Growth and development of poinsettia (Euphorbia pulcherrima) finished under reduced air temperature and bench-top root-zone heating

• Cold-finishing with RZH is proposed for energy-efficient poinsettia production.
• Cold-finishing with RZH ≥24 °C is effective for producing high-quality plants.
• Time to anthesis and time to marketablility decreased with increasing RZH.
• Careful cultivar selection is important when implementing cold-finishing with RZH.

Greenhouse heating is an essential requirement for holiday poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch.) production, especially during the finish stage. Previous studies have shown that cultivars with early response attributes (short day initiation to finish in 6–8 weeks), moderate to high vigor, and naturally large bracts are most suitable for reduced temperature finishing (RTF) that leads to energy savings. We postulate that RTF in combination with root-zone heating (RZH) can further reduce greenhouse heating costs by allowing for even lower air temperature set points. The objective of this study was to quantify how RTF in combination with bench-top RZH influences growth, development, and quality of six red poinsettia cultivars. Rooted cuttings of poinsettia ‘Prestige Early Red’, ‘Prestige Red’, ‘Premium Red’, ‘Infinity Red’, ‘Viking Red’, and ‘Bravo Bright Red’ were transplanted and grown under a 16-h photoperiod consisting of natural days with daylength-extension lighting until 01 Oct. and a mean daily temperature (MDT) of 21.5 °C [24/19 °C (12 h day/12 h night)] until 15 Oct. On 15 Oct., the air temperature set point was reduced to a MDT of 15 °C [19/13 °C (8 h day/16 h night)], and plants were placed on a bench without RZH or with RZH set points of 21, 24, or 27 °C; or plants were moved into a greenhouse without RZH and with a constant 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 ‘Prestige Early 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 generally longer for cultivars grown under RTF without RZH compared to all other treatments. Although plants grown under RTF with RZH matured at a similar rate to those in the commercial control, bract area index was reduced under RTF in most cases. For example, bract area of ‘Infinity Red’ was reduced by 27, 25, 18, or 19% when finished without RZH in ambient RTF or with a RZH set point of 21, 24, or 27 °C, respectively, compared to the commercial control. Stem elongation responses varied by cultivar. Our data suggest that when RZH temperatures >24 °C are utilized during poinsettia RTF, day/night air temperatures can be further reduced from the 20/13 and 21/17 °C (12 h/12 h), recommended by previous research, to 19/13 °C (8 h/16 h).