Efficacy and Volatility of Spring- and Fall-Applied Broadleaf Weed Herbicides.

Fall herbicidal timing for cool-season broadleaf weed control in turfgrass is usually recommended, but managers often need to control these weeds in the spring. In addition to efficacy, volatilization of spring-applied herbicides is of concern because of vulnerable landscape plants. This research evaluated the efficacy and volatility of Trimec Classic and several newer herbicides: Speedzone, Escalade II, Surge, Confront, 4-Speed XT, Cool-Power, and Imprelis SL and G. Herbicides were applied at their lowest label rates at six different timings in the spring and fall to turfgrass stands infested with dandelion (Taraxacum officinale). A rating grid was used to record weed levels 30 and 60 days after treatment (DAT), at season’s end, and the following spring. Volatility was compared in the laboratory and greenhouse by applying each herbicide at its highest label rate to flats of turf-type tall fescue (Festuca arundinacea), then placing two potted tomato (Solanum lycopersicum) plants over the treated turf. The tomato plants and turf were held in 66 L translucent containers in the laboratory at 22 C for 24 h, and tomato injury was evaluated in the greenhouse over 18 d. When applied pre-bloom in 2011, all herbicides gave  >88% control at 60 DAT, except Confront and Cool Power. When applied at full bloom, Imprelis SL and G, Speedzone, and 4-Speed XT gave ≥ 86% control at 60 DAT; all others gave <78%. When applied one month post-bloom, overall control was better than at pre-bloom or peak bloom, but 60 DAT data have not been collected as of this writing. In volatility studies, 2,4-D butyl ester caused tomato death by 18 DAT, while Speedzone, Cool Power, and 4-Speed XT caused intermediate damage. All these herbicides contained ester forms of a synthetic auxin. The remaining herbicides, containing amine formulations, caused little or no volatility damage to tomato.