Throughout the southeast, warm-season turfgrasses have broken dormancy and started greening up in response to increasing temperatures and day length. However, bermudagrass will not resume sustained, active growth until daytime and nighttime temperatures consistently reach the mid-80 and mid-60 degree Fahrenheit ranges, respectively. Unfortunately, the warm days and cool nights of spring are ideal for rapid growth of various winter annual weeds such as Poa annua, volunteer ryegrass and broadleaf weeds. Maintaining an acceptable level of weed control is a key management concern at courses throughout the region because of the negative impact weeds have on both playing surface quality and aesthetics.
At some courses, pre and postemergent herbicide treatments are conducted during the fall to control winter annual weeds. Yet, by mid-spring the residual activity of preemergent herbicides starts to break down and a flush of weed germination and establishment occurs. In areas that received abundant rainfall in December, a bumper crop of Poa annua is now developing. Frequent mowing is a standard recommendation to help maintain an acceptable level of control when managing winter annual broadleaf weeds. There are also a number of effective postemergent herbicides that can be used to control weeds. Dr. Bert McCarty, professor at Clemson University, recommends the addition of sulfentrazone in tank mix combinations to further improve winter annual broadleaf weed control. However, the rate of control will still tend to be slow and, because winter annual broadleaf weeds naturally decline with the onset of hot, humid conditions, it can be debated whether or not large-acreage, postemergent herbicide treatments for winter annual weed control are absolutely necessary.
Increased mowing frequency at a slightly lower height of cut can certainly help control Poa annua. Furthermore, Poa annua also will naturally decline with the onset of hotter temperatures in late spring and early summer. However, a single Poa annua plant can produce as many as 6,000 seeds that may remain viable in the soil for six years or longer. Thus, herbicide treatments in the winter and spring are advised to help prevent a progressive buildup of Poa annua in primary play areas. Unfortunately, the development of herbicide resistance by Poa annua is a major concern. Poa annua resistance to sulfonylurea, dinitroaniline and triazine herbicide classes, and the nonselective herbicide glyphosate, has been documented. On a positive note, there are newer pre and postemergent herbicide chemistries available that provide effective control of Poa annua. Yet, Poa annua can develop resistance in as little as six years following repeated applications of any herbicide. Therefore, the importance of regularly rotating herbicides in a resistance management program cannot be overemphasized. For more information, please see the article Poa annua Management on Golf Course Putting Greens.
Source: John Foy (jfoy@usga.org)
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