Hi, I am Millie Davenport, a horticulture Extension agent with the Clemson University Home & Garden Information Center.
Today we are to talk about annual bluegrass control in the lawn.
Each spring at the Home & Garden Information Center we get several calls about annual bluegrass control in the lawn. And whether you call it the common name annual bluegrass, Poa, Poa anna, or by its scientific name Poa annua, it is definitely a difficult weed to control. It is a good thing for homeowners to realize that they are not going to easily control this weed in the landscape. It is more of a fact that you really want to manage the population down to a tolerable level than actually eradicate it from the lawn.
The first thing with any pest control, whether it be a weed, an insect or disease problem that you are trying to deal with, identification is definitely the first key to making sure you are going to properly handle the situation.
First, it is a cool-season annual. So, it is going to start its lifecycle in the fall with seed germination at that time and then it is going to end its lifecycle in the spring. It has a clumping habit, unlike a lot of your other grasses which tend to spread. This one you are going to find growing in little clumps across your lawn. The leaves are nice and smooth on both sides and it is a light green color that helps you distinguish it from other grasses in the area. And the tips of those leaves have somewhat of a boat shape. The flowers themselves are a nice greenish to whitish color, and there are lots of them. That is going to make these weeds really pop out in the landscape.
Usually you are going to see this weed preferring areas with compacted soil, wet soil, or lawns with areas of high nitrogen applications. One, if you are over irrigating, you are creating a wet soil that is going to be more favorable for this weed versus your turfgrass. And if you are mowing too low, then you are making it a situation where those weed seeds will get enough light to germinate.
Now, the next step is to actually prevent the weeds from invading the landscape. And that is going to be monitoring the mulched beds around your trees and shrubs and with any plants you bring into the landscape.
Now, when you have done all that you can to prevent weeds from entering the landscape as well as properly maintaining your turfgrass, then the last step would be using herbicides when necessary. The problem with using herbicides at this time of the year, which is spring, is that these weeds are at the end of their lifecycle. Annual bluegrass, as you remember, is a cool-season annual, so it is going to start its lifecycle in the fall and end its lifecycle in the spring/early summer months as it gets warm. So, since it is going to die anyway, it is really a waste of your money to go out putting any type of products on this weed to kill it at this point. So, what I would do instead is to mark your calendar for the fall to use a pre-emergence herbicide. There are several pre-emergence herbicides available on the market. One that has done very well in trials is one that contains the active ingredients benefin + trifluralin. Now with that particular pre-emergence or any of them that you choose to use, you want to apply that first application in the fall when your temperatures get to about 75 degrees for a high, for several days. So, you want to do that first application then and then a second application 10 to 12 weeks later.
You should be aware that pre-emergence herbicides don’t distinguish between turfgrass seed and weed seed. So, if you are overseeding your lawn do be aware that this product could and will prevent those seeds from germinating.
One last option you have for controlling and killing the annual bluegrass in your landscape other than just hand pulling is using a generic form of the original Roundup, which is glyphosate. There are several generic forms of that product out there. A couple of brands are Quick Kill and Eraser. Now this is a non-selective herbicide, which means it is going to kill anything that it gets on. So, you have got to be really careful about where you use it.
A maintenance calendar is available on the Home & Garden Information Center web site, www.clemson.edu/hgic, for each turf species to help you follow the correct cultural steps for your specific turfgrass. Using that calendar, as well preventing the weeds from entering your landscape, as well as using herbicides properly are going to help you reduce the annual bluegrass population in your lawn.