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What Is The Best Humidity For Growing Cannabis Plants?

What Is The Best Humidity For Growing Cannabis Plants?

Cannabis is a high-value plant that has come to be primarily cultivated indoors due to its high sensitivity. Each stage of cannabis growth requires a specific kind of tune-up to achieve optimum growth and obtain high-quality yields. Humidity not only plays a roll in the growth of this incredible plant, but also in the proper curing of cannabis, and in proper storage containers and conditions while in storage.

While many cannabis growers identify sensitivity to the condition of the soil and the nutrients available for the plant, it’s environmental conditions that really matter. One of the environmental factors that affect the growth of cannabis is humidity.

Humidity can be defined as the amount of water vapor in the air. Generally, there are three types of humidity:

  • Specific humidity ‚Äď The specific humidity of an area is the ratio of the amount of water vapor to dry air.
  • Absolute humidity ‚Äď This is the actual amount of water vapor present in a given area.
  • Relative humidity ‚Äď This is the ratio of the actual amount of water vapor present in an area compared to the speculated maximum amount of water vapor that the area can hold.

In cannabis growth, as with any other plant, the relative humidity is used. It is also worth noting that humidity and temperature are interrelated. The amount of water vapor that air in an area can hold increases with increasing temperature.

What Factors Affect Relative Humidity Control For Cannabis?

There are three basic factors that weigh heavily in determining the correct relative humidity when growing cannabis plants. They are region, genetics, and stages of growth. Although there are lessor and differing situations, these three have the greatest impact.


Cold regions have less moist air due to the temperature relation stated above. Growing cannabis in these areas means that the relative humidity must be increased to an optimum level. On the other hand, hot or tropical regions require less control of humidity as most cannabis plants are highly favored by relatively higher humidity.


A good number of cannabis varieties have been developed over the years. Some cannabis species are adapted to warmer and more humid areas while others are adapted to cold and less humid areas.

Growing Stages

From germination to late flowering, the cannabis plants call for changes in the humidity levels. Since each stage comes with distinct metabolic reactions, the relative humidity needs also vary.

Why Is Humidity Important For Cannabis Plants?

To understand how humidity affects the growth of cannabis, let’s first understand the plant’s respiration process.

The sugars produced in the photosynthesis process must be converted to energy used to support plants’ growth. To convert the sugars, mainly glucose to energy and oxygen, later released to the environment, the cannabis plant must acquire carbon dioxide from the environment.

The carbon dioxide enters the plant via the stomata; tiny pores on the leaves. When this process occurs, some water from the plant’s water reservoir is lost to the environment. Relative humidity kicks in here as it dictates how much water the cannabis plant will lose to the environment.

Two scenarios can occur:

When the air is moist (high relative humidity)

The water concentration gradient is significantly reduced due to the higher amount of water vapor present in the air. This means that the cannabis plant will lose less water and hence remain in its best shape.

When the air is dry (low relative humidity)

The high water gradient created between the plant and the surrounding air causes high water loss. The marijuana plant will try to prevent this by closing the stomata, a process that compromises the vital respiration process.

In summary, relative humidity is vital to cannabis plants as it regulates the respiration process and its subsidiary activities such as water loss. For a cannabis plant to benefit fully from relative humidity, it must be maintained at an optimum level. It will also help if the humidity is also kept constant for an extended period.

Should Humidity Be Low Or High For Cannabis Plants?

Cannabis plants are grown in a relative humidity range of 70-40%. This is neither too high nor too low humidity. As discussed above, both extremes of relative humidity are potential causes of cannabis plant stagnation. It is also good to understand that high humidity may support the growth of mold that may affect the growth of your marijuana plants.

What Humidity Should I Have In My Grow Tent?

