A six‐parent diallel of maize (Zea mays L.) was evaluated for total leaf number (TLN), days to tassel initiation (DTI), and to pollen shed (DPS) in 18 diverse field environments and two phytotron environments to determine the contribution of photoperiod and temperature variables to the genotype ✕ environment interaction. Understanding the interaction of genotype, photoperiod, and temperature is essential, for ultimately the timing of tassel initiation and other major developmental events determines yield. For each trait, but particularly for TLN, the contribution of general combining ability (gca) to the genotype and genotype ✕ environment sums of squares far exceeded that of specific combining ability (sca). For both TLN and DTI, the linear effect of photoperiod was the primary climatic variable contributing to the gca ✕ environment interaction. The same rankings of gca effects in response to photoperiod were observed for TLN and DTI, with a range in parental sensitivity from 0.4 leaves and 0.0 days/h to 1.9 leaves and 4.9 days/h. For neither trait was there evidence of a critical daylength. The primary difference between TLN and DTI was that DTI was more sensitive to temperature, particularly average low temperature, than was TLN. Although the linear term for photoperiod also contributed significantly to the gca ✕ environment interaction for DPS, its effect was overshadowed by that of temperature.