Field Science

conducive to Poa annua infestation. Add to that the high level of maintenance ini- Controlling Poa tiated by Certified Sports Field Manager T.J. Brewer, and Poa has become an annua in sports turf increasing problem in recent years. Once it gets a foothold in cool-wet ■ By Dr. Nick Christians weather, Poa does what it does best in stress periods; it simply dies. Figure 2 is from a sports field at Iowa State uch of my career has been spent on trying to kill annual University. The clear outline of the Poa bluegrass, better known asPoa annua in golf course turf. annua can be seen as lighter colored While it does occur in other turf areas, such as lawns, it patches in the darker colored Kentucky has always been on the golf course where this species has bluegrass (Poa pratensis). So, now that it M presented the biggest problem. is here, how do you kill it? The very low mowing height and intense fields in most climatic zones. This has management regimes used on golf changed in the 2000’s, and the greater courses provide an ecological advantage intensity with which modern fields to Poa annua and it easily outcompetes are managed is resulting in Poa annua other preferred grass species. becoming a common complaint among This weed could always be found in sports turf managers. Figure 1 is from sports turf areas, but the less-extreme the Burlington Bees baseball field in management conditions that predomi- Burlington, IA. The Burlington field sits nated on most sports turf through the in a low area along the Mississippi River Fig.2. Poa annua in a sports field at Iowa 1900’s meant that Poa annua was usu- and is surrounded by trees and a fence State University. Courtesy of Tim Van Loo, ally not a major problem in athletic that results in a microclimate that is very CSFM, Iowa State.

Fig.1. Poa annua is becoming more common in sports fields each year as management levels of these areas intensifies. Courtesy of T.J. Brewer, CSFM, Burlington Bees, Burlington, IA.

