With yesterday as the official first day of Fall, is there anything better to discuss this morning than Fall Pre-Emergent Applications? More specifically…Why your lawn needs a second fall pre-emergent!
What is the goal of fall pre-emergent herbicides? The goal is to prevent winter annual weeds from germinating in your lawn this fall and over the winter. The goal is to have a clean, weed-free lawn throughout the winter and heading into the spring.
The most common winter annual weeds include henbit, chickweed, and poa annua. Each one of these have the ability to ruin the appearance of your lawn this winter and next spring if you skip fall pre-emergent applications.
Of the three most common winter annual weeds, poa annua is the most problematic.
Poa annua, commonly known as annual bluegrass, is the world’s most widespread weed.
Henbit is a winter annual broadleaf weed that germinates in the fall. Without fall pre-emergent applications you will have a carpet of weeds with purple flowers next March.
Annual bluegrass is a low-growing, lime-green, clumping, grassy weed with small white flowers. It is very noticeable in dormant Bermuda grass in late winter through the spring.
Poa annua begins to germinate when we have our first fall cool spell and there is an abundance of moisture. When nighttime temperatures drop into the low 60s and daytime temperatures stay under 85 degrees, annual bluegrass begins to germinate. (Typically, late September to early October). Optimal germination occurs once soil temperatures stay below 70 degrees for 4 consecutive days. (Commonly occurs by mid-October). The rest of the fall, winter, and spring, germination will continue making the poa annua a several month-long battle.
Best Practices:
A. Apply the first round of fall pre-emergent herbicide by early October.
B. Apply the second round of fall pre-emergent herbicide 6-8 weeks later.
Poa annua thrives in thin turf. The best defense for prevention is to develop a healthy turf.
Poa Annua in Warm Season Turf (Bermuda & Zoysia) is most troublesome during the winter when poa annua 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 in untreated lawns.
Poa Annua in Cool Season Turf (Fescue & Rye) is not as prolific since the turf is growing and thick in the cool season, but poa annua can still distract from a beautiful turf in the spring when it is producing seed heads. Maintaining a thick and healthy fescue lawn is important in the prevention of poa annua. For this reason, overseeding fescue in the fall is an important step in creating a thick turf and eliminating thin spaces ripe for weed germination.
Most concerning is poa annua has been developing an increasing tolerance for many pre and post-emergent herbicides.
Tolerance is the plant’s ability to survive and reproduce even though it has been treated with a herbicide.
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 November to early December, is critical in the control of poa annua. This application contains a different fall pre-emergent herbicide than is applied in late August through September. 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 turf grasses. 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 a lawn care application, 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. Effectiveness increases if the pre-emergent is watered into the soil. If you are applying your own pre-emergent herbicides, it is very important you read and follow label directions.
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. 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.
A second fall pre-emergent is an important step in making sure your lawn isn’t full of weeds in the late winter/early spring when the bermuda is dormant.
Maintaining a thick, healthy, properly maintained turf is a critical step in the prevention of weeds.
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 a healthy turf.
Our goal is to do all we can to strengthen your lawn and prevent winter annual weeds from littering your dormant lawn this winter.
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