Response of Chrysanthemums to Various Day–Night Air Temperature Differences/Drops and Root Zone Temperatures at the Same Daily Integrated Temperature

Temperature is one of the most important climate factors in greenhouse ornamental cultivation since it not only affects plant growth rates, quality and time of production cycle but also heating costs. It has been known that proper root zone heating could reduce energy consumption without compromising plant growth & quality. However, there has been little research on the interaction between root zone heating and air temperature management in ornamental crop production. Since the highest heating energy use usually occurs during the pre-morning or early morning periods, temperature regimes with temperature drop during pre-morning and/or early morning periods could reduce energy use. In this study, two growth chamber trials were conducted to investigate the response of Chrysanthemums to various temperature regimes with temperature drops at the same daily integrated temperature and their interactions with root zone heating for conserving energy while ensuring crop quality. Four air temperature regimes with different day-night temperature differences (DIFs, +6°C, +3°C, 0°C, and -3°C) were applied in 4 large growth chambers (one for each chamber). The different DIFs were achieved mostly by lowering the pre-morning or early morning temperature down to 13°C. The air temperatures in other periods during a 24-h period were also adjusted to maintain the same 24-h average temperature (19°C) or daily integrated temperature for all 4 temperature regimes. Four root zone temperatures (20°C, 23.3°C, 26.7°C, and 30°C, set-points) were applied inside each of the air temperature regimes (chambers) with 4 electrical root heating mats.

The air temperature regime with +3°C DIF at 24°C root zone temperature (actual bottom media temperature measured with needle thermocouple, 26.7°C set-point) had the highest biomass, leaf area and flower number as well as good flower size, and slightly taller plants. Therefore, this combination of air temperature regime and root zone temperature has achieved better plant growth and quality except for slightly taller plants. The air temperature regime has good energy saving potentials because of its low temperature during pre-morning (13°C) and early morning period (15°C).