Annual bluegrass (Poa annua L.) is a weed found throughout the United States on closely mowed areas, such as golf course greens and fairways. It is also becoming more and more prevalent on higher mowed turf, such as homelawns and athletic fields. Annual bluegrass has a long list of poor agronomic qualities, and there are few control options, which leave turf managers in a difficult situation.
Annual bluegrass, commonly referred to as Poa, is a winter annual, cool-season grass. The leaf tips are boat-shaped like the bow of a boat. Poa has a prominent membranous ligule and a shallow, fibrous root system. The plant oftentimes is lighter green than perennial ryegrass, Kentucky bluegrass and creeping bentgrass.
As a whole, annual bluegrass is a very diverse species consisting of different types. These include an annual type and a perennial type, and each has its own broad range of characteristics. The annual type behaves like a true annual in that it germinates in the fall, grows throughout the fall and winter, produces seed in the spring, and then dies in the heat of summer. It has a bunch-type, upright growth habit and produces more seed than the perennial type. The perennial type is a weak perennial and generally has a more prostrate, stoloniferous growth habit than the annual type.
An annual bluegrass plant can produce as many as 300 seeds that can lay dormant in the soil for years before germinating. Annual bluegrass seed germination and growth favors cool, wet conditions, which encourage establishment throughout both the fall and spring. Poa can grow well under moist conditions in full sun and partly shaded areas. It is particularly competitive against creeping bentgrass under shady conditions or against warm-season grasses in late fall through early spring when temperatures are cool. It can easily outcompete Kentucky bluegrass and perennial ryegrass in athletic fields during the spring and fall.
POOR AGRONOMIC CHARACTERISTICS
Annual bluegrass is lighter green in color and disrupts a dark green, uniform turf stand (see photo, page 32). Additionally, Poa produces seedheads in the spring at almost any mowing height (see photo, page 34). Both the light green color and the copious seedhead production lower the aesthetic quality of the turf.
Poa frustrates many turf managers because it lacks the ability to tolerate temperature extremes, particularly high temperatures. Poa tends to thin and die out during the heat and drought of summer. Conversely, it cannot tolerate prolonged cold winters and is susceptible to winter freezing and extended periods under ice cover.
In order to establish a healthy annual bluegrass stand, it is necessary for you to develop an intensive cultural management program. Your program should include increasing fertilizer rates, which will boost the quality of annual bluegrass turf. Poa also needs frequent irrigation to survive because of its shallow root system. It oftentimes needs fungicide applications as well due to its susceptibility to diseases such as anthracnose, brown patch, dollar spot, Pythium blight, snow mold and summer patch. The combination of these negative agronomic characteristics may prompt you to attempt to control annual bluegrass instead.
Your first control method should be to limit the chance of new establishment of annual bluegrass. If you are establishing a turf site, use quality, clean seed. If the turf site already has a few small patches of annual bluegrass, cut out small infestations and reseed open spots. During the summer, if you have small patches of annual bluegrass that die, take the opportunity to overseed with the desired turf species in early August and irrigate to encourage establishment because Poa will not germinate until the soil cools in the early fall.
To control annual bluegrass, practice management techniques that encourage the growth of the desired species. For example, increase the mowing height. The competitive ability of annual bluegrass increases as the mowing height decreases. It is amazing that annual bluegrass can tolerate mowing at ⅛ inch and still produce seed; few plants can survive under those conditions. Therefore, by increasing the mowing height, you will help the desired species compete. In addition, removing grass clippings during spring seedhead formation will reduce the number of seeds added to the soil.
Over-irrigated turf is an ideal place for Poa germination and survival. Annuals germinate from seed on the surface of the soil. Frequent irrigation keeps the soil surface wet and increases the chance of germination and establishment of annual bluegrass. Once germinated, Poa needs frequent irrigation due to its shallow root system and drought susceptibility. Take advantage of this by keeping irrigation to a minimum or by watering deeply and infrequently. Try to withhold irrigating until the desired species show the first signs of water stress. Infrequent irrigation will allow the surface to dry and reduce the possibility of successful annual bluegrass germination and establishment. The desired turf is better able to survive periodic drying due to its deep roots, while an annual bluegrass plant recently germinated from seed will not survive. If possible, allow a cool-season homelawn to go dormant during the summer, which will kill Poa while the desired turf will stay dormant until favorable conditions return.
Annual bluegrass thrives when you increase fertilizer rates, especially with fertilizers containing nitrogen and phosphorus. Applying fertilizers during the year that are high in phosphorous will encourage annual bluegrass establishment. Unless a soil test recommends otherwise, cut back on applying high phosphorus fertilizers.
