Fall is a chance for most superintendents to catch their breath, but those with warm-season playing surfaces need to be focused on their Poa annua control programs. While contamination is most obvious in the winter and spring months, those problems start now without good management. Fall preemergence applications are critical to managing Poa annua in warm-season turf, but they are not the only element of a successful program. Here are a few tips to help you deal with this troublesome weed:
Herbicides are necessary for good control, but remember that the best defense against any weed is a healthy and dense stand of turf, especially heading into winter. Poa annua will be more than happy to invade thin or worn-out areas. Raising the height of cut and minimizing stress as much as possible can go a long way toward keeping Poa annua at bay.
Poa annua pressure tends to be greater in areas that receive dense winter shade. Selective tree management will enhance the health of desirable warm-season turf and reduce the likelihood of contamination.
Herbicide control programs need to be based on local conditions and climate. There is no “one-size-fits-all” solution.
Properly timing pre and postemergence applications is critical for good control. Poa annua tends to germinate when soil temperatures are below 67 degrees F for five consecutive days, so applying preemergence products when soil temperatures fall to about 70 degrees F is a good rule of thumb (Taylor et al., 2021). Applying postemergence products immediately following germination or when plants are smaller will also improve control.
Recent research shows Poa annua has evolved herbicide resistance to multiple modes of action (Rutland et al., 2023). Tank mixing and rotating postemergence herbicides with a fall preemergence application can help delay the development of resistant populations. If taking this approach, the preemergence application should be made later in fall.
Some preemergence herbicides like dithiopyr have early postemergence control, while others do not. This can affect the timing of preemergence and subsequent postemergence applications, so be sure to always check the label.
Consider herbicide safety when applying products next to putting greens or in areas with both warm- and cool-season turf. Some products are safer to use than others in these situations.
Successful herbicide programs at courses with significant Poa annua pressure often include multiple applications and multiple modes of action. Relying solely on one preemergence application, or using the same postemergence product year after year will eventually lead to poor control.
Poa annua control is never going to be easy and no single application or maintenance practice will lead to success. You will need to use a variety of herbicide options along with taking steps to optimize the health and density of your desired turf. Please reach out to your regional USGA agronomist if Poa annua is a problem at your course.
References
Rutland, C. A., Bowling, R. G., Russell, E. C., Hall, N. D., Patel, J., Askew, S. D., Bagavathiannan, M. V., Brosnan, J. T., Gannon, T. W., Gonçalves, C., Hathcoat, D., McCarty, L. B., McCullough, P. E., McCurdy, J. D., Patton, A. J., Unruh, J. B., & McElroy, J. S. (2023). Survey of target site resistance alleles conferring resistance in Poa annua. Crop Science, 63, 3110–3121.
Taylor, D. R., Prorock, M., Horvath, B. J., & Brosnan, J. T. (2021). Modeling seasonal emergence of Poa annua in urban greenspace. Scientific Reports, 11(1), 18960.
Central Region Agronomists:
Zach Nicoludis, regional director – znicoludis@usga.org
Paul Jacobs, agronomist – pjacobs@usga.org