Growth and Development of Poinsettia (Euphorbia pulcherrima) under Reduced Temperature Finishing and Bench-top Root-zone Heating

Greenhouse heating is an essential aspect of holiday poinsettia (Euphorbia pulcherrima) production, especially during the finish stage. Previous studies have shown that cultivars with early response attributes (initiation to finish in 6 to 8 weeks), moderate to high vigor, and naturally large bracts are most suitable for reduced finish temperature (RFT) production. We postulate that RFT in combination with root-zone heating (RZH), can further reduce energy use. The objective of this study was to quantify how RFT in combination with bench-top RZH influences time to anthesis and marketability, bract area, and height of three red poinsettia cultivars. Rooted cuttings of poinsettia 'Early Prestige Red', 'Premium Red', and 'Viking Red' were transplanted and grown at day/night temperature of 24/19 °C (12 h/12 h) until 15 Oct. and under a 16-h photoperiod consisting of natural day lengths with day-extension lighting until 1 Oct. On 15 Oct., the air temperature was reduced to 19/13 °C (8 h/16 h) and plants were placed on a bench without RZH or with RZH set points of 21, 24, or 27 °C; or plants were moved to a greenhouse without RZH and an air temperature set point of 21 °C (commercial control). Time to anthesis from the start of short days was reduced by 6, 4, and 7 d, respectively, for 'Early Prestige Red', 'Premium Red', and 'Viking Red' when plants were finished on a RZH set point of 27 °C compared to a RZH set point of 21 °C. Time to marketability was significantly longer for all plants grown without RZH compared to all other treatments. Although plants grown under RFT with RZH matured at a similar rate to those in the commercial control, bract area index was significantly reduced in most cases. For example, bract area index of 'Early Prestige Red' was reduced by 166.7, 107.1, 62.9, or 93.3 cm² when finished without RZH or with a RZH set point of 21, 24, or 27 °C, respectively, compared to the commercial control. Stem elongation responses varied by cultivar. Change in height from the RFT initiation to anthesis was similar for 'Viking Red', whereas change in height of 'Premium Red' was significantly lower when plants were grown under RFT. Our data suggests that when RZH temperatures >24 °C are utilized during poinsettia finishing, air temperatures can be further reduced from the 20/14 and 21/17 °C (12 h/12 h) recommended by previous research.