Chapter 5 - Monitoring Temperature and Humidity

Monitoring Temperature and Humidity

Gardeners monitor heat and humidity levels by using thermometers to measure temperature and hygrometers to measure relative humidity. The desired ranges are:

Temperature: The best range is above 85 and below 95 degrees Fahrenheit with CO₂ injection. Some experiments indicate that maximum growth rate occurs at around 95 degrees with CO₂ at 1500 ppm. There are other experiments that indicate that CO₂ enhancement does not do much for the growth rate below 75 degrees. It is also recognized as a good practice to keep the temperature below 85 degrees if the ambient air is being used without CO₂ enhancement.
Humidity: In the literature, it is often stated that the grower should keep the relative humidity above 40% and below 65%.

The 65% point is OK as a rule of thumb, but problems can still occur in the high fifties and low sixties range. Most fungus stop growing when humidity levels are low and there is constant internal circulation from powerful oscillating fans.

Gardeners sometimes overlook the humidity build-up during the dark hours. Some growers have their fans set on the same timer as the lights. However this is the time when molds

are most likely to grow. To prevent attack the fans should be left on during the dark hours to prevent mold.

Some gardeners use a small heater to keep the temperature up and the humidity down during the lights’ off cycle. A dehumidifier serves both purposes, raising temperature and lowering humidity.

As the temperature drops, the relative humidity increases. Hot air can hold more water than cold air. The same amount of water dissolved in cold air creates a higher humidity than when dissolved in hot air. An automatic humidistat operating a blower or dehumidifier can be left on during the dark hours. The well-equipped growth chamber with remote hygrometers and thermometers allows the grower to observe the conditions in the grow room during the dark hours, without disrupting the dark cycle.