Glyphosate tolerance of eastern white cedar: Third year results

Little is known about the herbicide tolerance of eastern white cedar (Thuja occidentalis L.). To determine the sensitivity of cedar seedlings to timing and concentration of herbicide applications, glyphosate was applied to 2-year-old seedlings at three concentrations (1.04, 2.07, and 4.14 acid equivalent (ae) kg ha -1 ) at three times (July 28, August 10 and 31), at a research site in north central Ontario. Seedling survival, growth, and biomass three years after spraying were compared with those of seedlings in manual weeding (competition/weed free via manual weeding) and control (no weeding and therefore always competition/weeds) treatments. Only glyphosate applied at 4.14 ae kg ha -1 significantly reduced, by 27%, third year seedling survival. Pattern of sensitivity of cedar seedling growth to glyphosate was diameter>height. Seedlings treated with 2.07 and 4.14 ae kg ha -1 glyphosate had significantly less root biomass than those in the control plots, whereas only cedar treated with 4.14 ae kg ha -1 glyphosate had lower shoot biomass. Application timing made no difference in survival, growth, or final biomass. All glyphosate and the no weeding control treatments reduced root, shoot, and total biomass of cedar relative to the manually weeded seedlings.