GENERAL CULTURAL PRACTICES (Enviromental Conditions Required) – TEMPERATURE

Temperature: The optimum growing temperatures (day and night) vary for different plant species and different varieties within a species. For example, if you decided to grow tomatoes:

• Germination and post-emergence temperatures = 23-25 C (74-77 F)
• Production optimum temperatures = 22 C (72 F) day/20C (68 F) night.

One of the major reasons for a greenhouse is to control temperature. Air at 70° F will hold twice as much moisture as air at 50°F. In the range of temperatures encountered in a greenhouse, for every 20° F rise in dry bulb temperature, the water-holding capacity of the air doubles, and the relative humidity is reduced by one-half. This relationship is important in managing humidity in the greenhouse. Most of the crops grown in a greenhouse are of tropical or subtropical origin or are grown at inclement times of the year. The greenhouse is used to maintain an optimum night temperature (above freezing). This is usually accomplished with some kind of heating system. During the day, however, the greenhouse structure can actually interfere with temperature control by trapping heat. Therefore, various cooling methods must then be used. Temperature is a measure of heat energy. The energy balance or movement of energy into, within, and out off a greenhouse occurs by three different methods:

1. Conduction: diffusion of thermal energy through a continuous medium, the rate of which depends on the properties of the medium. Heat energy movement is always from a region of high temperature to lower temperature.
2. Convection: diffusion of thermal energy between two dissimilar materials, usually between a gas and liquid, gas and solid, or liquid and solid. Heat energy movement is always from a region of high temperature to lower temperature.
3. Radiation: radiation heat transfer occurs when electromagnetic energy leaves one object and is intercepted and absorbed by another. This differs markedly from conduction and convection. All objects emit radiation, warmer objects more so than cooler objects. The wavelengths usually considered to be involved in radiation heat transfer are in the infrared band.

Radiation Cooling: On clear, cold nights, plants and other objects within a greenhouse will lose heat to the outside by radiation cooling. Because objects within the greenhouse are much warmer than those outside, they lose heat by emitting infrared radiation through the glazing and into the clear sky. Under such conditions, the foliage temperature may be 5°F cooler than the surrounding air. Radiation cooling occurs very little when skies are cloudy.

Condensation: When the foliage of plants in the greenhouse are cooler than the surrounding air, moisture in the air may condense on the leaf surfaces. This is most common in the spring and fall when days are bright and warm and nights are cool and clear. The moisture that collects on the leaves serves as an ideal medium for the germination of several disease-causing spores, mainly powdery mildew. Condensation also occurs on the inside pf poly-covered greenhouse when the inside air is warm and humid and the outside air is cold. Under these conditions, condensation can rain-down on the foliage creating an environment for diseases.

Greenhouses heat up during the day for two reasons: 1) the greenhouse effect, 2) because the greenhouse is an enclosed space. How much a greenhouse heat up during the day depends on how much solar radiant energy is coming in through the glazing, what happens to that energy, and how much is retained.