Relative humidity (RH)
The relative humidity of the air influences the functioning of the stomata, among other things. Cannabis flourishes the best with an RH of 60-70%. At higher RH percentages, the stomata have problems getting rid of excess water. At a lower RH, the stoma keep releasing water until the plant dries out. At that moment, the stomata close. Then, the intake of CO2 stagnates, and plant growth is impaired. The relative air humidity is also influenced by the temperature in the growing space. In the chart below, you can see the number of grams of water which can be absorbed in a 25 m3 room (for example: 3 x 3 meters, and 2.5 meters high). Absorption in grams of water (degrees C) 0 degrees 120 10 degrees 240 20 degrees 460 25 degrees 630 30 degrees 840 35 degrees 1120 40 degrees 1460 It may be concluded from this chart, that with every rise of 10 degrees in temperature, the air humidity doubles. Ventilation influences the relative humidity. Ventilating a space makes the RH fall. In some cases it’s necessary to install a humidifier in the grow room. The best results can be achieved by using a discharge fan with a variable speed control. This way, you can easily regulate the quantity of air to be removed. When the plants are in the dark, the temperature is lower (the lamps don’t give off any heat). So, you would expect the relative humidity to fall (less moisture can be absorbed by the air). But this is not the case; RH increases in the dark. The plants breathe out water in darkness. Therefore, sufficient ventilation must be provided. Too high a humidity level provides considerable risks for the health of the plants. Generally, pests and diseases (see Chapter 8) have a better chance with a high humidity level. Too low an RH is also risky; the plants can easily dry out. Prevention is better than cure . . . Finally, it should be stated that young seedlings and clones generally perform better at a humidity level of 65-70%. Their root systems are not yet developed well enough to take in water fast enough. A higher humidity insures that the young plants will be protected from drying out.