14 SportsTurf | March 2015 www.sportsturfonline.com Field Science

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

16 SportsTurf | March 2015 www.sportsturfonline.com years. I have conducted research on most is generally limited to use on creeping of these. While some appeared promis- bentgrass, or on perennial ryegrass. It can ing, and I occasionally got excited about also be used on bermudagrass turf over- the results, every one of them has had seeded with perennial rye. These are very the same problem. They work on some limited situations in athletic fields, and its biotypes of Poa annua and not on others. use in sports turf is quite limited. There are literally thousands of biotypes The newest postemergence, selective of this species. While these biotypes are product is PoaCure (methiozolin). This all genetically classed as Poa, they vary product is also known in the industry in widely in their response to postemer- the US as the “Korean product” because gence herbicides. It’s not unusual to see Fig.5. Tenacity (mesotrione) turning Poa it comes from Moghu Research Center multiple types on the same golf course annua white in Kentucky bluegrass turf. in South Korea. PoaCure has a new and it is assumed that this is also the Courtesy of Tim Van Loo CSFM, Iowa State. mode of action and works by inhibiting case on sports turf. This is Poa annua’s cell wall formation. This product has strength and our downfall. It always has tion within the limits of the label will be been very promising in early testing in a way of getting around us. If a product necessary. golf course turf. It has been used under is found that controls 99% on a given site Xonerate (amicarbaxone) is an herbi- an experimental use permit (EUP) for (I rarely see more than 80% control) the cide used primarily for the removal of Poa limited testing in 34 states this year in remaining Poa that is resistant to the her- annua from both cool- and warm-season the US. They hope to begin marketing bicide will produce seed and simply turn perennial grasses. While the golf market in 2016. It has provided the best results over the population. was the primary thrust of its develop- of any new product that I have seen for This is the primary reason why I wrote ment, it is labeled in many states for other many years. However, as more results are earlier that I do not think that we will turf areas, including sports fields. Its use being reported, it is evident that there is ever see the “magic bullet” herbicide that in the golf market has been somewhat variability in its control of various Poa will completely take out Poa. This genetic limited because of phytotoxicity problems biotypes. This product is generally more variability is the best guarantee of sur- with creeping bentgrass. For use on sports effective in fall than in spring. It can be vival that a weed can have. fields, follow the label very carefully. It used for control in Kentucky bluegrass These postemergence herbicides can should not be applied to Kentucky blue- turf, although Dr. Askew reports varia- also be quite variable in their effect on grass when air temperatures are expected tions in phytotoxicity among different desirable turf species. One of the most to exceed 85 F within 3 weeks of its cultivars of Kentucky bluegrass in tests effective postemergence herbicide that application. It can also damage seedling conducted in Virginia. Research on sports I worked with has been Prograss (etho- Kentucky bluegrass and should not be fields will be an important issue once it fumesate). This product is very effective applied to Kentucky bluegrass areas that has reached the market for golf course at killing most Poa biotypes from peren- are less than 12 months from establish- use. The company has not tested it in the nial ryegrass. The catch is the perennial ment. It also should not be applied to turf US on sports turf, but they report that it ryegrass. It easily damages Kentucky that has been treated with Embark within is widely used in Korea to take Poa from bluegrass and other species commonly the past 3 months. It is best applied for Kentucky bluegrass soccer fields. used in sports turf areas. If your field is Poa annua control in the spring, when it To this day, the most effective solution perennial ryegrass only, Prograss may be can be applied at 2 oz/acre in 14 to 21 day to the problem has been the develop- the product that you need, but this is rare. intervals, or split into 4 one oz/acre appli- ment of Roundup (glyphosate) Ready Tenacity (mesotrione), a recent cations separated by 7 day intervals. creeping bentgrass. This method involves postemergence material from Syngenta, Dr. Shawn Askew of Virginia Tech the genetic manipulation of creeping can also be effective in some situations. reports that a combination of Tenacity bentgrass. Scientists from O.M. Scotts This product, though, is limited to at 4-5 oz/A in a tank mix with Xonerate Co., Marysville, OH, and Monsanto, St. use in Kentucky bluegrass. It will kill at 1 to 1.5 oz/A is proving to be a good Louis, have successfully inserted a gene bentgrass and can damage perennial combination for control of Poa annua in into creeping bentgrass that makes the ryegrass and tall fescue. This product Kentucky bluegrass sports fields. grass tolerant of glyphosate. Creeping does hold some promise for Kentucky Velocity (bisprybic-sodium) is bentgrass is normally susceptible to bluegrass sports fields, but again, the another postemergence herbicide that glyphosate, as are most weeds found biotype problem with the Poa is a shows promise for Poa annua control in in bentgrass turf. The susceptible spe- concern. I have seen some promising creeping bentgrass turf on golf courses. cies include Poa annua, and this makes trials and some that were less promising It can cause discoloration of creeping it possible to remove the Poa without (Fig. 5). With Tenacity, persistence is bentgrass and there are clearly biotype damaging the bentgrass. My students important and more than one applica- differences in the response of Poa. It and I did a lot of work at Iowa State on www.stma.org March 2015 | SportsTurf 17 Field Science the concept and found it to be highly However, this remains the most promis- effective. The sale of glyphosate tolerant ing technology that I have worked with creeping bentgrass is still under review by in my career and hopefully it will be regulators in the US federal government released in the future. and to date, it has not been released. Scotts is currently developing gly- This process is not without draw backs. phosate tolerant Kentucky bluegrass. For instance, Poa annuacould develop These cultivars are still under develop- resistance to the glyphosate over time. ment, but they may provide one of the

Fig.6. Fraze mower used to remove grass and the upper layer of soil from practice sports field at Iowa State.

best opportunities for the selective con- trol of Poa in Kentucky bluegrass sports turf areas that have been available to date. It is likely to be a few years before these cultivars are ready for the market. A potential ‘non-chemical” method that can offer some relief to the prob- lem in Kentucky bluegrass fields is fraze mowing, which quickly removes the upper layer of soil, including Poa annua seed, followed by reestablishment with Kentucky bluegrass seed (Fig. 6). In 2014, CSFM Tim Van Loo performed a success- ful test of this procedure on one of Iowa State’s practice fields. While the objective of this was not to control Poa, it is appar- ent that this procedure can potentially be used on older Poa-infested fields. It is apparent that the removal of Poa annua from any turf area is a very complex issue. The solution varies with a series of factors, including the primary grass species on the field, climate, weather, soil type, and location. There is no single solution. The solution may require some experimentation by the sports turf man- ager to find the right solution for their particular situation. ■ST

Dr. Nick Christians is a professor of horticulture at Iowa State University. His area of research interest is in the develop- ment of naturally occurring herbicides for the turf industry. He also conducts research on reducing the pesticide and fertilizer input for the management of quality turf in sportsturf, golf and lawn care industries. For a list of citations for this article, visit www.sportsturfonline.com.T

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