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

Herbicidal timing for cool-season broadleaf weed control in turfgrass has traditionally been recommended for the fall, but turfgrass managers are often under pressure to control these weeds in the spring or summer. In addition to efficacy concerns, volatilization of spring-applied herbicides is of interest because surrounding dicotyledonous plants are more vulnerable in spring than fall. This research evaluated the efficacy and volatility of Trimec Classic and several newer herbicides: Speedzone, Escalade II, Surge, Confront, 4-Speed XT, and Cool-Power. Herbicides were applied at six different timings in the spring and fall. All were applied at the lowest labeled rates to turfgrass stands infested with dandelion (Taraxacum officinale) or white clover (Trifolium repens). The experimental design was a randomized complete block design with three replications. The applications were made with a two-nozzle (XR8002VS) CO₂ -powered backpack sprayer, operating at 30 psi, to deliver a spray volume of 364.6 L ha^-1. A rating grid was used to record weed levels 30 days after treatment (DAT) and at season’s end. 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 under plastic translucent covers in the laboratory for 24 hr, and then tomato injury (determined by visual symptoms of epinasty, leaf curling, and necrosis) was evaluated in the greenhouse over an 18-d period. For the volatility study, the  herbicides were applied using a spray chamber operating at a volume of 299.3 L ha^-1. 2-4 D butyl ester was used as a volatile control. First-year results of this two-year study will be presented.