2019 ASHS Annual Conference

Poinsettia (Euphorbia pulcherrima) flowering can be delayed by exposure to supra-optimal temperatures – a phenomenon termed “heat delay”. The objective of this research was to evaluate the interaction of temperature and photoperiod on poinsettia flower development. The experiment included three day temperatures (20, 24, and 28 °C), four night temperatures (16, 20, 24, and 28 °C) and five night lengths (10, 11, 12, 13, and 14 h) for a total of 60 treatment combinations. After 17 d in the temperature and photoperiod treatments, all plants were moved to one greenhouse with an inductive environment (14-h nights and 24 °C day/20 °C night temperatures). Two cultivars were used for this experiment – Orion Red (heat tolerant) and Prestige Red (heat sensitive). Time to first color, visible bud, and anthesis were recorded. Progress to anthesis (days/flower) was calculated as the reciprocal of days to anthesis. At the 10 h night length, no progress to anthesis occurred for either cultivar. For Prestige Red, progress to flower increased as night length increased from 11 to 14 h and as night temperature decreased from 28 to 20 °C. Providing long night lengths with black curtains during September is often viewed as a technique to avoid heat delay. Our data demonstrate that growing Prestige Red with a 13 or 14 h nightlength will reduce the time to anthesis when compared to natural night lengths of 11 to 12 h that occur September. However, high night temperatures will cause a significant delay in flowering when compared to more moderate temperatures even when poinsettias are provided a 13 to 14 h night length. Orion Red did not demonstrate a delay in flowering at high night temperatures, and the rate of progress to flower increased at a higher rate at shorter night lengths, e.g., 11 and 12 h, than Prestige Red. This explains why Orion Red is categorized as an early-season cultivar that normally flowers in early November while Prestige Red is a late-season cultivar that flowers in late November when grown under natural night lengths. A model to predict flower development based on ambient temperatures and photoperiod will be presented.