Efficacy of imazapyr and glyphosate in the control of non-native Phragmites australis

The cosmopolitan common reed (Phragmites australis) has been expanding into previously unoccupied wetland habitats throughout North America. This invasion by a non-native haplotype of Phragmites has become a major concern due to a reduction in plant diversity, reduction of faunal biodiversity, and changes in ecosystem structure. A randomized complete block design was used to compare the efficacy of two herbicides, glyphosate (Rodeo, Dow AgroSciences, IN, U.S.A.) and imazapyr (Habitat, BASF Corporation, NC, U.S.A.), on 1-ha Phragmites monoculture in a shallow borrow pit. Six foliar experimental treatments were applied consisting of (1) 2% glyphosate formulation, June application; (2) 2% glyphosate formulation, September application; (3) 2% imazapyr formulation, June application; (4) 2% imazapyr formulation, September application; (5) 5% imazapyr formulation, June application; and (6) 5% imazapyr, September application. Experimental plots were monitored yearly for two years after treatment. Relative importance values (RIV) were determined to assess the efficacy of herbicide treatments. We report that imazapyr foliar application is statistically superior to glyphosate in reducing Phragmites RIV, with no significant differences between the 2 and 5% formulations. Both herbicides are more effective in reducing Phragmites RIV if applied early in the growing season (June). No significant differences in non-Phragmites plant recolonization were observed between herbicide treatments over the two-year time course. These results suggest that imazapyr is superior in reducing Phragmites RIV, and that earlier applications of herbicides may be more effective on Phragmites. However, managers must note that adjacent nontarget plant species may be negatively affected by earlier treatments.