Tolerance of Native and Ornamental Grasses to Over-the-top Applications of Topramezone Herbicide

Research was conducted to determine the tolerance of multiple native and ornamental grass species and one ornamental sedge species to over-the-top applications of the postemergence herbicide topramezone at three locations in the southeastern United States in 2016 and 2017. Fully rooted liners of selected grass species were outplanted into research plots in Apopka, FL; Dallas, TX; and Knoxville, TN in late spring, allowed time to establish (approximate to 1-2 months) and then treated with two applications of topramezone at either 0.05 or 0.10 kg a.i./ha at 6-8 weeks intervals. Results showed that species including Andropogon virginicus (broomsedge), Schizachyrium scoparium 'The Blues' (little bluestem), Tripsacum dactyloides (eastern gamagrass), and Tripsacum floridanum (florida gamagrass) exhibited the greatest tolerance to topramezone with < 10% injury to no injury being evident after each application of both herbicide rates tested. Chasmanthium latifolium (wild oats), Eragrostis elliottii 'Wind Dancer', Muhlenbergia capillaris (pink muhly), and Spartina bakeri (sandcord grass) were significantly injured (50% injury or greater) at both herbicide rates. Average injury observed on Panicum virgatum 'Shenandoah' (red switchgrass) (ranging from 39% to 100% injury) and Sorghastrum nutans (indian grass) (ranging from 0% to 40% injury) was higher in Florida than in Tennessee (injury ranging from 23% to 43% on red switchgrass and 0% to 10% on indian grass). Similarly, Pennisetum alopecuroides (dwarf fountain grass) showed higher tolerance in Texas (ranging from 0% to 34% injury) compared with those observed inTennessee (ranging from0% to53% injury). Topramezone injury to Carex appalachica (appalachian sedge) was <= 18% following two applications at both rates tested. Although no injury was observed in appalachian sedge following a single application up to 0.1 kg a.i. in Florida, plants succumbed to heat stress and accurate ratings could not be taken following the second application. Because of variability observed, tolerance of red switchgrass, indian grass, dwarf fountain grass, and appalachian sedge to applications of topramezone deserves further investigation. There is potential for future use of topramezone for control of certain grass and broadleaf weeds growing in and around certain ornamental grass species. However, as there was significant variability in tolerance based on species and differences in cultivars, testing a small group of plants before large-scale application would be recommended.