Marijuana Humidity

Achieve Optimal Humidity Levels In Your Marijuana Garden

Keeping your  marijuana grow room at the proper humidity level is a balancing act all cannabis growers face.

Not enough humidity, and your cannabis plants won’t thrive. Too much humidity, and mold and mildew become a concern.

Humidity is water vapor held in the air, and it’s described using different measurements: absolute humidity, specific humidity and relative humidity.

Marijuana growers are concerned with 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 marijuana grow 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.

Depending on where you live, humidity problems are most likely seasonal.

But they’re also created when plants transpire moisture into your grow room atmosphere.

If you’ve got fast-growing plants in deep water culture, using C02, and running a hotter than average grow room, your plants could create humidity problems because they’re taking in and transpiring so much water.


For many indoor marijuana grow environments, humidity runs too high. This is because of the mass amount of vegetation crammed into a relatively small area.

Marijuana plants transpire moisture at surprising rates.

One obviously good investment is a dehumidifier.

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 saves you the extra cost of buying an atmospheric controller.

Many  brands of dehumidifier suck, so be sure to read customer reviews and product evaluations before you spend your money.

A professional dehumidifier good enough for a normal-sized marijuana grow op will cost at least $250.

High-powered exhaust fans are also used as a defense against higher humidity in marijuana grow rooms.

An exhaust fan can double as temperature and humidity control. If you decide to use an exhaust fan for humidity control, invest in an atmospheric controller with a humidity setting.

Of course, another way to control humidity is to have dedicated air conditioning for your marijuana grow op. This has the added advantage of temperature control.

Note there are times when your marijuana plants transpire unexpectedly, during the dark cycle for example.

A humidity controller ensures operation of fans and/or dehumidifiers when the relative humidity gets out of the desired range regardless of the temperature.


Most marijuana grow rooms are broken into different stages such as seedlings/cloning, veg phase and grow phase.

At these different stages, marijuana requires different photoperiods, food, and even humidity levels for optimal results.

In general, higher humidity levels may beneficial through most of grow phase but are detrimental in bloom phase.

Remember, each marijuana strain and even individual cannabis plants are different, so humidity requirements can vary. The following is a good generalization and should be used as a starting point in your experimentation.

Clones or Cuttings. Extremely high humidity for the first 3-7 days after cannabis clones have been taken is most effective for negating drought stress. I’ve had the most success with 80-95% relative humidity in a cloning dome.

After clones develop roots so they can intake and transport water properly, you can lower the humidity and take them out of their dome.

Most marijuana strains like vegetative growth to be in the 55-65% relative humidity range. Because there are no flower sets established, the worry about mold is not as prevalent.

Some marijuana strains may dry out or have leaf problems if they’re growing in too arid an environment, and you may even have to get a humidifier to boost humidity.

If you’re worried about molds or mildews for any reason, including the fact that you might be growing marijuana that is prone to molds and mildews, keep your humidity lower than 60% in grow and bloom phase.

During bloom phase, and especially if you’re seeing dense, fat buds, you want humidity lower than 58% but not lower than 51%.

Gray mold loves to grow inside dense marijuana buds.

If you see mold or mildew in your marijuana grow room, or are worried about an outbreak, take a look at the humidity conditions that favor mold or mildew, and then alter your humidity levels to directly go against those favorable conditions.

Also be sure to invest in sufficient aeration fans so you blow fresh air into the cannabis canopy. Stagnant, humid air is bad for buds.

Controlling humidity is just one of many duties you have as a successful marijuana grower. Keep your grow room humidity at the right levels, and your marijuana plants will do much better for you.

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