One successful approach is to allow the warmth of the sun to raise the daytime temperature and then to compensate for this by lowering the night time temperature. Another proposed strategy is to lower the daytime temperature and then save energy by raising the night time temperature while using an efficient thermal screen.
In the experiments described here, young plants of 'Covington' and 'Irvine' were grown at set points of 12, 15, 18 and 21 degsC by day from the onset of short-day treatment. Ventilation was set 2 degsC higher and the same average CO2 concentration was maintained in all treatments. In the treatment with a daytime temperature setting of 18 degsC, the night set-point was also 18 degsC and the night settings in the other three treatments were adjusted so as to produce the same average temperature.
A day temperature of around 15.5 degsC was actually achieved in the treatment with a 12 degsC setting and this required a night temperature of at least 20.5 degsC to compensate. Flowering was delayed by this treatment, which suggests that this approach should be adopted with caution.
Effects of Day and Night Temperature on Chrysanthemum Morifolium: Investigating the Safe Limits for Temperature Integration by Adams, Valdes and Fuller (2009). Journal of Horticultural Science & Biotechnology 84 (6): 604-608. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.