Reproductive growth and development of cool‐season grasses is primarily driven by air temperature and photoperiod. This study was conducted near Salina, KS, to model the effects of growing degree days (GDD), day of the year (DOY), and cropping systems on first‐ and second‐year intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] (IWG) in Kernza perennial grain production systems. In 2018 and 2019, GDD and DOY were highly correlated with reproductive growth and development (i.e., mean stage count, head meristem height, and leaf, stem, and head biomass fractions). Row spacing, fertilization, and intercropping with alfalfa (Medicago sativa L.) did not influence IWG reproductive growth and development. Across years, DOY more closely predicted reproductive stages than GDD, indicating a greater response to photoperiod than air temperature. After stem initiation, the fraction of total biomass allocated to leaves decreased, whereas stem and head biomass increased in response to GDD and DOY. At anthesis, stem biomass exceeded leaf and head biomass. Parameterizing GDD and DOY models for management in IWG dual‐purpose, Kernza grain production systems will require additional datasets from many locations and environments.