What is the Best Way to Deal with Annual Bluegrass (Poa annua)?

Jeff Higgins, Ph.D.
Director of Market Development
Pursell Technologies Inc.

             Fall is a great time of the year.  It’s the time of year for football season, holiday season, tree foliage is changing colors, and for many turfgrass managers it is hunting season.  After the hectic spring and summer fighting crabgrass and goosegrass, the fall of the year is a good time to get away from that golf course or athletic field and think about something else.  For many turfgrass managers, that is exactly what they do.  However, fall of the year is also a good time to think about weed control.  Many of our troublesome weeds that cause us turfgrass problems in the spring are a result of poor control or lack of attention in the fall.  One such example is annual bluegrass (Poa annua). 

             Annual bluegrass is one of the most invasive and troublesome weeds that turfgrass managers deal with.  It can be found on practically every golf course and athletic field in the United States.  Attempts to control this species are usually met with failure.  As a matter of fact, many would consider Annual Bluegrass Control to be an oxymoron.  For those non-Auburn graduates, an oxymoron is defined as a “figure of speech combining seemingly contradictory expressions”.  That is why most turfgrass managers have learned to just live with this competitive weed pest for lack of any better strategy. 

             Firstly, let’s discuss a little bit about the biology of annual bluegrass.  By understanding it better, we have a much better chance of controlling or managing annual bluegrass.  The name annual bluegrass is a misnomer! There are actually two species of annual bluegrass that can and do infest highly maintained turfgrass.  The “true” annual type has the scientific name Poa annua var. annua, while the perennial type is called Poa annua var. reptans.   These two species can be separated by their morphological characteristics.  The annual type has a more upright growth habit, prolific seed production, and a seed dormancy mechanism.  The perennial type has a more prostrate growth habit, less prolific seed production than the annual type, and no seed dormancy mechanism.  The perennial type will produce stolons, thus giving it the more prostrate growth habit. The flowering habit of both types includes both self- and cross-pollination.  Early contaminants in turfgrass stands tend to be cross-pollinating types but continued turfgrass culture often leads to large populations of the perennial type.  

            Research has shown that on a single golf course, there can be several ecotypes of annual bluegrass.  In one such study, the various ecotypes were correlated to irrigation or watering practices.  For instance, in the rough where no irrigation was used, the annual bluegrass present was primarily of the annual type having a one year life cycle.  However, on the greens where frequent irrigation was applied, the predominant annual bluegrass type was more perennial in nature and tended to live for more than one year.

            What really makes annual bluegrass such a weed problem is its reproduction potential.  It is one of the few species that can produce viable seed one day after pollination has occurred.  Most other species require weeks or months of seed ripening before that seed can be viable.  With annual bluegrass, mowing one day after pollination would still result in the production of some viable seed.  Thus, annual bluegrass actually “overseeds” itself every time you mow!

             Research has estimated that annual bluegrass in a golf green will produce between 14,000 and 65,000 viable seeds per square foot.  If you were to seed creeping bentgrass at a rate of 1 pound per 1,000 square feet, which is equivalent to approximately 8,000 seeds per square foot, only about one-half to one-eighth the amount of seeds that annual bluegrass is capable of producing.  In addition, most of the annual bluegrass seed will germinate during the year or growing season in which they are produced and a small percentage of the seed will remain dormant for longer periods of time ensuring a “seed bank” or reservoir of seed.  This is why annual bluegrass is so hard to control!

             Another aspect of annual bluegrass that ensures it to be one of the most troublesome weeds every year is the fact that it can easily adapt to different environmental conditions.  For example, consider a fairway that is partially shaded by surrounding trees.  The shaded environment will select for those annual bluegrass plants that are best able to tolerate the shade.  Obviously, the desirable fairway turfgrass that was established is limited as to what was planted, but each year the annual bluegrass produces seeds from the plants which have best survived the shaded conditions.  The seed produced should logically contain some seed that are even better adapted to those conditions than were the parent plants.  Thus over time, the annual bluegrass will produce plants that are better adapted to the shaded conditions than the annual bluegrass that was present there five years earlier.  This same scenario can be played out with other environmental conditions across the golf course or athletic field.

