Poa Annua And Trivialis
No one trying to build a uniform fescue lawn likes to wake up one morning to find the development of unwanted weeds in their yard. It’s even worse when these winter weeds can pose a risk to newly developing fescue seed, not to mention when they end up being rather difficult to control. Poa annua and poa trivialis are two such weeds that develop in the fall and winter, potentially crowd out baby fescue grass that is also trying to develop just after germination, and throw many homeowners and turf management professionals for a loop when it comes to their control.
“Poa annua” and “poa trivialis” are the scientific names for two common plants in the bluegrass family otherwise known as “annual bluegrass” and “rough bluegrass,” respectively. As its name would suggest, annual bluegrass is an annual weed, meaning that its life cycle is completed in the span of one growing season. On the other hand, rough bluegrass is perennial, which means that its life cycle spreads over the course of several years. Poa annua and poa trivialis do have their own unique characteristics, but they are often bundled together and painted with the same broad stroke under the name “poa.”
A winter lawn that is mostly a mix of poa annua and poa trivialis (light green) with occasional fescue growth (dark green)..
How Did This Poa Weed Get in my Lawn?
When we look at any weed issue that develops in a lawn, there are two key factors to consider: A. In what possible ways might this weed have been introduced to the lawn, and B. What about this lawn creates a habitable environment for this weed? In the cases of poa annua and poa trivialis, there is usually a straightforward answer for both questions.
Poa is actually an extremely common weed for homeowners in RVA to see in their lawn. According to an article from Ohio State University, poa annua is one of the five most widely distributed plants in the world. A plant that has this much international success is doing something right evolutionarily. This tells us that, unfortunately, poa can come from just about anywhere:
● – Dormant poa seeds can exist in the soil of a lawn for years, waiting for the right conditions to germinate and take over.
● – Seeds can encroach on a lawn from neighboring property like many other weeds often do.
● – Poa can actually be a contaminating weed in many cool season turf seed bags that are applied in the fall.
So if poa is so prevalent, why does it show up so aggressively in some yards, but not others? Poa is very exclusively a cool season weed, which means that it germinates and develops in the fall, then dies off once temperatures rise in the spring. Poa annua’s ideal lawn condition is one that has been shallowly watered, has compact soil, and is being mowed below three inches.
Unfortunately, mowing low and watering shallowly are only done in a properly maintained fescue lawn (the lawn of choice in RVA) during the fall, when poa is make its grand entrance. It ends up being a catch-22; following proper fescue seeding aftercare is vital to getting proper germination for your seed, but it also ends up supporting the germination of poa annua. Additionally, once poa has established itself in a lawn, it can be very difficult to drive out.
A clump of poa annua with seedheads.
What Threat Will Poa Pose to my Lawn?
While most weeds are more a symptom of a bigger lawn issue than an issue for the lawn themselves, poa annua and poa trivialis can in fact be a threat to a healthy lawn because of how aggressively it grows during the fall and winter. As stated in a Purdue Extension publication, from late fall into early spring, poa annua can and will outcompete all other types of turf. This can be detrimental to a fescue lawn in RVA because as baby fescue grass tries to develop after fall seeding, it is crowded out and killed off by the aggressively competing poa.
The biggest issue with fescue being crowded out by invasive poa is that once the poa dies off in the spring, all that will be left in its place is a bare spot in the lawn. Because most fescue varieties do not have the ability to spread laterally and fill in their own holes, this bare spot will remain in the lawn until it is reseeded in the fall. At that point, new poa is likely to develop, and the cycle will continue all over again. In the meantime, weeds that take advantage of bare spots like crabgrass, dandelions, and others will develop continually during the spring and summer.
Turf type tall fescue (dark green) being crowded by a mix of poa annua and poa trivialis (light green).
What Can I Do about POA
Because they are in the bluegrass family, poa annua and poa trivialis are more difficult to treat than standard weeds due to the limited types of selective herbicides that target poa without damaging the desired species of turf in the lawn. These selective products like Tenacity and PoaConstrictor can be effective in reducing and eliminating that season’s crop of poa annua, but it can be a several season process to completely remove it from the environment due to its high seed production. Because poa needs to be treated in the window of time during which baby grass is still sensitive after fall seeding, herbicide application should be done cautiously if at all. Additionally, specialty weed control products are often a higher cost and priced as an ancillary treatment by professional companies.
Due to the cost and timing concerns when treating poa annua and poa trivialis chemically, the best method for reducing its occurrence is to reduce the conditions that encourage its growth. By mowing at a high setting, watering deeply, and employing a turf management program like PPLM’s that reduces compaction and thickens turf with organically based, carbon rich treatments, a lawn will be more habitable for the desired fescue and less habitable for undesired poa annua, poa trivialis, and other weeds. A healthy lawn is a happy lawn, and a happy lawn will have a better chance of keeping the poa at bay.
A mix of poa annua (light green) and fescue (dark green).
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