Seasonal Susceptibility of Boreal Plants to Glyphosate I. Blue-Joint Grass and Black Spruce

Seasonal susceptibility and posttreatment recovery of blue-joint grass (Calamagrostis canadensis [Michx.] Beauv.) to glyphosate (N-[phosphonomethyl] glycine) were examined in a young black spruce (Picea mariana [Mill.] B.S.P.) plantation. In 1990, two rates of the herbicide (1.1 and 1.7 kg ae/ha) were applied on July 19, August 1, 15, and 29, and September 10 and 29 using a backpack sprayer. Although differences between rates were marginal, time of application strongly influenced post-treatment cover of blue-joint grass. Compared to the untreated control, glyphosate applied between August 1 and September 10 significantly reduced cover of blue-joint grass for 3 yr after treatment and increased growth of black spruce for at least 5 yr. The optimum period of glyphosate efficacy corresponded to the end of blue-joint grass's full flowering and beginning of aboveground senescence. North. J. Appl. For. 17(4):141–148.