treatment. Caution: Every preemergence herbicide can harm turf health and quality, so always read and understand label instructions.
Common weeds
Crabgrass emergence is initiated in the spring by soil temperatures of approximately 57°F at a 1" soil depth. In the mid- to deep-South, most preemergencecrabgrass applications are applied
in late January through early March. Preemergence herbicides can be tank mixed with postemergence, non-selective or selective herbicides for “reachback” of already germinated weeds. Additions may include a broadleaf herbicide such as Trimec or Metsulfuron. Many superintendents may add an ALS inhibitor to their preemergence herbicide (such as Monument, Revolver or Katana) for postemergence control of annual bluegrass; these herbicides need relatively warmer conditions to work best. Simazine is another common addition at this timing. Caution should be used when applying preemergence herbicides with potentially damaging non-selective herbicides, such as Roundup or diquat, for fear that the residual effects of the preemergence will hinder regrowth and recovery of injured turfgrass. Commercially available mixtures, such as Echelon (prodiamine +