Humidity is defined as water vapor held in the air. Humidity can be presented in different measurements: absolute humidity, specific humidity and relative humidity. Medical marijuana growers are concerned with the latter, relative humidity. Relative humidity is expressed as a percentage of the water vapor in the air at a specific temperature compared to the maximum capacity of water vapor at that specific temperature. For example, your room’s humidistat reads 60% humidity at 75° F. This means the water vapor level is at 60% of the maximum water vapor level possible at 75° F.
For most indoor grow environments, humidity tends to run on the high side. This is because of the mass amount of vegetation we cram into a relatively small area. Plants transpire moisture at surprising rates, which can give an unsuspecting grower a sea of new problems. The best investment against high humidity is, you guessed it, a dehumidifier.
A good dehumidifier can be a life saver from molds and mildew. Make sure you get a dehumidifier capable of handling your room size. Generally speaking, bigger is better. Buying a dehumidifier with a built in humidistat will save you the extra cost of buying an atmospheric controller.
High-powered exhaust fans can also be used as a defense against higher humidity. A good fan can double as temperature and humidity control and, in some grow rooms, may be all you need. If you decide to use an exhaust fan for humidity control, I highly recommend investing in an atmospheric controller with a humidity setting. There are times when plants transpire unexpectedly, during the dark cycle for example. A humidity controller will ensure the room gets exhausted when the relative humidity gets out of the desired range regardless of the temperature.
Most medical marijuana grow rooms are broken into different stages: cloning, vegetation and blooming. At these different stages, medical marijuana requires different photoperiods, food and even humidity levels for optimal results. Through my experimentation, I have found higher humidity levels are beneficial through earlier stages and can be detrimental in later stages. Remember, all plants are different and, depending on a plant’s particular lineage, humidity requirements can vary. The following is a good generalization and should be used as a starting point in your experimentation.
Clones or Cuttings. I have found that an extremely high humidity for the first 3-4 days after clones have been taken is most effective for negating stress. I have had the most success with an 80-100% relative humidity. After the first few days, the cuttings should be able to acclimate to the ambient humidity of the vegetative room.
Vegetative Room. Although this may sound high, I have found most medical marijuana strains like their vegetative growth to be in the 65-75% relative humidity range. Because there are no flower sets established, the worry for mold is not as prevalent. I found with some varieties I had to add a humidifier to maintain a 70% relative humidity for optimal vegetative growth. If your room shows any sign of mold or mildew I do not recommend running a humidity above 65%. Experimenting with higher humidities is definitely something to try when all other environmental factors are in check.
Flowering Room. I always recommend trying to keep the humidity of your flowering room under 60%. I try to run 50-60% relative humidity in my flowering room. Solid air movement and a quality dehumidifier can help you achieve this goal fairly easily. The flower sets in a medical marijuana plant are made up of tightly packed vegetation unable to transpire moisture as easily as large shade leafs. Keeping the humidity below 60% hinders molds and mildews from targeting your tightly packed nugs.