Humidity is probably one of the most important air quality parameters to control but it is the most poorly designed and executed. If it’s too humid, mold growth can destroy the entire crop. If it’s not humid enough, plants can get stressed and die.
Nutrient absorption: humidity levels influence how much water marijuana plants absorb which influences the number of nutrients it absorbs. A low humidity level causes plants to drink more resulting in “nutrient burn”, which gives the leaves a yellow or burnt color.
Altered growth: marijuana plants in early stages need a higher humidity level whereas more mature plants prefer lower humidity levels. Moisture is one of the factors that control the opening and closing of the stomata. Dry air causes the stomata to close to limit the loss of water.
What Humidity level for which growing stages?
Overview: Drying: While the marijuana is drying, relative humidity of 50 percent should be maintained. This stops the plant from drying out too quickly because the water leaves the cannabis slowly.
Lights On and Off: During the Light-On period, humidity levels should be as per the set point (indicated in the above table). As soon as the lights are turned off, the plants will release moisture into the air, the RH% level in the air must be reduced and HVAC systems should respond with de-humidification mode.
Young Sprout: Seedlings and clones a high humidity levels of 65-80%. Because the root system is not developed, high humidity allows the absorption of water through the leaves.
Vegetative growth: Humidity levels can be reduced by 5% per week (acceptable range: 40-70%). Now the roots absorb more water, and the evaporation by the leaves refreshes the plants, so moisture content in the air can be reduced. Flowering: Humidity levels should be lowered to between 40 and 50%, as flowers can be affected by high humidity levels typically above 55% RH. Each grower will have their own recipe at this stage since moisture content in the air will affect the appearance and flavor of the crop.