2018 ASHS Annual Conference

Plants of ‘Prestige Red’ poinsettia (Euphorbia pulcherrima Willd. Ex Klotzsch) were grown in greenhouses in Overton, Texas USA to determine if photoperiod could be used to hasten flowering under high temperature conditions. Flowering of this cultivar is known to be delayed by 4 weeks of high temperatures under natural photoperiod during floral initiation. Plants were potted in early September and pinched to 5 nodes two weeks later for natural season flowering. Plants were exposed to either 0, 2, or 4 weeks of a high temperature treatment consisting of a daily average temperature of 27C starting at the time of the pinch. During the high temperature treatment, half the plants were exposed to a natural photoperiod and the other half were exposed to a 9 hour short day with a 15 hour night using black cloth. For the rest of the forcing period, an average daily temperature of 22C was maintained. Dates of first bract color, visible bud, and anthesis were recorded for each plant. There was no difference in number of days to anthesis from start of treatment for plants with 2 weeks of high temperature at either photoperiod compared to those with 0 weeks of high temperature. However, short days hastened anthesis for plants with 4 weeks of high temperature when compared to plants with 0 or 2 weeks of high temperature under either photoperiod. These relationships held for days to visible bud from treatment as well. When considering days to first bract color from treatment, 4 weeks of high temperature delayed color development on plants under natural days. But, this effect was reversed by short day exposure during the high temperature treatment, though no further hastening by short days was seen. This indicates that short days can be used to reverse the delaying effects of high temperature on flowering.