Greenhouse - Effects on Crops When Considering Growing at Lower Temperatures I

Although prices have dropped recently, continued high energy prices are prompting many greenhouse growers to consider lowering their greenhouse temperature setpoints to reduce their monthly heating costs. This is the first post on the subject taken from an article from the Michigan State University Greenhouse Alert Newsletter.

Temperature influences crop timing, plant quality and energy consumption, as well as other issues. When determining a growing temperature, it is important to understand how temperature influences plant growth and development so that growers can optimize their production schedules and still produce high quality plants on time.

Temperature controls crop timing

Within the temperature range of most greenhouses during the winter, plants develop leaves and flowers progressively faster and faster as temperature increases. Thus, turning down the thermostat during the day or night will delay crop timing. In other words, if you grow plants at cooler-than-normal temperatures, production time will increase. This means that, to finish a crop on the same time as last year, you must begin growing the crop earlier in the year at the cooler temperature compared to the date you would need to start your crop at a warmer temperature.

Plants respond differently to temperature

As temperature decreases, there is some temperature at which a plant ceases to develop. This temperature is called the base temperature, and varies from crop to crop. For example, the base temperature for seed petunia is about 39°F (4°C), which means that at or below this temperature, petunias stop growing. For seed vinca (Catharanthus), the base temperature is much higher, around 50°F (10°C). Vinca placed in a 45°F (7.5°C) greenhouse will not develop leaves or flowers, but a petunia will continue to grow (just slowly).

As temperature increases above the base temperature, plants grow faster and faster. Figure 1 illustrates the time it takes for petunia and vinca plants to flower at various temperatures. As temperature increases above the base temperature, a small increase in temperature can make a big difference in the time to flower. As we get to warmer temperatures, the same increase in temperature has a smaller effect on accelerating flowering.

Lowering the temperature by 5 degrees has a somewhat small effect at warm temperatures, and has a larger effect at cooler temperatures. The effect of lowering the temperature also depends on the crop. For example, lowering the temperature from 65 to 60°F would take petunia about 13 days longer to flower, and would take vinca about 30 days longer to flower.

Adapted from "Temperature effects on crop timing and plant quality" by Erik Runkle,
Horticulture, in the October 28, 2005 edition of the Michigan State University Greenhouse Alert Newsletter.