Both Poa annua (annual bluegrass) and Poa trivialis (rough bluegrass) are undesirable weeds in lawns. They share similarities like light green color and boat-shaped leaves but differ in their growth and control methods. Poa annua is an annual grass that spreads by seed, whereas Poa trivialis is a perennial that spreads by stolons. Controlling these weeds requires different approaches due to their distinct growth habits and environmental preferences.
Poa Annua vs Poa Trivialis
Seedhead Production
- Poa annua: Produces prolific seedheads, especially noticeable in May and June.
- Poa trivialis: Rarely produces seedheads when mowed, spreading primarily via creeping stolons.
Growth Pattern
- Poa annua: Annual bluegrass, spreading by seeds that can remain dormant in soil for years.
- Poa trivialis: Perennial grass, spreading through above-ground stolons.
Preferred Environment
- Poa annua: Thrives in full sun, often dying in the summer heat but reseeding in the fall.
- Poa trivialis: Prefers shaded areas, forming dense mats that can outcompete desirable grasses.
What Is Poa Annua?
Poa annua, annual bluegrass, is one of the most widely distributed plants on the planet, which underscores why controlling these weeds is so difficult. Poa annua is a cool season grass that looks similar to Kentucky bluegrass, which is why it can be difficult to identify. However, it features a lighter green color than Kentucky bluegrass and is an annual plant. That means it will die, but each plant can leave behind thousands of weed seeds.
Typically, annual bluegrass plants die in the intense summer heat, but their seeds ensure a new crop of weeds. Poa annua seedlings will emerge in the fall and then flower in the spring. Most people rely on pre- and post-emergent herbicides to control annual bluegrass effectively.
What Is Poa Trivialis?
Poa trivialis is a perennial plant, so it doesn’t die yearly like annual bluegrass. Poa trivialis, known more commonly as rough bluegrass, goes dormant during the winter. Rough bluegrass spreads via its stolons rather than by seed, unlike annual bluegrass. It forms dense mats of grass stems that can choke out the growth of your desirable grass, like perennial ryegrass or tall fescue. Also light green in color, Poa trivialis tends to thrive in shaded areas of lawns.
It can be difficult to kill Poa trivialis because gardeners mainly have to rely on non-selective herbicides, but these chemicals can also kill grass and other nearby plants. Unfortunately, the best way to kill Poa trivialis is using a non-selective herbicide, as selective pre-emergent and post-emergent herbicides are mainly ineffective in controlling rough bluegrass.
How can I identify Poa trivialis?
Poa trivialis, known as rough bluegrass, is light green, spreads via stolons, and can develop a reddish hue. It stands out in late summer when it turns brown, often resembling a fungal infection.
Does Tenacity work on Poa trivialis?
While Tenacity can weaken Poa trivialis, it is generally ineffective for complete control. Non-selective herbicides are often required.
What causes Poa trivialis?
Poa trivialis thrives in shaded, moist areas and can invade even healthy lawns. Overwatering and bare spots increase the risk of infestation.