             Annual bluegrass has another morphological characteristic that contributes to its success as a weed.  The fact that annual bluegrass is a very shallow rooted plant has an affect on its biology and survival. Many of the problems associated with the management of annual bluegrass can be better understood by considering the effects of the shallow root system that it possesses.  Annual bluegrass has always been considered to have poor heat tolerance due to its shallow root system.  Research has been conducted that show some truth to this assertion.  The shallow root system has a large impact on the heat stress experienced by these plants.  The only way that plants can cool themselves is through transpiration (water loss via the leaves).  With a shallow root system, even a turfgrass plant that was well watered the night before may transpire all of the available water during the day and undergo heat and drought stress that same day.  This is why golf course superintendents are forced to “syringe” greens during the heat of the day during the summer months.  This type of irrigation management only favors the survival of annual bluegrass as well.  Annual bluegrass has problems surviving when irrigation is withheld and the soil allowed to become dry.  In addition, annual bluegrass does not respond well to alternating wet and dry cycles. 

             Surface disturbance also favors annual bluegrass.  Surface disturbances in the form of either low mowing heights, vertical mowing, coring, ball marks, cleat divots, spike marks, and general wear also will facilitate the initial encroachment of annual bluegrass into established turfgrass.  Continued surface disturbance is important for the survival of annual bluegrass and constant renewal of plants from the seed bank.

             Soil fertility and soil pH are also important in the survival of annual bluegrass.  Like most turfgrasses, annual bluegrass favors a soil pH between 5.5 and 7.0 and responds well to a balanced fertility program.  Research has shown that annual bluegrass population increased as soil phosphorus levels increased from intermediate levels to very high levels.  The same response was observed with increasing levels of soil potassium.  Nitrogen is important to the establishment and survival of annual bluegrass.  Ammonium based fertilizers, such as ammonium sulfate tend to stress the annual bluegrass and may predispose it to diseases such as anthracnose.  The acidifying effect of these types of fertilizers may explain this phenomona since acid soil conditions reduce the solubility of important elements such as phosphorus.  Because ammonium sulfate also contains sulfur, it is especially deleterious to the survival of annual bluegrass.  Sulfur will begin to affect the health of annual bluegrass at levels above 1.5 pounds of S per 1,000 square feet per year.  Urea, nitrate, or natural organic nitrogen sources may be the preferred forms of nitrogen for encouraging annual bluegrass.  We need more research in this area to fully understand the effects of nitrogen source on annual bluegrass.

             So, how do we manage annual bluegrass in turf?  While trying to control annual bluegrass is a losing battle, we currently have the tools to “manage” this weed species at a level that will reduce its impact upon the turfgrass and make it a minor component of the turfgrass stand.  The first step in an annual bluegrass management program is to determine the amount of annual bluegrass infestation that is present in your turfgrass.  If your turfgrass stand is young, then perhaps annual bluegrass has not become very widespread and will be more easily managed.  But, if your turfgrass is mature and has large populations of annual bluegrass present, then its management is going to be much more difficult.

            In terms of control of annual bluegrass with chemical measures, there are two distinct approaches, either preemergence or postemergence herbicides can be utilized.  Preemergence herbicides have existed for quite a while and have been used; however, they alone have not been very effective.  This should not be surprising due to the perennial nature of many annual bluegrass plants found in highly maintained turfgrass.  As good as the current preemergence herbicides are, it is just difficult to achieve 100 percent weed control year in and year out with a preemergence herbicide program.

