Seasonal Timing of Glyphosate Application Influences Control of Poa trivialis

Rough bluegrass (RBG; Poa trivialis L.) is a perennial cool-season turfgrass and a problematic weed in coolseason turf due to suboptimal color, invasive stoloniferous growth, and sensitivity to heat and drought. Naturalized populations spread vegetatively during routine aeration and from contamination in seed lots (Reicher et al., 2011). Bispyribac-sodium (Velocity) is the only product currently labeled for selective RBG removal in cool-season turf and is effective, but it can damage desirable species (McCullough and Hart, 2011) and is only labeled for sod-farm and golfcourse use. Nonselective herbicides are often the only option for RBG control in home lawns, but it is unclear if efficacy varies with seasonal application timing, like RBG response to bispyribac-sodium (McCullough and Hart, 2011). Rough bluegrass persistence is anecdotally reported after latesummer glyphosate application. Adkins and Barnes (2013) observed better Kentucky bluegrass (Poa pratensis L.) control with spring treatments of imazapic plus glyphosate, but tall fescue (Festuca arundinacea Schreb.) control was better following summer applications. Therefore, our objective was to determine if the seasonal timing of glyphosate application influences RBG control.