Managing Poa annua greens
I once overheard a turf professional say to another, "Our bentgrass-management techniques must be good for Poa, because all we have is Poa." The implications of statements like this have led me and other researchers to examine the benefits of actually using Poa annua as a golf-green turfgrass. After all, many greens already consist of more Poa than bentgrass, and certain strains of this species have excellent properties for greens use. However, if you wish to manage your greens to favor Poa rather than bentgrass, how do you devise a management program? Where do you get your information? We actually know little about management techniques that enhance Poa annua's growth and performance.
One thing is certain: Poa annua is a very different species from bentgrass. Unfortunately, we still lack a uniform supply of a strain (or blend of strains) that I call "greens-type" Poa specifically adapted to golf greens. This would provide a standard for performing the needed trials and extensive experimentation to determine cultural needs of Poa. Until this happens, we must rely heavily on the experience of superintendents who manage Poa greens.
Defining Poa annua The Poa you find on today's golf courses is highly variable. It consists of many different strains, and 10, 20 or even 30 or more can grow on a single golf green. This is what gives the patchy appearance to old Poa greens. Greens-type Poas fall within the subspecies known as Poa annua ssp. reptans. They result from adaptation over a long period and typically occur on old golf greens built in the 1920s or earlier with a long history of close cutting. Wild and weedy types of Poa (Poa annua ssp. annua) occur on greens that are relatively new (5 to 10 years old) and on greens where management practices continuously obstruct the establishment of Poa. This type is truly a weed. Intermediate types (that I call "turf-type" Poas, which also are ssp. reptans) occur on greens of intermediate age (10 to 30 years old) and greensmowed at heights greater than 1/8 inch.
Greens-type Poas typically exhibit extremely high shoot density and fine leaf texture and will persist in sun or shade. Unfortunately, they often are susceptible to diseases. They do, however, show variation for disease resistance, color, seed-head production, fall color retention and many other traits, so the potential for breeding improved varieties exists. Recently, The Pennsylvania State University initiated a Poa breeding program aimed at developing commercial sources of greens-type Poa (see photo, bottom of page 14). The management recommendations I discuss here are a collection of observations based on our breeding program and from the many discussions I've had with superintendents who have greens-type Poa on their greens.
The evolution of Poa annua Certain golf courses I've visited in the Northeastern United States, unlike most other courses, have greens dominated by a single strain of greens-type Poa. These greens tend to be uniform in appearance and playability and produce few seed heads. When I ask about their management techniques, these superintendents often reply that they do very little, especially in terms of cultivation. They spray fungicides for disease, use light, frequent applications of liquid fertilizer and, if they aerate at all, they do so only once a season, generally during the fall.
Conversely, I have visited courses with a history of root-zone problems on their greens that required the superintendent to constantly core and spike throughout the growing season to enable drainage and root growth. These greens were uniformly covered with a seed-head-producing strain of Poa, the likes of which I have rarely seen. Literally, the greens were white with seed heads. Growth regulators, which superintendents often use on Poa greens to reduce seed heads, were ineffective in these cases for an unusual reason. Regulation of seed-head production requires critical application timing, before visible flower-culm elongation. However, this particular strain of Poa was developing its seed heads beneath snow cover. Thus, it was impossible to treat with PGRs during this critical time.
These interesting examples are exceptions. Most courses I've visited have greens containing a collection of Poa strains, with patchiness being the norm (see photo, below left). These observations suggest that the types of Poa that reside on a green are a direct reflection of the management that occurs there. For example, the more you cultivate a green, the more open ground is available for Poa seed to become established. Conversely, greens that receive little or no cultivation provide few opportunities for seedlings to become established. Therefore, Poas that invest more of their resources in vegetative growth, rather than in seed production, become the dominant strain. The patchiness of most Poa greens might, then, be a reflection of management programs that fluctuate, at times favoring vegetative Poas, at other times favoring seeded types. One way or another, however, Poa seems to have a quest to dominate the green's putting surface by seed or through vegetative growth.
With this background of Poa evolution in mind, let's discuss the primary and supplementary cultural practices I've found that encourage good-quality Poa on greens. Admittedly, it is difficult to isolate any one cultural practice and discuss it without connection or reference to others. Therefore, the discussion will contain some overlap among cultural practices.
Managing for Poa annua * Mowing. Mowing is, without a doubt, the strongest selective factor on a golf green, and Poa seems to become specifically adapted to the mowing height of the green on which it grows. Generally, greens-type Poas do not grow vertically much above the height of cut to which they are adapted. Moreover, we have not yet observed a limit to the closeness of mowing to which Poa can adapt. We have collected our highest-quality strains of Poa from the most closely mown greens. In fact, we discovered one strain that we mow at 1/8 inch, which we don't believe we've ever clipped--its leaf tips simply don't get that tall!
* Vertical mowing. Some greens-type Poas tend to become puffy, which requires vertical mowing to correct. Although this needs more study, we believe this puffiness is a result of extremely high shoot densities, which buckle the putting surface, rather than a buildup of thatch and organic matter. Among other factors, puffiness relates to the amount of nitrogen fertilization. Thus, the higher the fertility, the more likely it is that you'll need to perform vertical mowing.
