Why do many consider poa annua, commonly known as annual bluegrass, the world’s most problematic weed?
It is the world’s most widespread weed.
It is showing increasing resistance to currently available pre-emergent herbicide chemistry.
Poa annua is a winter annual that first germinates in the fall when nighttime temperatures drop into the 60s, daytime temperatures stay under 85 and there is an abundance of moisture.
The only positive to the dry fall we are having is it may reduce the development of poa annua.
Optimal germination occurs when soil temperatures dip below 70 degrees. This week soil temperatures dipped below 70 degrees for the first time this fall.
Poa annua continues to germinate through the fall, winter and spring making it a several month-long battle. A single plant can produce up to 2,000 seeds.
Poa annua is a low growing, lime green, clumping, grassy weed with small white flowers. Everyone has experienced annual bluegrass. It is most troublesome in short cut warm season turf, bermuda and zoyia, during the winter when it is growing and seeding heavily, and turf is dormant. During the winter it does not have to compete with warm-season turf for light, water and nutrients which allows it to spread quickly when not treated. In fescue it is not as prolific since the turf is growing and thick in the cool season, but when present and flowering, it will distract from a beautiful turf.
Most concerning is poa annua has been developing an increasing tolerance for many pre and post emergent herbicides. Tolerance is the plants ability to survive and reproduce even though it has been treated with a herbicide.
Tolerance is scary to a lawn care professional when you are talking about weeds and herbicides!
Herbicide resistance in poa annua first developed in Japan in the 1980s with the first occurrence in the US in Mississippi and Tennessee in 2004.
Poa annua is a clumping grassy weed with small white flowers.
Here are a few recommendations on how to best overcome annual bluegrass in your lawn:
Problem - A single fall application of a herbicide is not enough for the prevention of poa annua because germination can occur from September to March. Also, repeated use of the same herbicide will result in herbicide resistance.
First Solution – Our application #7, in October and November, is critical in the control of poa annua. This application contains a different fall pre-emergent herbicide than is applied in Application #6. Additionally, we use a herbicide in Application #7 that has post-emergent abilities on poa annua.
Second Solution – Application #1 in January and February contains herbicides for control of poa annua that can only be used on dormant warm-season turfgrasses. Once we reach March control of annual bluegrass is more difficult.
Poa annua will thrive in thin dormant turf during January and February without effective fall prevention.
Problem – Lawn care application effectiveness is reduced when the pre-emergent herbicide is not watered into the top 1” of the soil.
Solution – Always follow the watering instructions when we do your lawn care applications, but especially when the application includes a pre-emergent. Pre-emergent herbicides are not effective at preventing germination when they remain on the soil surface.
Problem – Unhealthy, thin turf is a breeding ground for poa annua and many other weeds. Therefore, annual bluegrass is more of a problem in dormant warm-season turf than it is in cool-season turf. Annual bluegrass also prefers compacted soil.
First Solution – Applications during the growing season for warm-season turf contain fertilizer that thickens the turf reducing the area for poa annua seeds to germinate and develop in the fall.
Second Solution – Do not cut warm-season turf short for the winter. Maintaining a healthy amount of top growth will inhibit some seeds from reaching the soil surface. Annual bluegrass is much more troublesome in short cut, warm-season turf during dormancy.
Third Solution – Aeration in the early summer on warm-season turf and in the fall on cool-season turf will result in a thicker, healthier turf. Aeration will reduce the compaction and strengthen the root system of your lawn by allowing air, nutrients, and water to reach the root system.
Maintaining a thick, healthy, properly maintained turf is critical to the prevention of poa annua.
Heavy reliance on herbicides to control poa annua increases the likelihood that it will develop herbicide resistance particularly if you are relying on a single fall pre-emergent application of the same herbicide year after year.
In the spring after lawns have greened up, controlling poa annua can be very difficult without damaging the turf.
Without two fall pre-emergent herbicide applications, poa annua will be prolific the next spring when the turf is coming out of dormancy.
Effective prevention of poa annua is the result of two differing fall pre-emergent applications and healthy turf.
Our goal is to do all we can to strengthen your lawn and prevent poa annua from littering your dormant lawn this winter.
We are committed to taking steps to overcome the herbicide resistance of annual bluegrass.
We need your help in following watering instructions, maintaining proper mowing heights, and adding aeration to your annual services.
Lorne Hall
Hall | Stewart Lawn + Landscape
(405)367-3873