Poa annua, 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 lawns. Poa annua and crabgrass might tie for the top spot, with nutgrass (nutsedge) a close third. 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 Poa annua is the small clump of fine, dark green blades and tasseled seed stalks that become visible in winter and early spring in dormant lawns, such as bermudagrass, which turn straw color in the winter. In fescue and other cool season lawn grasses, 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.
When not mowed, the seed stalks of Poa annua 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.
Methods of Control
Unfortunately, there is no chemical that is totally effective for killing this highly invasive grassy weed in lawns. But you can take steps to prevent it.
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, here's some things to consider:
- I would first recommend testing your soil to identify any nutrient deficiencies or pH problems and taking steps to remedy them. We offer soil test kits or you can have your soil tested through your local Extension service.
- If soil moisture, compact soil or drainage is a problem take steps to correct these issues perhaps consulting and working with a reputable landscape contractor.
- 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 want to consider growing groundcover plants where grass will not grow.
Chemical Control
When cultural methods don't work to eradicate all or most of the Poa annua in your lawn chemical control can be effective. 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. That being said, 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.
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.
There are many types of weed preventers on the market that are effective in controlling Poa annua, and which will also prevent many other types of grassy and broadleaf weeds. I use Lebanon Team 2G Weed Preventer or combination lawn fertilizer + weed preventer products. For effective Poa annua prevention, apply a weed preventer in the early fall (Sept-October depending on your location) to prevent fall germination. Then another application in late winter and/or early spring (January-Feb) to prevent spring germination This will keep the Poa annua seeds from sprouting.
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.
- Before Poa annua plants produce seeds, spray the ones growing in non-lawn areas with a glyphosate-based weed killer such as Hi-Yield Killzall. This should help to reduce the Poa annua population on your property.