Annual bluegrass (Poa annua L.) or simply “Poa” is a cosmopolitan weed of turfgrass. Annual bluegrass is very easy to identify due to its boat-shaped leaf tips and tall membranous ligule. It’s easy to identify from a distance because it produces patches in the turf that are distinctly lighter green in color compared to Kentucky bluegrass or perennial ryegrass. Also, it oftentimes has a seedhead on it, regardless of the mowing height.
It is, of course, closely related to Kentucky bluegrass and rough bluegrass. This is one of the factors that have made the development of an effective and selective herbicide to control annual bluegrass a difficult task. Always considered the bane of the golf course superintendent, annual bluegrass is becoming a more prevalent weed issue both in home lawns and also in athletic turf. It tends to be a more severe problem as the intensity of turfgrass management increases and thus some of the increase in annual bluegrass pressure in athletic fields can be attributed to the increased number of intensely managed athletic surfaces.
Annual bluegrass ecology
Annual bluegrass is able to survive in so many climates due to multiple adaptation strategies. The success of annual bluegrass as a weed is due in no small part to the fact that it produces copious amounts of seed. A single annual bluegrass plant might produce up to 2,000 seeds and these seeds can remain viable in the soil seed bank for many years. And, mowing shorter won’t reduce seedhead production, since annual bluegrass can produce viable seed when mowed at putting green height. Many weed species require exposure to light, as might occur following injury to the turf via disease, insects, or traffic in order to germinate. But research conducted by McElroy et al. in 2004 showed that annual bluegrass can indeed germinate in dark.
Annual bluegrass produces a large amount of seed that can germinate under less than ideal conditions and survive for a very long time in the soil. It does this by sacrificing its ability to produce a strong, healthy plant with a deep root system. Most annual bluegrass plants have a winter annual life cycle – the seed germinates in fall and the plants put most of their energy into growth that will lead to seed production in the spring. This growth is at the sacrifice of tillers and a deeper root system. Because of this, annual bluegrass is designed basically to grow rapidly, produce a lot of seed and then die. Of course, these dead patches of annual bluegrass in the turf are a problem visually. Also, while annual bluegrass is alive, it tends to be more susceptible to environmental extremes or insect and disease pressure. So, managing a field with annual bluegrass is a problem because of all of the potential extra inputs that are required to keep the annual bluegrass alive.
If the annual bluegrass plant is so frail, how can the species be such a successful weed? The answer lies in the large amount of plants that are produced from seed. But, in addition, there is a lot of biotype diversity with annual bluegrass. This even includes the fact that there are two recognized varieties – Poa annua var. annua, which is “annual” annual bluegrass and Poa annua var. reptans, which is “perennial” annual bluegrass. On low-maintenance athletic fields, the annual biotypes dominate. What we think happens on highly maintained fields is that first the area is “colonized” by annual biotypes and then subsequent plants from seed take on the characteristics of the perennial variety. Variety reptans does produce more shoots, spreads by stolons and has a more prostrate growth habit. But it still has the lighter green color and susceptibility to insect and disease pests that the annual biotype does.
Annual bluegrass control strategies
There are several preemergence herbicides that are labelled for control of annual bluegrass, including benefin, bensulide, dithiopyr, oxadiazon and prodiamine. Each of these products can result in 80 to 90 percent control. But, there are several potential issues with their use on an athletic field, not the least of which is that they will prevent the germination of desired grasses when seeding or overseeding.
Best control is achieved with an early fall application. But, this will not allow you to overseed during the fall. While Poa mainly germinates in the fall, it’s quite opportunistic and some plants germinate during other times of the year, thus escaping the herbicide application. Also, this would not be an effective strategy against perennial biotypes. Mesotrione (Tenacity) is labelled for use as a preemergence herbicide at time of seeding/overseeding and is also labelled for suppression of annual bluegrass germination.
