Cool-Season Lawns & Winter Weeds

WHAT IS POA ANNUA?
Poa annua, a.k.a., annual bluegrass, is a winter annual grassy weed that germinates in
early/mid-fall and dies out in early spring. Like any green plant, soil temperatures and
available soil moisture truly drive its germination and eventual death.

Soil temperatures at or below 67°F, 0.5” of available soil moisture, and shorter, cool
days, help this weed explode with growth. Poa annua has a high light requirement to
germinate. This means when you have a thin stand of grass, the sunlight can pass through
to the soil and the exposed annual bluegrass seed can germinate.

WHY IS POA SO DIFFICULT TO CONTROL?
Specifically in cool-season lawns, like Tall fescue, Poa annua is weed enemy
number one. As previously mentioned, Poa annua is a winter annual grassy weed that
germinates in the fall. Unfortunately, or fortunately, we’re also aerating and seeding cool-season
lawns in the fall, as we need to reestablish any plants that may have been lost
over the summer months, so unfortunately, we cannot apply a Preemergent to control this
pesky weed. Lastly, this weed is a prolific seedhead producer, especially when it’s mowed
short.

HOW CAN THIS WEED, AND OTHERS, BE CONTROLLED?
The best way to decrease any weed pressure is to increase the density of your
desirable lawn grass. Increasing density is quickly achieved by mowing your lawn at
the correct height and cadence. We recommend the tallest setting your mower is
capable of, for all cool-season grass types. This is especially important as we head into
the cooler, winter months, for many reasons.

Lastly, when seedheads do occur, we advise mowing any affected sections last and
collecting the clippings. This will prevent the further spread of the weed species.
Also, be sure to clean the mower so you do not introduce weed seeds on your next mow.

HOW DO WEED SEEDS GET ON MY LAWN?
Weed seeds are small and can travel by wind and water. They also travel during mowing
sessions and by animals. Also, weeds are always naturally occurring in a lawn because
lawns are grown as a monoculture. In Mother Nature, this system does not exist and that is
why chemistries like Preemergent herbicides are utilized. These chemistries help suppress
the growth of weed seeds by creating a prohibitive growth layer in the soil, which keeps
your lawn as clean as possible throughout the season.

Like all chemistries, these have a soil life, and their protective layer eventually breaks
down and weed seeds are free to grow. Lastly, these chemistries do not know the
difference between a desirable seed and a weed seed; it will inhibit all seed growth