The ability to predict when a cool-season forage grass cultivar will begin infl orescence emergence under different ecogeographical conditions would allow plant breeders, agronomists, and grass-seed marketers to better position that cultivar into a forage production system. Our objective was to determine the ecogeographical factors (longitude, latitude, elevation, day of year when average daily temperature exceeds 0°C for fi ve consecutive days [DOY at 0°C], cumulative growing degree-day [GDD], photoperiod, and cumulative photosynthetic active radiation [PAR]) that have the greatest effect on grass maturation in the spring. Infl orescence emergence was monitored in established cultivars of festulolium (× Festulolium spp.), orchardgrass (Dactylis glomerata L.), ryegrass (Lolium perenne L. and Lolium multifl orum Lam.), tall fescue (Festuca arundinacea Schreb.), and timothy (Phleum pratense L.) at eight locations in North America during the spring of 2004 and 2005. As latitude increased, the day of year when grasses reached 1% infl orescence emergence (DOY) also increased, while cumulative GDD and PAR decreased. Latitude, cumulative PAR, and DOY at 0°C were more closely correlated (r 2 ≥ 0.67) to the onset of infl orescence emergence than the other variables. Latitude combined with the inverse transformation of PAR provided the best prediction of when these grasses would initiate infl orescence emergence (validation R2 for all species ≥ 0.83).