Growth regulators have shown some promise for the gradual removal of Poa from golf courses, but so far this strategy has received limited attention in sports turf. The Type I growth regulators, such as mefluidide, while effective at suppressing seedhead formation on annual bluegrass, also may have some phytotoxicity issues, depending on your species and cultivars of turfgrass.
More sports field managers who are using Type II growth regulators as part of their field maintenance programs have reported good results, such as increased density and wear tolerance. If this becomes a more widespread management practice, it may follow that recommendations for the use of growth regulators for gradual removal of annual bluegrass from sports surfaces may be developed.
There are several herbicides that are currently registered for postemergence control of annual bluegrass. Of these, the first herbicide to be registered was ethofumesate (Prograss), which is labeled for both pre and postemergence control. Results are generally favorable but inconsistent. It was discovered that the half-life of ethofumesate can be much less in turfgrass compared to bare-soil, which could limit its use as a preemergence herbicide when applied to turfgrass. Because of this, best control is achieved using it as a postemergence herbicide and making three applications on 2-3 week intervals in the fall, following the flush of fall germination.
Mesotrione is labelled for the suppression of annual bluegrass when used as a preemergence, but not for control postemergence. To date, results of research on postemergence control have been inconsistent. At the Ohio Turfgrass Foundation Research and Education Center, it was discovered that three applications of mesotrione every two weeks would provide, on average, as high as 60 to 70 percent control. However, control would be very good in some of the research plots and the same rate would result in poor control in another plot. Research is being conducted to determine if there is a rate or timing recommendation that could be developed for using mesotrione postemergence, either alone or in combination with another herbicide.
Research with amicarbazone (Xonerate), has shown very favorable results. For bluegrass, it’s labelled for use either twice at 2-ounces product per acre or 4 times at 1-ounce product per acre. For ryegrass and tall fescue, it’s labeled for use twice at 2- to 4-ounces per acre. Though the control can be good, note that both Kentucky bluegrass and perennial ryegrass can be sensitive to this product, so it’s best to use the lower label rate recommendations and also avoid spraying when temperatures exceed 85 degrees. Best control is achieved using 4 applications at 1-ounce product per acre every 7 to 10 days, beginning in the spring (late April – around the time crabgrass begins to germinate).
Dr. John Street’s research at Ohio State has shown that meostrione (2-ounces per acre) combined with Xonerate (1-ounce per acre) will give excellent control and also allows you to reduce to two applications at a 3-week interval. Research at Ohio State has also tested a schedule of 3 applications at 1.3-ounces product per acre with favorable results. Carefully read and follow label recommendations as to the use of the product and also when desired turfgrass can be overseeded following application.
Future control strategies
Research on methiozolin (Poa Cure) has produced very favorable results and this product is very safe to bentgrass. It offers both pre- and postemergence control. The product is still in the experimental stage and is being trialed mostly on golf course putting greens, with an anticipated launch of 2016 (research to date has not focused on the use of this product on sports turf). There are reports of variability in control among different Poa biotypes and also perhaps some variations in phytotoxicity among different high cut turfgrass cultivars. After the product is released, there will likely be a major focus on testing for use on athletic fields.
Roundup Ready bentgrass was developed many years ago and is still under review by the U.S. government. If sale of this product is allowed to go forward, then it’s likely that the company that developed it, OM Scotts (Marysville, Ohio), will also be allowed to market Roundup Ready Kentucky bluegrass, which is reportedly in development. While it’s always possible that annual bluegrass could develop resistance to glyphosate, this might offer the best hope for finding a truly effective annual bluegrass control solution.
To successfully deal with annual bluegrass, be aware that there is not a single management solution. A variety of cultural management practices that favor the desired turfgrass while discouraging annual bluegrass should be practiced. In addition, while the biotype diversity of annual bluegrass results in there not being one clear choice, the use of herbicides to selectively remove the annual bluegrass is becoming increasingly more important.