Getting Rid of Annual Bluegrass or Poa

Q. So how do we eradicate poa Anna?

Annual bluegrass in a hybrid bermudagrass home lawn. Notice the discoloration by the Poa when the bermudagrass is actively growing.

A. Poa annua or annual bluegrass is easy to control in landscapes but extremely difficult to control in most lawns. If this is in a lawn such as a cool season grass like tall fescue, Kentucky bluegrass or perennial ryegrass it is very difficult to control. This is the worst situation. If it is growing in 100% Bermuda grass it is much easier to control. If it is growing in a landscape, around trees and shrubs, it is easy to control with mulch.

 

Annual bluegrass persisting in hybrid bermudagrass on a golf course during the winter months. Annual bluegrass stays green during the winter in colder climates while the bermudagrass becomes dormant, dies, or turns brown.

Annual bluegrass has evolved. 

This grass is very different from what it used to be 50 years ago in many locations. In the old weed control manuals 30, 40 and 50 years ago they all said the same thing. It is a winter annual, the seed germinating toward the end of summer, flowering or producing seed heads during the fall and winter months and the seed laying dormant or sleeping through the summer. It repeats this cycle over and over. Annual bluegrass is a very poor competitor with mulch but it is an excellent competitor when it grows among other grasses. When I went to school, this is what I learned annual bluegrass was.

 

Annual bluegrass seedhead.

Annual bluegrass has evolved in certain landscapes and climates where it now persists as a perennial in warm climates and will produce seed at different times of the fall and winter months. This is very apparent on many golf courses and a nightmare for golf course superintendents. This grass can be mowed very short, 1/4 inch or less, so mowing it out will not happen. It loves environments where the grass is mowed short and kept wet. It is also very strong in soil environments that are compacted without much airspace between soil particles. I would compare it, from an evolutionary standpoint, to the cockroach.

Controlling it in cool season lawns.

When it grows in cool season lawns, the usual methods of control are pre-emergent weed killers or herbicides. If this is a home lawn, you will have an advantage because you can apply these pre-emergent herbicides several times during the year and that's what you will probably have to do, starting in late summer and through next spring. 

Look at the pre-emergent herbicides available to you and select one that can be applied to a lawn and says that it controls annual bluegrass. Make the first application of pre-emergent herbicide in the middle of late summer. In the hot, southern Nevada climate this would be about mid July or early August. Read the label and it will tell you when to make a second application and how much to apply.

In the past, Scotts has made excellent products for weed control in lawns.

Pre-emergent herbicides slowly degrade over time. You want to apply this herbicide a short time after its peak control period. It will tell you on the label how many weeks to wait after this first application before the second one is made. There is some residual from the first application so you will make the second application at some rate less than the first application. I repeat, the label will tell you how much to apply and how long to wait before applying the second application. Continue this cycle of applying, waiting and repeating all winter long (if the label allows it) and into early spring. That should kill most of the seed. If you see some young plants that were missed by the herbicide application (they are usually lighter green and grow faster than the surrounding grass) then pull them by hand. They will pull out easily because they have very shallow roots.

Annual bluegrass biotype producing short rhizomes,
adapting like a cockroach to a changing environment.

If this is a bermudagrass lawn, kill the annual bluegrass during the winter months after the bermudagrass has turned totally brown (dormant). Do not overseed this bermudagrass lawn with a winter lawn this year, and possibly the next year, until you get control of the bluegrass. By the way, this is also an excellent way to control other weeds including tall fescue growing in a bermudagrass lawn. After the bermudagrass lawn has turned all brown, and you have mowed it, spray the lawn and the annual bluegrass in it with Roundup herbicide. The Roundup will kill the annual bluegrass because it stays green during the winter months (cool season grass) but the bermudagrass is dead above ground. The Roundup will have little to no effect on the dormant bermudagrass.

This is a core aerator that punches holes in the lawn.

Management methods that help to suppress annual bluegrass are aeration (punching holes in the lawn with a core aerator) and waiting longer between irrigations during the late summer, fall and winter
months. Aeration helps open the soil which annual bluegrass does not like. As I said before, annual bluegrass likes it wet because it has a very shallow root system. Waiting longer between irrigations helps keep the soil dry and minimizes its invasion.

Preventing annual bluegrass from entering your landscape and lawn should be done in the future. The seed is transported to the landscape on shoes that walk across the seed heads during the fall and winter months. If a landscape has annual bluegrass and people walk across the seed heads and then walk into your landscape, they will transport the seed and that's where it starts. It is also transported to lawns in the same way as well as the lawnmower. If a lawnmower was used on a lawn that had annual bluegrass in it and it was seeding, I guarantee
100% that this seed will be brought into your lawn if it's the next lawn that is mowed. Mowers should be cleaned thoroughly between lawns to slow the spread of weed seeds such as annual bluegrass and diseases.