Poa annua, also called annual bluegrass, can be a very annoying and persistent weed in home lawns, golf courses, and even crops. It germinates from seed in the fall, usually in late September with sufficient rainfall.
There are 4 types of Poa species:
Kentucky bluegrass (poa pretensis,) a desirable cool-season turf grown in more northern climates that are cool and moist, such as coastal areas or intermountain areas.
Rough bluegrass (poa trivialis) is a less desirable turf/weed species that does well in cool moist areas, but isn’t very heat or drought tolerant.
Bulbose bluegrass (poa bulbosus) is usually only found in northern California turfgrass.
Annual blue grass (poa annua) is the type we are concerned with. Unlike the other species, annual bluegrass can survive close mowing and still reseed. Unfortunately, it seeds as early as 6 weeks after germination, although most seeding occurs in the spring. Poa annua seeds rapidly and profusely, making it the pest that it is!
We control poa annua with a pre-emergent in our fall weed control application. If you start a lawn care program in the spring, you will have missed the fall pre-emergent and may have a problem with poa annua weeds. Luckily, it will burn up once the temperatures warm up, but it can be quite an unsightly nuisance while it’s there. We can spot treat it while it’s still cool – unless it’s in fescue which is a cool season grass just like poa annua. Anything we use to harm the poa annua, will also harm your fescue, so there’s no alternative except to tolerate it until the warmer weather. Your fall application will suppress it for next year.
How you can help control it: Given the prolific nature of poa annua, it’s best to take precautions. After mowing an area infested with it, rinse off your lawn mower, both the deck and underneath where the blades are, before resuming mowing so you don’t carry poa annua seeds to a clean area. Good watering and mowing practices will insure that your turf is thick and healthy and will also slow the spread of poa annua. Thick turf shades the soil, making it more difficult for poa annua seeds to germinate.
Poa annua trivia: Pebble Beach Golf Links, located on the Monterey Peninsula in California is one of the most highly-regarded golf courses in the world, and its putting greens are poa annua. Some people, Tiger Woods included, don’t like poa annua on greens, because different strains of the grass grow at different rates during the sunny part of the day. So a smooth green in the morning, may be a little less smooth or downright bumpy in the afternoon.