The effects of day and night temperatures (15 to 35C) and three irradiance levels [50% of ambient, ambient, and ambient plus 12 mol·m-2·day-1 of supplemental photosynthetic photon flux (PPF)] on development of Catharanthus roseus `Grape Cooler' were determined. Time to flower decreased by 30 days and leaf-pair unfolding rate (LUR) increased linearly as average daily temperature increased from 18 to 35C. Flower size was greatest when plants were grown at 25C. Supplemental light decreased days to flower and increased flower size. Flowering occurred when nine leaf pairs were present on the plant. Using the inverse of the LUR curve, i.e., days per leaf pair, the number of days to flower could be predicted at any time during plant development based on plant leaf number.