Winter is nearly here and deciding whether to continue trying to control weeds can be difficult due to the possibility of creating large bare areas that may last through the spring. When temperatures start to drop, turfgrass begins to change its focus from top growth to storing carbohydrates in order to better survive the winter. This physiological response can be advantageous when trying to get rid of difficult-to-control weeds like dallisgrass (Paspalum dilatatum) and tropical signalgrass (Urochloa subquadripara).
Turf managers often struggle to control certain weeds during the summer for a number of reasons. It may just be that control is nearly impossible at that time, or they may miss an important timing window for a second application, or they don’t make necessary follow-up applications at all. Sometimes missing the window can be just as bad as not making a subsequent application. Also, the target weed may get misidentified, which can lead to the wrong products being used.
Tropical signalgrass is regularly mistaken for crabgrass, bull paspalum and other grassy weeds. Identification is easier when the inflorescence is fully formed, as only tropical signalgrass has racemes (branches) at angles near 90 degrees to the axis. A second distinctive characteristic is rooting at the nodes into the soil. Also, quinclorac controls broadleaf signalgrass and a variety of crabgrasses but does not even dent tropical signalgrass. Herbicides like Celsius, Certainty, Dismiss, Manuscript, Plateau, Pylex, Roundup, Tribute Total, Xonerate, Sencor and Revolver are used for controlling tropical signalgrass in bermudagrass and efficacy is usually significantly better in the fall.
In South Florida, we do not normally get killing freezes so many annual weeds turn into perennials and become much more difficult to control. Luckily, the open growth characteristic of tropical signalgrass usually allows bermudagrass to survive under the canopy. I have killed large patches that looked like they would turn to dirt but was often surprised with how fast they filled in with bermudagrass.
I have also conducted research on dallisgrass and found that fall applications clearly worked the best. Adding ammonium sulfate to the application at a rate of 1.5-3.0 pounds per acre can add 10%-20% to the control as long as it’s compatible in the spray mixture.
Focusing on proper weed identification, using proven products and their label recommendations, applying simple tank mixes that have proven effective on small test areas, and proper timing of initial and repeat applications will help you succeed at controlling even the most stubborn weeds. If you are having trouble identifying or controlling tropical signalgrass, or other weeds at your course, reach out to your regional USGA agronomist.
Southeast Region Agronomists:
Chris Hartwiger, director, USGA Course Consulting Service – chartwiger@usga.org
Jordan Booth, Ph.D., agronomist – jbooth@usga.org
John Rowland, Ph.D., agronomist – jrowland@usga.org