Annual bluegrass is highly competitive in compacted soils. Highly trafficked areas such as sports fields and greens oftentimes will have a significant annual bluegrass population. Ideally, reroute traffic to allow recovery of the desired species. Because that is not always possible, you can also use core aerification to relieve compaction, giving other turf species a better opportunity to compete.
Overall, practice management techniques that favor the desired turf species. This allows the turf to be more competitive against annual bluegrass and slows contamination of the turf site with new Poa. Remember, a dense, actively growing turf is a good defense against annual bluegrass establishment. However, even with properly practiced cultural control methods, you may also need to use a herbicide to achieve the desired level of control.
Preemergence herbicides work by inhibiting the growth of susceptible plants that are germinating from seed. Once you apply the chemical activate it by irrigating in order to create a chemical barrier or zone in the soil. Annual bluegrass seedlings germinating in this zone absorb the preemergence herbicide, killing the plant. Table 1, page xx, lists some commonly used herbicides labeled for Poa control. For a more complete list, see “Chemical Update: Turfgrass herbicides,” Grounds Maintenance, January 2003.
|Product||Herbicide||Pre-emergence control||Post-emergence control||Reseeding should be delayed after herbicide application (in weeks)|
|Team Pro||Benefin+Trifluralin||Y||N||8 to 16|
|Dimension||Dithiopyr||Y||N||12 to 16|
|Prograss||Ethofumesate||Y||Y||0 to 6|
|Barricade||Prodiamine||Y||N||16 to 48|
|Y = yes N = no|
As with all pesticide use, application timing is important. To control winter annual grassy weeds such as Poa, you should apply the pre-emergence herbicide in late summer to early fall. They are applied at this time because annual bluegrass seed germinates when soil temperatures are consistently below 70 F. You should apply a preemergence herbicide prior to the soil reaching this temperature consistently. That said, it may also be necessary for you to apply a second preemergence herbicide in the spring to control any spring-germinating annual bluegrass. With this type of program, you'll see gradual reduction in the annual type of Poa over many years.
It is also necessary to keep in mind the makeup and size of the annual bluegrass population you are trying to control. Preemergence herbicides will control only annual bluegrass geminating from seed and not mature plants. Thus, these herbicides will control the annual type of Poa, but not the perennial type. If the annual bluegrass patch is small enough that the desired turf could fill in without reseeding, simply apply a preemergence herbicide to prevent the Poa from germinating. If the annual bluegrass patches are large, controlling a large population of the annual type will result in a large thin or bare area.
Keep in mind that preemergence herbicides affect seed of the desired species as well, so you should delay between application of a preemergence herbicide and seeding with the desired species. Check the product label for seeding delays, which can be significant.
PREEMERGENCE PLUSPOSTEMERGENCE CONTROL
Two herbicides labeled for selective control of annual bluegrass are Kerb (pronamide) and Prograss (ethofumesate). Kerb is a restricted-use preemergence and early postemergence herbicide that is fairly slow-acting. It is used in the southern United States on bermudagrass, zoysiagrass, St. Augustinegrass and centipedegrass. Prograss is a preemergence and postemergence herbicide. It is mainly used in the northern United States on perennial ryegrass, Kentucky bluegrass, creeping bentgrass, tall fescue and, to a lesser extent, in the southern United States on St. Augustinegrass and dormant bermudagrass. Two to three applications of Prograss applied between September and December are recommended per year with applications spaced three to four weeks apart. You may not be able to see results in the fall, but you should be able to see them the following spring.
Prograss' specific mode of action is unknown, but we do know Prograss causes a reduction in leaf waxes, the protective layer against water loss. This reduction may make Poa more susceptible to desiccation during the winter. There are reports that Prograss also may cause slight growth inhibition in Kentucky bluegrass, a darker green color on treated turf and a reduction in severity of dollar spot.
Our research has shown that Prograss is taken up through the foliage (see Figure 1, above). Thus, when used postemergently, you should apply Prograss in a carrier volume that gives good foliar contact without excess runoff. Likewise, you shouldn't water-in Prograss applied postemergently immediately following application because it is not root-absorbed. The rate of Prograss absorption into annual bluegrass increases rapidly during the first three days after application but then levels off. This suggests that you should delay mowing or irrigation for 72 hours after application (if possible) to allow for maximum absorption, which may improve Poa control. Furthermore, our research shows that Prograss does not move out of the treated leaf. Therefore, good coverage of the entire plant is necessary to maximize control.
Annual bluegrass has numerous negative qualities that make management difficult. Unfortunately, there is no single technique for successful control. However, combining cultural and chemical control methods may help slowly reduce the Poa population, allowing for a higher-quality turf stand.
Eric Kohler, Ph.D., is a post-doc in turf science at Purdue University (West Lafayette, Ind.).
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