This study measured the detailed phenologies of 27 bluebunch wheatgrass (Pseudoroegneria spicata) populations from six seed transfer zones (STZs) raised in a common garden to test whether existing STZs created using a combination of plant morphology and “first-day” phenological measurements adequately capture population-level variation in season-long, detailed phenologies. The number of individual plants within an STZ not producing harvestable seed during peak harvest levels indicated that 10–27% of individuals from a seed lot could be excluded from a single-pass harvest. The findings generally support current STZ delineations for P. spicata, but they also suggest sourcing from multiple populations and harvesting with multiple passes when resources permit.