Temperature Changes

Plants have optimal temperatures, where they are most productive, and base temperatures, where plant growth stops. Reducing the greenhouse temperature will save energy on a daily basis but may slow plant growth, requiring a longer growing period and, ultimately, more weeks of heat and additional energy. Plants respond more to the average daily temperature than to short-term temperature fluctuations, which allows the use of a temperature differential between day and night. Since 70 to 80 percent of the heating occurs at night, lowering the night temperature will reduce energy costs. If the daytime temperature is increased by the same amount that the night temperature was lowered (assuming an equal day and night length), the average temperature is the same and plant growth should not be affected substantially, provided the temperatures are above the base temperature for the plants being grown. If the day (high temperature) and night (lower temperature) lengths are different, then a weighted average will need to be used.

An example: if daytime temperature is 75°F for 10 hours and night-time temperature is 60°F, then the weighted average is
                                            ((75 x 10)+(60 x 14))/24 = 66.3°F.

For cool-season plants like pansies, the base temperature is 34°F and the optimal temperature is 72°F, while for warm-season crops the base and optimal temperatures are 50°F and 95°F, respectively.
The temperature difference between day and night temperatures is often referred to as DIF. As DIF increases, plants tend to elongate faster, which may be undesirable and require the use of plant growth regulators. A 10°F DIF will result in about a five percent energy savings. Growers will need to weigh the cost of growth regulators versus energy savings.