Greenhouses are controlled environment buildings. The main part of the environment that is controlled is the greenhouse temperature.
The greenhouse furnace and the ventilation fan(s) are controlled by thermostats. The thermostats should be placed in or just above the crop canopy in more or less the middle of the greenhouse. If the thermostat is placed near the crop canopy, the grower will know that the plants are receiving the proper temperatures.
Many people feel the most convenient place to locate a thermostat is near the furnace, the ventilation fan(s) or near the door so anyone can change the setting easily. Unfortunately the temperature near the door, near the furnace, near the fan or near a ventilation louver is not representative of the greenhouse.
Once the thermostat is placed near the crop canopy it should be protected from direct sun so it does not give a false reading from solar gain. Use a few pieces of Styrofoam to shield the thermostat. It would be best to place the thermostat inside an aspirated box. This is a small box with an air inlet on one end and a small fan on the other end. Greenhouse air is pulled through the box to give the thermostat a correct evaluation of the greenhouse temperature.
Thermostats are not perfect, they may not read the correct temperature. It is important for a greenhouse operator to have a standardized thermometer in the greenhouse to check on the real greenhouse temperature. Anyone can standardize a thermometer by placing it in a glass of ice water. The thermometer should read 32 degrees F if it is accurate. It would be best to have this thermometer in the aspirated box, as well.
The best type of thermometer to use is a Max-Min thermometer. These should be available at farm and garden stores. The thermometer tube is U-shaped with a maximum and minimum temperature scale. The tube also has a small piece of metal enclosed on either end of the mercury. As the mercury rises to reach the highest temperature of the day, it pushes the metal piece. As the temperature falls, the mercury falls, but the metal stays in place. The same happens for the lowest temperature of the day. Thus if the greenhouse operator checks the greenhouse once a day, he/she can determine the highest and lowest temperature that occurred in the greenhouse since the last time they checked and reset the thermometer.
The thermometer is reset by gently shaking the indicators back against the mercury or by the use of a small magnet. It is best to reset the thermometer when the actual temperature is between the daily high and low. Two or three such thermometers could be placed around the greenhouse to give an indication of hot or cold spots and, thus, the uniformity of the greenhouse environment.
If the greenhouse operator also takes a moment to write down the temperature on a piece of paper, they will have a good record of the temperature performance of their greenhouse over the spring period. A record of the high and low temperatures each day is an outstanding way to identify problems that may have occurred with the heating or ventilation systems.
Aspirated boxes are not common is small greenhouses, even though they work very well. There are alternatives. Place the max-min thermometer in a small Styrofoam cooler, that has holes in the sides, and float the cooler on the water next to the plants. This should give a good measure of crop temperatures each day. Place other max-min thermometers at other locations for more temperature information. A greenhouse operator could also place a few quarts of water in a similar cooler and place a standard thermometer in the water. The water in the cooler will equilibrate to the average daily temperature of the greenhouse. If this thermometer is checked and logged daily, more temperature information will be gained.
The temperature of the water in the float bed is quite important to know during germination and the early stages of plant growth. Simple floating aquarium thermometers are an inexpensive way to determine the water temperature. Unfortunately, experience has shown that there are great variations in water temperature in a float bed so regular observation of the water temperature may not be predictive for crop response.