* Fertility. When fertilizing Poa, we feel it is necessary to use a liquid fertilizer. Even small granular fertilizers routinely leave "freckles" on Poa that are not apparent on similarly treated bentgrass greens. This freckling likely relates to the relatively high shoot densities of Poa, but it may also indicate a higher nutritional requirement than for bentgrass. We need more research on the nutritional requirements of greens-type Poa to understand this.
* Disease management. Higher fertility levels may have benefits for Poa that are not apparent for bentgrass. On a recent trip to the Pacific Northwest, for example, I observed that superintendents who used lower rates of nitrogen (2 to 3 pounds of nitrogen per 1,000 square feet per year) had to keep alert for the threat of anthracnose disease. Anthracnose can be a devastating disease on Poa, attacking during times of poor, stressful growing conditions, usually during late summer. However, neighboring superintendents that provided higher levels of nitrogen (5 to 6 pounds) experienced little or no anthracnose. The price, in this case, of avoiding anthracnose was the frequent vertical mowing the higher fertility program required. Thus, providing higher fertility for Poa, though beneficial, created a tradeoff.
New fungicides are a great help to managers of Poa greens. Some of these materials have helped fight Poa's basal-rot anthracnose problems (azoxystrobin, Zeneca's Heritage, is the one I hear of most often for this use) to the extent that some Northeastern and Mid-Atlantic superintendents are reconsidering their Poa elimination programs in favor of maintaining their existing Poa.
Genetic-based resistance to disease is a great advantage for any turfgrass. Certainly, our breeding program focuses on screening for sources of natural resistance to disease.
* Irrigation. Irrigating Poa seems to work best with light, frequent applications. Unfortunately, many extremely close-cut greens must rely on extensive hand watering, which can be hard on the budget. This is because, in general, Poa is shallow-rooted. However, we've seen strains whose rooting potential rivals that of bentgrass, so this is another characteristic that breeders may be able to improve.
Often, our breeding work on Poa raised more questions than it answered. For example, how deep should a sand-based root zone be? Does bentgrass use all 11 inches of a standard sand-based green? With Poa generally being a shallower-rooted species, could we build a shallower root zone that will provide a perched water table closer to the surface? Until we can answer questions like this, superintendents simply will have to shift some of the mowing budget to hand watering if they wish to work with Poa.
* Overseeding. Until breeders develop suitable, commercially available strains, overseeding with greens-type Poa is not an option unless you collect and clean the seed yourself. I have seen superintendents overseed bentgrass into nearly pure Poa greens during late summer. At first, I thought this practice was wasteful because these superintendents' greens essentially remain pure Poa year after year. But I eventually learned they were using bentgrass as a nurse crop to hold the green over through stressful periods until the Poa made it to more favorable growing conditions. This technique may add some variability to the putting surface for short periods, but aggressive, close mowing removes the bentgrass by mid-fall.
One possible reason why close mowing is useful for removing bentgrass from Poa greens is that the vertical leaf-extension rate of greens-type Poa is much less than that of bentgrass. Thus, Poa may remain more competitive than bentgrass by retaining more of its photosynthetic tissue below the mowing height. However, I don't believe that it's just the tolerance to close mowing that makes Poa so competitive with bentgrass. I believe that greens-type Poa's high shoot density and relatively upright growth habit makes it more tolerant of wear than bentgrass. This contradicts the common observation that Poa is intolerant of wear. However, I commonly observe its early domination of newly established bentgrass greens in highly trafficked areas.
One of the most important management tools for Poa greens is a Poa nursery. Nothing looks worse on a Poa green than a repair job that uses sod from a bentgrass nursery. To create a Poa nursery, roll out and topdress aeration cores you've collected from your greens, two or three deep, and use a light overseeding of bentgrass to knit the sod together. Then maintain it as you would a normal green.
PGRs for seedhead reduction Poa's tendency to produce seed heads under even the closest mowing heights has enabled it to evolve its compact, high-quality greens types. However, seed heads detract greatly from putting-surface quality. One plant growth regulator (PGR), mefluidide (PBI/Gordon's Embark), suppresses seed-head formation on Poa greens. Application timing--about 2 weeks before seed-head appearance--is critical for proper seed-head control. Future varietal development of greens-type Poa annua will necessarily require some seed production for the purposes of commercial production. Thus, PGRs to control spring-time seed-head formation will likely be necessary for all such commercial varieties with the exception of vegetatively propagated types.
Maintaining Poa greens can be a rewarding challenge. A well-maintained Poa green is beautiful and an asset to the golf industry. Consider that most of the last 10 to 15 U.S. Open golf tournaments were played on Poa rather than bentgrass greens. Nevertheless, we still lack a uniform supply of Poa that possesses those traits that we believe are "ideal" (high shoot density, low to no seedhead production, deep rooting, disease and insect resistance, tolerance of extreme temperatures, etc.). When we finally have such a source, we can develop more firm management recommendations for this species. Until then, we must rely on the experience of superintendents who've dealt with Poa to maximize the quality of existing Poa in our greens.
Dr. David R. Huff is a turfgrass breeder at The Pennsylvania State University (University Park, Pa.).
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