It is estimated that 80% of the cost of heating a greenhouse is incurred at night. Given long winter nights, heating become a major cost in producing plants in Northern climates at a time when fuel costs are rising all the time. Lowering greenhouse temperatures can have some positive effects: branching is stimulated, number of flowers and size of flowers tends to increase, development rate of insects pests is reduced. If the night temperature is reduced while maintaining the same daytime temperature, quality increases as measured by dry-weight increase. On the other hand a cooler regime has negative effects such as: lengthening the production time to flowering (more in some varieties than others), it increases the possibility of disease because some diseases are more active at cooler temperatures (e.g. Phythium, Rhizoctonia, Botrytis, Thielaviopsis) and wet leaves and greenhouse surfaces favour the spread of disease. While plants will generally require less water at cooler temperatures, any free water takes longer to evaporate and roots are at greater risk from root rots. Chemical activity is lower and therefore any curative effects will take longer. The pros and cons of these factors needs careful examination also in relation to the DIF (difference between night and day temperature) if we intend to save heating costs without reducing productivity and quality. Ohio Florist’s Association Bulletin No. 865 www.ofa.org E-mail ofa@ofa.org