SportsTurf provides current, practical and technical content on issues relevant to sports turf managers, including facilities managers. Most readers are athletic field managers from the professional level through parks and recreation, universities.
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Field Science 16 SportsTurf | March 2015 www.sportsturfonline.com This is a subject that I have often writ- ten about for the golf industry, but this is a first specifically for sports turf manag- ers. A recent article appeared inCanadian Groundskeeper (http://www.kenilworth. com/publications/cg/de/201411/files/6. html) and parts of the basic informa- tion in this article also appear in the Canadian article. To understand Poa annua, it is best to begin with its biology. This species appears in the botanic literature as a winter annual, which means that it ger- minates late in the summer and into the fall, lives through the winter as a mature plant, and in the spring it produces a seed crop and simply dies. It is better adapted to low mowing heights than most of our turf species and tends to crowd them out at low mowing heights. It thrives under an intense management system. Lots of fertilizer and water helps it get a foothold, whereas turf that is subjected to moisture stress and lower fertility levels rarely develops a problem. It can pro- duce a quality stand of turf in cool and wet conditions, but the fact that it dies in mid-season can be a real problem for summer sports, such as baseball (Fig.3). It can also produce seed at any time of year when it is actively growing, even at the lowest mowing heights. Most of the other grasses that are used in sports turf cannot do this. This ability to produce seed gives Poa an ecological advantage over other grasses and it slowly takes over intensely managed turf. If all Poa annua were a true win- ter annual, we could control it with preemergence herbicides, but much of it is a weak perennial that can survive the summer. Even where it survives, it is still an annoyance in sports turf because of its difference in color and texture from other grasses. Patches are easily picked up on high definition TV signals, even if it still healthy. It really stands out when it is dead (Fig. 4). I began my career 40 years ago as an optimist concerning the control of this species. Each turf show launched a new herbicide for Poa annua control and it appeared that it would only be a matter of time until this species would no lon- ger be a problem. That turned out to be incorrect. Four decades later, we are still searching for that herbicide or manage- ment strategy that will provide a solution. My years of experience have left me less confident that we will ever find an answer to the problem. It is likely that we will never see a completely effective "magic bullet" that will eliminate it. The earliest attempts to control Poa annua was to simply cut it out when it appears. This still works, but it is very labor intensive and only those with the highest budgets can generally follow this procedure. Preemergence herbicides have long been proposed as the solution and they can provide some control. However, the seed for Poa can live for years in soil and plants will emerge every time there is an opportunity for germination, such as in a cleat mark on a sports field. If it were a true winter annual, preemergence herbicides could work, but there are also many perennial biotypes in most areas, and the living plants are not affected by preemergence materials. Plant growth regulators (PGR) have also been promoted as possible controls. The first attempt was to use the Type I materials, like Embark (mefluidide) to inhibit seedhead formation. No seed- heads, no Poa was the concept. While Embark was an excellent seedhead inhibitor, this strategy was not effec- tive. Again, seed can live in the soil for years, and while Embark will inhibit seed production for a while, Poa can produce seedheads under nearly all conditions throughout the season. Embark was also highly variable and could result in phy- toxicity to the desirable grass. The next phase was the use of Type II PGR's, or gibberellic acid (GA) inhibitors, such as Trimmit (paclybutrazon), and Cutless (flurprimidol). These products are effective in selectively slowing the growth of the Poa in creeping bentgrass fairways. Their use can be an effective as part of an integrated management pro- gram designed to discriminate against the Poa annua. The Type II PGR's do provide some relief in creeping bentgrass turf on golf courses, but this is usually not a sports turf grass and their effec- tiveness is more limited in sports turf situations. Many experimental and commer- cially available postemergence, selective herbicides have been developed over the Fig.3. What happens to Poa annua in sports fields when it gets hot. Courtesy of Tim Van Loo, CSFM, Iowa State. Fig.4. Baseball field with dead Poa annua in summer. Courtesy of T.J. Brewer, CSFM, Burlington Bees, Burlington, IA.