Like my wife, who simply could not wait until after Thanksgiving to put up the Christmas decorations, Poa annua could not wait to make its annual appearance in the Southeast and has burst onto the scene like a 16-foot-tall inflatable Santa Claus. The bright green clumps of this incredibly adaptable grass (considered a weed in much of the Southeast) are now dotting warm-season playing surfaces throughout the region. Understanding Poa annua and what makes it so adaptable is a complex topic; however, the climate, herbicide resistance issues, and other factors all contribute to its proliferation in the Southeast. In fact, Poa annua grows so well in the region at this time of year that it was often left untreated and used as a kind of winter overseed in years gone by. Thankfully, there are some steps you can still take if earlier preemergence herbicide applications have not worked out as planned and you’re dealing with an explosion of Poa annua.
Document the problem: Whether you have breakthrough issues every year or are just now dealing with it, you want to identify and manage possible contributing factors. Recording details like post-application irrigation or rainfall, tank mix partners, weather conditions, and timing of cultural practices may help uncover an issue. Ensure application equipment is properly calibrated and functioning and map areas with the most Poa annua to prepare for next year. If you suspect resistance, an herbicide resistance diagnostic lab can confirm it. Remember that it’s extremely unlikely for resistant Poa annua plants to appear in straight lines. Patterns like that usually point to application error.
Herbicide options: Fortunately, there are options for treating Poa annua in non-overseeded warm-season turf. If the window has closed for applying traditional preemergence products but Poa annua is showing up in your area, applying indaziflam or pronamide plus a triazene can be an effective strategy. Products containing foramsulfuron and amicarbazone are common postemergence options and there are some combination products like Tribute Total that can work well too. Significant resistance issues have led some courses in the Southeast to experiment with products like bensulide and methiozolin. The Resist Poa project published a great article that includes a comprehensive list of all herbicides that control Poa annua.
Dormant applications of nonselective herbicides: This is a good way for courses in areas where warm-season turf goes dormant to rotate in different herbicide modes of action. Glyphosate offers good nonselective control of Poa annua in dormant bermudagrass. However, glyphosate resistance is a big concern among superintendents and leading many to opt for glufosinate, another nonselective herbicide that is effective and works on a different site of action than glyphosate. Flumioxazin is another option for dormant bermudagrass applications.
Adjust cultural practices: Poa annua loves shade, water, fertilizer, stressed-out turf, poorly timed cultural practices, fungicides and low mowing heights – among other things. Keeping these things to a minimum will create an environment that favors the slowly growing or dormant bermudagrass rather than Poa annua.
Turf scientists continue working hard to find new solutions for the problem of Poa annua, especially when it comes to resistance. Sixteen university-based researchers are collaborating on the Resist Poa project and continue their good work identifying issues and delivering new control strategies. One of the best resources for golf course superintendents that I’ve come across on developing and implementing a sustainable integrated weed management program for herbicide-resistant Poa annua was published by the Resist Poa team in 2023.
At the recent Agronomy, Crop, and Soil Science Societies International Annual Meeting, several engaging research presentations explored new ways of controlling Poa annua. One particularly interesting study was presented by graduate research assistant Juan Romero of Virginia Tech and looked at thermal methods of control, including using boiling water and radiant heat to treat Poa annua growing in dormant warm-season turf. Researchers found that applying radiant heat biweekly 2 cm above the turf surface reduced Poa annua density by 87% (Romero et al., 2024). While turf scientists work hard to find new chemical and nonchemical management options, it’s important for turf managers to do their part and keep an eye out for resistance issues. Have plants screened if you suspect resistance is occurring, and stop using herbicides that are no longer effective.
Poa annua doesn’t need to be like that unwelcome guest who shows up at your front door around the holidays each year holding a suitcase and a fruitcake. Evaluating your herbicide program and overall management strategy, then adjusting things accordingly, will help you keep those little green clumps to a minimum.
Reference
Romero, J.R., Godara, N., Koo, D., & Askew, S.D. (2024). Thermal treatments for Poa annua control in dormant warm-season turf [Abstract]. ASA, CSSA, SSSA International Annual Meeting, San Antonio, Texas. https://scisoc.confex.com/scisoc/2024am/meetingapp.cgi/Paper/157504
Southeast Region Agronomists:
Jordan Booth, Ph.D., senior director, USGA Course Consulting Service – jbooth@usga.org
Chris Hartwiger, director, Agronomy – chartwiger@usga.org
Chris Neff, agronomist – cneff@usga.org