 Preemergence herbicides can provide an effective approach to annual bluegrass control if the turfgrass manager is willing to apply them regularly and in fact the annual bluegrass is the annual type.  Annual bluegrass germinates in the fall and throughout the winter and spring until early summer.  In some climates, it can germinate year around.  The difficulty in controlling annual bluegrass with a preemergence applied herbicide is finding the appropriate “window” of application.  Many times, the annual bluegrass has either already germinated or is of the perennial type.  In either of these instances, preemergence herbicides are going to be ineffective.  In addition, with the perennial type having no seed dormancy requirement and the ability to germinate at any time after the seed has ripened makes it even more difficult to apply these herbicides in a timely manner.  The optimum temperature regime for annual bluegrass germination has been shown to be an alternating day/night temperature of 85/65.  These types of temperatures can occur during several periods of the year in the southern US.  Therefore, a preemergence herbicide program for annual bluegrass requires at least three applications per year, with one in the fall, one in the winter, and one in the spring.  Another reason for these multiple applications is the fact that a single application with any of these preemergence herbicides will only provide 4 to 6 weeks control of annual bluegrass.  Rubigan (fenarimol) is a systemic sterol inhibitor fungicide that has also been used to control annual bluegrass as a preemergence treatment.  Rubigan can be applied to bermudagrass turf prior to overseeding at a maximum use rate of 8 ounces of product per 1,000 square feet.   However, if annual bluegrass has already germinated or established in the perennial type, Rubigan will not provide any postemergence control.  Other sterol inhibitor fungicides have shown similar preemergence herbicide activity on annual bluegrass.

 An effective annual bluegrass management program must include proper cultural practices as well as the effective use of preemergence and postemergence herbicides.  Until only recently there has not been an effective postemergence herbicide program for the control of annual bluegrass.  Some effective postemergence approaches to controlling annual bluegrass have emerged over the past ten years. 

 One approach is the use of plant growth regulators (PGR’s), which work by inhibiting gibberellic acid synthesis in plants.  These PGR’s regulate annual bluegrass more severely than creeping bentgrass, thus altering the competitive balance between these two species and allowing the bentgrass to compete with the annual bluegrass.  This approach has been adopted by several golf course superintendents because it is relatively easy.  The PGR’s do not kill the annual bluegrass, but merely discolor it.  Use of these PGR’s will be more effective and discoloration of the annual bluegrass less severe if these products are applied at low use rates and repeat applications are made.  Sometimes these PGR treatments can be too injurious depending on the temperature.  In these cases, it is sometimes necessary to apply antidotal treatments of either soluble nitrogen and/or gibberillic acid to help mitigate discoloration and injury.  The use of PGR’s also requires the adoption of several other important management practices, including clipping removal and gradual reduction in the irrigation amount.  Another important factor is nitrogen fertilization since creeping bentgrass can get by with less nitrogen than annual bluegrass. 

 A second approach to postemergence control of annual bluegrass is the use of selective herbicides.  Prograss (ethofumesate) is a selective postemergence herbicide that has been used safely for control of annual bluegrass.  Other herbicides have been tried and tested in the past without consistent results. The biggest problem with postemergence herbicides is safety to the desired turfgrass.  This too is a concern in certain situations with Prograss.  The use of Prograss on perennial ryegrass (overseeded bermudagrass) has worked quite well with timely applications.  The difficulty in using Prograss for postemergence control of annual bluegrass comes from two sources.  First, Prograss must be used under the right conditions.  It is recommended that Prograss not be used unless there are at least 75% of the desired turfgrass present in the turfgrass stand.  The second difficulty in using Prograss is that it has an unusual mode of action that requires multiple applications in the fall of the year with results appearing the following spring.  It is speculated that the reason for this is related to temperature and possibly growth stage of annual bluegrass during the fall of the year.

Obviously, there is no silver bullet for the control of annual bluegrass and complete eradication of this weed is impossible.  However, we now have more tools than ever to combat this weed and our challenge as turfgrass managers is to integrate these control strategies with other management practices to reduce the annual bluegrass populations over time.  Management practices that favor annual bluegrass should be avoided and those that favor the desired turfgrass should be implemented.  We as turfgrass researchers are also challenged by the perennial type of annual bluegrass to come up with more information and better control strategies.  We are currently conducting some very good research on perennial annual bluegrass types at Auburn University to better understand its biology and control.  As a matter of fact, if any of you have had a problem historically controlling annual bluegrass, it may be of the perennial type.  If so, give me a call and maybe we can collect some of the annual bluegrass and maybe even put out a research trial to determine better how to control it.  The next five years shall determine if we as researchers have turned the corner on annual bluegrass control or whether it has turned the corner on us!