Root-zone and plant canopy temperatures were continuously monitored as a poinsettia (Euphorbia pulcherrima Willd. ex JSI.) crop was grown in the greenhouse under warm day/cool night [(+) DT-NT] or cool day/warm night [(-) DT-NT] temperature regimes. Day temperatures were imposed from 0900 to 1700 hr. Light levels photosynthetic photon flux (PPF) and outside ambient air temperatures were also monitored. Temperature differences between the root-zone and plant canopy microenvironments were most extreme during the night-to-day and day-to-night temperature transition periods. The temperature difference between the plant canopy and the root zone following temperature transition periods had been previously identified as a critical factor affecting stem elongation. Overall poinsettia height was consistently shorter under the (-) DT-NT than under the (+) DT-NT environment.