Pennsylvania Flower Growers #235
The critical daylength for poinsettia, a qualitative short-day plant, cannot be stated precisely because both temperature and the cultivar influence the response to photoperiod. In general, as the temperature is in creased from 60 to 80°F, shorter photoperiods are required for flower initiation (Langhans and Miller, 1960). For the cultivar 'Barbara Ecke Supreme', the critical daylength for flower initiation is more than 13 hours at 60°F, between 12% and 12% hours at 65 and 70°F, and 9 to 10 hours at 80°F (Larson and Langhans, 1963). In contrast, the cultivar 'Paul Mikkelsen tends to initiate flower primordia under conditions that prevent flowering of 'Barbara Ecke Su preme' and other cultivars. At 60°F, flowers are initiated at photoperiods up to 16 hours; at 65°F, initiation is delayed slightly only at photoperiods of 16 hours; at 70 and 80°F, plants tend to remain in a vegetative condition as the daylength is extended to 16 hours — eventually flowers are initiated but at a higher node on the stem (Kofranek and Hackett, 1965). The main difference in the control of flower initiation between these two cultivars is their response to temperature. If temperatures are high (72° minimum — 85°F maximum), the length of the lighting period required to interrupt the dark period for preventing flower initiation is similar for both cultivars — one hour or more. If temperatures are low (59° mini mum — 75°F maximum), however, 'Barbara Ecke Supreme' is maintained in a vegetative state with a 2-hour light interruption at ten foot candles, whereas, 'Paul Mikkelsen initiates flower primordia even with a 4-hour light interruption. A higher light in tensity (35 foot candles) did not alter the response of either cultivar (Hackett and Miller, 1967).
Keywords: Flower primordia flower initiation extraneous light frequency of premature flower init