There are tough weeds, and then there are ultra-tough weeds. Annual bluegrass (Poa annua), otherwise simply known as "Poa," is in the upper five in terms of persistence, resiliency and objectionable characteristics in most parts of the U.S. In case you needed any convincing, let's briefly go over why it's so deserving of its ranking.
First, persistence. Of the thousands and thousands of weed seeds in the soil under golf course turf more than 10 years old, a generous percentage is annual bluegrass. As a result, when good conditions exist, it germinates and competes with desirable turf species, such as creeping bentgrass or Kentucky bluegrass. In addition to coming back year after year, Poa annua is a prolific seed producer, a characteristic that serves to replenish the supply.
Next, resiliency. A resilient grass is one that has the capacity to thrive under tough growing conditions for other grass species. In this case, Poa tolerates low mowing heights and is competitive under wet, compacted, heavily trafficked conditions.
The last consideration is what characterizes a grassy or broadleaf plant as a weed - the objectionable features it possesses. Poa annua is lighter in color than other desirable grasses. As a winter annual in most areas, it yellows, fades and often dies in the heat of summer, interfering with the purpose and function of the turf surface. That said, Poa annua is tolerated and/or encouraged in many situations, and if that's the case for you, consider moving on to the next article in this issue.
The more researchers work with annual bluegrass, the more they learn about the complex variation within the species, with millions of naturally occurring biotypes within the two known species of annual bluegrass (Poa annua annua and Poa annua repens).
The herbicidal effects on Poa annua in a creeping bentgrass fairway are obvious in these faraway and up-close photographs.
ALL PHOTOS: BY JOHN C. FECH, UNL
The problem with the biotypes is that variability exists amongst them, far more so than the differences in characteristics between various cultivars of Kentucky bluegrass; differences in terms of winter or summer tolerance, spring green-up, disease resistance, texture and color are documented. With this in mind, manifestations of Poa annua responding differently to management practices or herbicide applications are consistent with the variability within the overall gene pool.
Unfortunately, the gene pool is not constant, as it is a naturally occurring entity rather than a manipulated genetic phenomenon such as improved cultivars of desired turfgrass. The result is a constantly changing annual bluegrass population. As it turns out, this may be a force to be reckoned with.
Traditionally, control failures from applications of various products or the outcome of reduced control from the same active ingredients over time are normally explained as responses due to weather or adverse environmental conditions. While this remains plausible, natural changes in the biotypes of a given Poa annua population could also be responsible for less-than-effective responses.
Poa annua is obvious in low-cut recreational turf.
Reduced control from herbicides could be due to changing annual bluegrass populations. For example, during the spring a certain herbicide may be quite effective on the naturally occurring biotypes present at this time of year. However, when the same product is applied in the summer, it could be less effective due to the changes in biotypes and makeup of the overall Poa annua population.
Ongoing research at the University of Nebraska-Lincoln is investigating the effects of herbicides applied throughout the year on the population of annual bluegrass present in a three-state study. The research is extremely preliminary, but is using genetic techniques to determine how populations change due to geography, season and management - and how these potential changes affect control from herbicides or growth regulators. The results of this initial work are encouraging in terms of learning more about the factors responsible for the success or failure of various treatments.
Since annual bluegrass and the environments vary dramatically across the country, within states and even within the same golf course green, control recommendations are extremely site-specific and likely require experimenting on your own. Additionally, new strategies and products are being developed quickly to further complicate the issue. Any guidelines heard about or seen anywhere should be used cautiously until a level of comfort is experienced on your own site. Be sure to always read and follow label directions when using these control products. Following are a number of strategies that have been successful in the Midwest.
For Kentucky bluegrass fairways, three fall applications of Tenacity at the rate of 5.3 ounces per acre, made no more than two weeks apart, in addition to a traditional pre-emergence herbicide applied in late summer and late fall have been shown to be effective, albeit somewhat inconsistent. The inconsistency is exactly why Tenacity does not carry a label for use on annual bluegrass but can be used on Kentucky bluegrass for a wide range of other weeds.
During the middle of summer, Illinois research suggests multiple applications of Tenacity made as often as twice per week at 2 to 3 ounces per acre can be effective. Tenacity may occasionally damage certain cultivars of perennial ryegrass, so on-site testing is a must.
Amicarbazone (XONERATE) also shows promise for controlling annual bluegrass in Kentucky bluegrass/perennial ryegrass fairways, but researchers are still dialing in rates, intervals and timings.
Poa annua in a golf green.
For creeping bentgrass fairways, two or more early summer applications of Velocity at 4 to 6 ounces per acre applied three weeks apart can be effective, especially in more stressful summers. Additional fall applications as well as late summer pre-emergence herbicides should improve control.
For perennial ryegrass fairways, two early summer applications of Velocity at 4 to 6 ounces per acre applied three weeks apart should be effective, as well as many more combinations of rates and intervals as listed on the product label. In a similar regime with other turf, a fall pre-emergence herbicide application will be beneficial. As with bentgrass fairways, Velocity applications may be repeated in early fall.
PROGRASS can also be highly effective when applied in the fall, since higher rates can be used on perennial ryegrass than on other turf species.
Mefluidide (Embark) has been on the market for nearly 25 years for use on fairways. The plant growth regulator, which debuted in 1988, has a history of Poa annua seedhead suppression and is currently undergoing a resurgence.
When renovating fairways, seed as soon as possible in late summer to facilitate establishment of the desired species before the emergence of annual bluegrass. Reading the product label is key to the success of renovation in that specific instructions are outlined as to the application timing of products such as PROGRASS, Tenacity or Velocity after seedling emergence. The same is the case for the applications of pre-emergence herbicides to suppress germination of undesirable species.
Currently, there is no herbicide labeled to eradicate Poa annua on creeping bentgrass greens. But there are specific plant growth regulators - paclobutrazol (Trimmit) or flurprimidol (Cutless) - and superintendents have found success using them to control Poa annua. Applications of paclobutrazol or flurprimidol can be made throughout the summer according to product label directions.
Possible label changes to existing products and/or the new experimental methiozolin may expand control options in the future.
All products can be variable from season to season and within season, producing phytotoxic effects. Specific environmental conditions and characteristics of the desirable turfs being grown may lead to undesirable results from the approaches to Poa annua control outlined above. Testing on a small scale or in localized areas is recommended.
Evaluation of the implementation of appropriate cultural practices and effective combinations of plant growth regulators and herbicides is difficult work; we should all be thankful to diligent researchers across the country for their efforts. Many years of research will be needed to provide more thorough answers to the basic question of how best to control unwanted Poa annua.
John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln.