By Eric Reasor, Ph.D.
Southeast Research Scientist
The Problem
Poa annua, also known as annual bluegrass, is one of the most prolific and adaptable weeds in the world. It can be problematic in all turf situations from golf course putting greens to home lawns in both cool- and warm-season turfgrasses. As the name suggests, annual bluegrass is classified as an annual weed that germinates in spring and fall and then dies off in the summer heat. However, it can perennate when growing in favorable conditions (e.g., high moisture, shade, and mowing height).
Poa annua is most notable for its negative effects in turfgrass such as light green color, pest susceptibility, surface disruption, and excessive seed production. Cultural control practices including deficient watering, limiting nutrients, and cultivation can aid in reducing the Poa annua population, but herbicide applications are inevitably needed due to its competitiveness and adaptability.
Herbicide control of Poa annua is often required when cultural control practices do not provide acceptable control. Pre-emergent (PRE) herbicide applications in late-summer to early-fall prior to Poa annua germination, and post-emergent (POST) applications in late-winter have traditionally been the two herbicide application strategies. However, Poa annua has developed significant resistance to herbicides used at these timings.
Herbicide-resistant Poa annua populations have increased the past decade. More control failures have been reported with resistance as the likely culprit. So much so that the United States Department of Agriculture funded a multi-million dollar grant to universities across the U.S. to study the issue. For more information on the USDA-SCRI Annual Bluegrass Collective, visit ResistPoa.org.
Herbicide resistance issues in turf include both pre- and post-emergent herbicides. Resistance to herbicide mode of action Groups 2 (ALS inhibitors), 3 (microtubule inhibitors), 5 (photosystem II inhibitors), and 9 (EPSPS inhibitors) are some of the most reported. The reliance on herbicides and the current application patterns will continue to select for herbicide-resistant populations if strategies are not changed moving forward.
Moving Forward
Tank-mixes of PRE and POST herbicides have provided the most consistent Poa annua control in research trials (university and PBI-Gordon) across multiple sites and resistant populations. Tank-mixes of PRE and POST herbicides have been used in the past, but those mixtures have focused around non-selective herbicide use in late-winter dormancy when Poa annua plants are mature and are more difficult to control.
The application timing of the PRE+POST is critical for resistance management and long-term control. Instead of a late-POST application with a non-selective during dormancy, shift the application timing to an early-POST timing soon after the initial germination flush. Monitoring soil temperatures and volumetric water content, and scouting for plants can help predict the fall germination flush. Recent research in Knoxville, TN demonstrated annual bluegrass emergence most rapidly occurred when soil temperatures fall below 70°F with ample soil moisture (especially rainfall; Jim Brosnan, personal communication, 2021).
Bermudagrasses and zoysiagrasses will likely not be dormant enough for safe non-selective POST applications at the early-POST timing; therefore, selective herbicides for Poa annua are needed in the fall PRE+POST applications. The sulfonylurea herbicide family has many active ingredients for its selective control in bermudagrass and zoysiagrass. Flazasulfuron, foramsulfuron, rimsulfuron, sulfosulfuron, and trifloxysulfuron are examples of efficacious active ingredients.
Katana® Turf Herbicide (active ingredient flazasulfuron) is a sulfonylurea, ALS-inhibiting (Group 2) herbicide for selective Poa annua control. It is highly effective for post-emergent control with the addition of residual control during cooler weather. However, many annual bluegrass populations in the southern U.S. have developed resistance to ALS inhibitors. PBI-Gordon and university research trials the past five years have focused on fall/winter applications of Katana tank-mixed with pre-emergent herbicides in warm-season golf course fairways and greens.
Katana (Group 2) tank-mixed with herbicides such as bensulide (Group 8), dimethenamid (Group 15), indaziflam (Group 29), pendimethalin (Group 3), prodiamine (Group 3), pronamide (Group 3), and simazine (Group 5) have provided long-term acceptable annual bluegrass control when applied at the aforementioned early post-emergent timing. Furthermore, Katana, bensulide, and pronamide can be used on bermudagrass greens.
Poa annua control with Katana®
Poa annua is a problematic weed in turfgrass that often requires herbicide applications. Herbicide resistance has increased and herbicide use patterns should be shifted to a fall/winter application after the first germination flush. Combine a post-emergent herbicide application with a pre-emergent herbicide for the long-term control and resistance management.