Poa annua grass, also known as annual bluegrass, is an annual grassy weed commonly found in lawns, but can be found in garden beds as well. It is among the top three most difficult weed grasses to control in southern lawns. Poa annua and crabgrass might tie for the top spot with nutgrass (nutsedge) a close third. That being said, with the introduction of new chemical products that are very effective at controlling crabgrass and nutgrass, Poa annua takes the top spot. Aside from the fact that there is really no chemical that is totally effective for controlling this highly invasive grassy weed, one Poa annua plant can produce several hundred seeds in one season, and the seeds can lay dormant for several years before sprouting.
The identifying characteristic of annual bluegrass is the small clump of fine, dark green blades and tasseled seed stalks that become visible in winter and early spring in the Southeast...later further north. Poa annua is very easy to identify in dormant warm season grasses, such as Bermuda, because its foliage will be dark green during the winter while the rest of the lawn is light beige. When not mowed, the seed stalks can become quite tall, maybe 6 or 8 inches in height, however the plant will quickly produce shorter seed stalks when mowed, and this is what makes it so difficult to control.
In fescue and other cool season lawns, which stay green during winter, annual bluegrass is a problem because when it dies back in hot weather it leaves unsightly brown spots in the lawn during spring and summer.
Methods for controlling Poa annua in lawns
There are several methods for controlling Poa annua however prevention is the most effective. There is only one truly effective way to kill this pesky grassy weed. When it comes to prevention, there are two basic methods.
KILLING Poa Annua In The Lawn
So you've tried every lawn weed killer on the market, including Image, with less than effective results. Maybe the weed discolors a little but it comes back with a vengeance the next year. There is only one sure-fire method for killing Poa annua and this involves spraying it with a glyphosate product such as Killzall Super Concentrate or Roundup. I don't recommend Roundup because there's just too many strengths of the stuff on the market (designed to sell you less product for more money) and even the Roundup Super Concentrate is about two times the price of Killzall. Glyphosate-based products will kill Poa annua, and just about any other type of plant, and herein lies the problem. Glyphosate is very effective for use to kill Poa annua in landscape beds, where it can be carefully sprayed, avoiding over-spray on the foliage of shrubs, trees and other plants. But, when it comes to the lawn, not only will the glypohosate kill the Poa annua, it's likely to kill your grass. By no means would I recommend spraying glysphosate in a fescue or other cool season lawn, and I would only recommend very careful spot spraying in totally dormant warm season lawns, such as Bermuda; a lawn grass which has the most difficult time competing with Poa annua. And, besides, if your lawn is infested with Poa annua, it could take days to spot spray all the clumps. Too, to avoid battling next years crop, you'd have to kill all the Poa annua before it goes to seed.
PREVENTING Poa Annua In The Lawn
Prevention is by far the most effective means of controlling Poa Annua, and there are two basic methods for doing so. It's best to use both methods simultaneously. Even then, this isn't a battle that can be won overnight. Even when on a strict, specifically timed weed prevention program, and depending on the type of lawn grass, it could take up to two years to eliminate it entirely from your lawn.
Dense Turf
A healthy, dense turf is the best weed preventer. When Poa annua and other weeds are out of control in your lawn this could mean that your lawn grass is unhealthy or weak. If your lawn grass is unhealthy or weak, this is most likely due to problems with the soil or the amount of sun exposure. If the pH of the soil is too acid or too alkaline, or there is a nutrient deficiency, or the soil is too compact, or if the soil remains constantly wet or soggy, these conditions can weaken lawn grass, leaving it susceptible to invasion from weeds or damage from disease.
If a healthy, weed-free lawn is what you're after, I would first recommend testing your soil to identify any nutrient deficiencies or pH problems and taking steps to remedy them. Soil test kits are available at your local nursery and garden center or you can have your soil tested through your local Extension service. If soil moisture or compact soil is a problem your local nursery and garden center professional or reputable landscape contractor can provide information about and/or services to improve soil drainage. If lack of or too much sunlight is a problem this will need to be adjusted as well. Limbing up trees can help to provide more sunlight. If sun exposure cannot be changed you might wat to consider growing groundcover plants where grass will not grow.
Chemical Control
Okay, so you have a healthy or at least somewhat healthy lawn and Poa annua is still everywhere...this leaves only one alternative: prevention. Successfully preventing Poa annua over the long term is entirely possible. In order to keep their customers happy, golf courses do so through a a specifically timed weed prevention program. The trick is all in the timing and knowing about the life cycle of Poa annua.
In the South, Poa annua seeds are produced in the early winter and spring but germinate in the fall or late winter. So, the timing of applications of weed preventers is very important.
A weed preventer, also know as "pre-emergent," is an herbicide that will prevent the Poa annua seeds from germinating. There are many types of weed preventers on the market that are effective in controlling Poa annua, and many other types of grassy and broadleaf weeds. Consult with your local nursery and garden center professional as to what weed preventers they carry. For effective Poa annua prevention, apply a weed preventer in the early fall (Sept 1 to October 15) to prevent fall germination. Then another application in late winter and/or early spring to prevent spring germination This will keep the poa annua seeds from sprouting. In the do-it-yourself lawn programs our nursery and garden center provides for its customers, we suggest a December and February application of weed preventer. Our lawn care programs also include weed preventer and fertilizer combination products for spring and fall applications.
Other Prevention Tips
- Poa annua seeds can be spread from yard to yard or from one part of the yard to another. When Poa annua seeds are visible on plants make sure to wash seeds out from the underside of mower decks when moving from an infested part of the yard to a clean part. Also, if you have a lawn maintenance service mowing your grass, insist that they clean under their mower decks before mowing your lawn.
- Keep in mind that Poa annua seeds are tough and can survive many seasons in the soil without germinating. Eradication of Poa annua takes time: upwards of two to three seasons. Even then, it;s a good idea to remain on a weed prevention program to prevent germination of weed seeds blown in by the wind or carried and dropped by birds.
