Annual bluegrass (Poa annua) is a prevelant component of many cool-season turfgrass mixtures in Michigan. Objections to annual bluegrass include its prolific seedhead production in the spring of the year. Mefluidide or Ethephon applications can be very effective at limiting the expression of seedheads, however, proper application timing can be difficult. Calendar-based application timings are often unreliable. Linking application timing to developmental models like growing degree days can provide much more consistent results. This study tracked growing degree day accumulation for multiple application timings over a four-year period. Mefluidide or ethephon were applied a 12 different timings in each year. Calendar date, growing degree days and soil temperatures were recorded for each application timing. Annual bluegrass seedhead production was measured twice per week during the main spring production period. Results were used to identify which application timings provided the highest level of control with the least amount of turfgrass injury.