Poa Annua – also known as Annual Bluegrass – is a winter annual grass that can spread very quickly. An infestation of Poa Annua is very noticeable as it begins to flower and push seeds. To say that this nuisance grasstype is less than desirable for lawn care nuts would be an understatement.
Controlling Poa Annua is not a one-and-done approach. The seeds of Annual Bluegrass can lay dormant in the soil for several years before emerging, so constant control and cultural practices are required for several years.
So that begs the questions, what is Poa Annua and how do I kill it and control it? This post should help you cool season grass owners.
What is Poa Annua (Annual Bluegrass)?
Poa Annua is a cool season annual bluegrass that germinates in the fall when soil temperatures begin to drop to 70 degrees and below. Poa Annua lives overwinter in a vegetative state and begins to grow rapidly the following the spring, generally through June. As the summer temperatures begin to climb, Poa Annua will flower and produce seeds. Annual Bluegrass will soon die off once they drop their seeds, and the cycle beings all over again at the end of summer/early fall.
Poa Annua can also grow in clumps. It has a more lime green color that makes it easier to identify in a darker green and well maintained lawn. This, coupled with the whitish flowering seed heads makes this grass really stick out.
Is it Poa Annua or Kentucky Bluegrass?
As previously mentioned, Poa Annua will begin to sprout seed in May and into June. Unfortunately this is also around the same time as when Kentucky Bluegrass and Tall Fescue begin to seed, too. Confusing Annual Bluegrass with Kentucky Bluegrass at this time of coming to seed is a very common question.
Like Kentucky Bluegrass, the seed of Poa Annua grass will emerge from the top of the grass. The most common differentiator between the two is that the seed of Poa Annua will grow out more horizontally, where Kentucky Bluegrass will be bunched more vertically. Kentucky Bluegrass will also have more seeds per stem in a more slender shape; the Annual Bluegrass will appear to have fewer seeds on shorter stems.
How To Kill Poa Annua
There are some herbicides that will selectively kill Poa Annua in lawns. While it is not specifically labeled on Tenacity Herbicide label, it will damage the plant. Also keep in mind that Tenacity may leave bleaching in your lawn during its peak time to shine.
Round-Up or glysophate will also kill Poa Annua. But since this is a non-selective herbicide, it will also kill any other plants and desirable grasses that it comes in contact with. Avenger Herbicide is my personal favorite. It’s also a non-selective herbicide but an organic spray that is made from citrus oils. A concentrate of 4 parts Avenger to 1 part water will also kill Poa Annua in 1-2 applications. But like Round-Up, this, too, will also kill any other plants that it comes in contact with.
How To Control Poa Annua
Your best bet against Poa Annua is pre-emergent control and cultural practices. The most effective control of Poa Annua is by applying a pre-emergent such as Prodiamine (liquid or granular) or Dithiopyr (generic Dimension) towards the end of August, or when soil temperatures approach 70 degrees. For Massachusetts residents, you should apply Prodiamine by the third week of August.
With a fall pre-emergent strategy, you will not be overseeding your lawn. That means no core aeration and no spreading of grass seeds. Both Prodiamine and Dimension will not only help prevent Poa Annua from germinating, but also your new grass, too. Overseeding is my most favorite time of the cool season year, but his method of bypassing overseeding in lieu of pre-emergent application will be your best bet reducing the Annual Bluegrass in the lawn over time.
Poa Annua Prevention – Cultural Practices
Poa Annua dies off in the summer as outside temperatures are consistently 80 and above. This Annual Bluegrass has shallow roots and prefers shady and damp areas. If you water deeply and infrequently, you will help prevent these ideal conditions for Poa Annua.
Mow Your Grass Tall
Poa Annua is a short annual grass. If you follow best practices for most cool season grass types and mow your lawn between 3-4 inches, you’ll make it more difficult for annual bluegrass to survive.
Poa Annua Pre-Emergent Herbicides
Poa Annua Weed Killer (Non-Selective, Post Emergent Herbicides)
- Avenger Organics Weed Killer Concentrate
- Gly Pho-Sel Pro 40%
- Glysophate Plus
- Tenacity (not specifically labeled)
Poa Annua Frequently Asked Questions (FAQ)
Poa Annua is a winter annual cool season grass that germinates in the late summer and early fall when soil temperatures start to fall below 70 degrees.
Since Poa Annua is a winter annual grass, the best way to control Poa Annua is by applying a pre-emergent at the end of summer/early fall, and the following spring. This will help prevent the germination of annual bluegrass. For post emergent herbicides, a non-selective herbicide such as glysophate will kill Poa Annua. Poa Annua is not labeled on Tenacity Herbicide, but it will damage the plant.
Poa Annua and Kentucky Bluegrass can look very similar when the grasses begin to flower and seed. Poa Annua is an annual cool season grass where KBG is a perennial. The seeds that shed from Poa Annua germinate later in the summer to early fall, where the KBG seeds are sterile and do not germinate.
Poa Annual also grows in clumps and its green color is much lighter than Kentucky Bluegrass. KBG is known for is deep dark green that can sometimes appear blue.
Poa Annua is also known as Annual Bluegrass.
There’s a lot in common between Poa Annua (Annual Bluegrass) and Poa Trivialis (Rough Bluegrass) but the main difference are that Poa Annua is an annual grass where Poa Triv is perennial. Poa Annua also flowers and produces seeds where Poa Triv generally does not.
Both grasses require cultural and chemical practices to manage. The clumping lime green color are similar but Poa Annua flowers and seeds in the summer, and germinates in the fall. Poa Annua survives overwinter in a vegetative states and emerges, and thrives, the following spring. Poa Trivialis is a perennial grass and rarely produces seeds.
The label of Tenacity reads that both bentgrass and Poa Annua are “sensitive to Tenacity applications.” We have used two applications of Tenacity with a non-ionic surfactant on Poa Annua and have seen the grass turn white, then brown and ultimately die. However, this is not a guarantee to kill poa annua or preventing its seeds from shedding and germinating.
The label of Tenacity reads that when used as a pre-emergent against Poa Annua, it will act as a suppressant. It’s always best to use Prodiamine in the late summer and following spring to help prevent the germination of poa annua. However, using Tenacity (Mesotrione) at seeding is a good measure in helping to suppress annual bluegrass.