Abstract: Outside its native range, smooth cordgrass is a noxious weed in estuaries throughout the world. Trials were conducted to evaluate smooth cordgrass control with imazapyr at various timings, rates, and spray volumes, and with several surfactants; comparisons were made with glyphosate, the sole herbicide registered in the United States for estuarine use. Across years, all sites, application dates, and experiments, it was found that smooth cordgrass control with imazapyr at 1.68 kg/ha was better and more consistent than with glyphosate at 8.4 kg/ha. Control with imazapyr at 0.84 kg/ha was comparable to control with glyphosate at 8.4 kg/ha. The time between application and tidal inundation over the canopy affected the efficacy of imazapyr less than that of glyphosate. There was a lack of consistent effects across dates of application. For imazapyr, a higher application volume (467 L/ha) usually provided better control than did the low volume (93 L/ha), but excellent control was achieved using ultra-low application volumes (23 and 47 L/ha) if the dry time was greater than 12 h. Several surfactants provided comparable efficacy for imazapyr.
Nomenclature: Glyphosate; imazapyr; smooth cordgrass, Spartina alterniflora Loisel. #3 SPTAL.
Additional index words: Aquatic weed management, estuary, surfactant, ultra-low volume.
Abbreviations: MAT, months after treatment; MHHW, mean higher high water; MLLW, mean lower low water; RCB, randomized complete block design; SE, standard error.