When you cultivate cannabis in a grow tent, conditions such as humidity and temperatures must be appropriately controlled. While by now we know that different growing stages of the cannabis plant require different levels of humidity, it would help if you kept the humidity in your grow tent relatively high for cannabis seedlings. Several factors affect the amount of humidity in your cannabis grow tent. These include:


If the lights of your grow tent are always on, the temperature of the contained air is raised and hence the relative humidity. Poorly lit grow rooms have cold and dry air (low humidity).


If your grow tent is adequately ventilated, the relative humidity is reduced due to increased air circulation. Ventilation is required for mature cannabis plants that require low relative humidity.

Frequency of Watering

Watering cannabis plants in a grow tent increases the plant’s water intake through the roots. High uptake of water by the plant increases the water loss rate through the leaves, making the room more humid. The induced moist environment, in turn, regulates the respiration process keeping the marijuana plant healthy. Again, watering should be optimized to maintain the grow tent in the recommended level of humidity.

What Humidity Is Good For Cannabis Flowering?

The recommended relative humidity for a flowering cannabis plant is 40-50%. The moisture is reduced to this level mainly to prevent the growth of mold that is very vicious at this stage.

Did you know that mold infestation on one cannabis flower can ruin your entire harvest? Lowering the humidity in this stage is, therefore very crucial and should not be overlooked. The humidity can also be reduced below 40% when the cannabis is close to harvest. This, however, depends on how your cannabis plants respond to changes in humidity.

In conclusion, it can be said that humidity is an essential condition in the growth of cannabis plants. Always start with a high relative humidity of about 70% for your cannabis seedlings and reduce the moisture gradually until the plant is ready for harvest, where the humidity should be around 40%.

Luckily, humidity levels in cannabis growing spaces can be detected easily using manual or automatic hygrometers. If your cannabis plants are in a grow tent, there is also an advantage of automatic reduction of humidity levels. Commercial size dehumidifiers and fans are used for this purpose.

16 comments on What Is The Best Humidity For Growing Cannabis Plants?
  • Dr. Peeper
    Dr. Peeper

    I live in a very dry climate so I actually have to supplement humidity for a large chunk of the grow. Off the shelf humidifiers don’t cut it for me in a 4×4. I was going through a gallon plus a day. The same applies for drying and trimming. Plus when it’s hot the Central AC makes it even worse. I use a 6" AC INF Exhaust, and a 4" AC INF Intake. I do not want to have to screw with vents. I made my own humidifier in a 27-gal tote using parts from House of Hydro. 5-Disc floating atomizer comapred to my original 1 lousy disc. It’s controlled using an AC Inf Controller 76 and ducts straight into the tent. I still maintain negative pressure but am able to move the heat out of the tent very effectively. It’s still upper 90’s right now in late Sept and power is expensive as hell in Cali. They make 12 disc. Toss in some lights and lasers and you have a Pink Floyd concert.

    September 29, 2022
  • That70sDude

    Unless you have your exhaust fan fill force but in a 4 √ó 4 I have been advised and successful opening a single vent, keeping 6" Infinity at 3 max and still get the right amount of negative pressure. I think a full on carbon filter is waist until flowering, but that‚Äôs me. I use a small washable filter with mesh wraps. When plants in full flower at 2nd ‚Äď 3rd week I open a 2nd vent‚Ķthe 1st opposite the exhaust and the 2nd on a perpendicular wall. Never the wall by the exhaust. That‚Äôs also when I install the AC Infinity Carbon filter. I also learned to only use a 0ppm water, R/O or distilled or Pure water from discount store. Measure the PPM because of it has any you can bet it‚Äôs cal Calcium‚Ķmany people think the have PM but plants don‚Äôt care for it either. These are suggestions from my initial errors

    July 21, 2022
  • Andy

    As with any 60+ humidity loving plant, just spray the covering, inside growth and especially outside growing early. Always try and cover early plants, late plants need more air flow. Keep the humidity up early and your usually on the money.

    June 28, 2022
  • Jesse F Fiordimondo
    Jesse F Fiordimondo

    If your using a tent I found soaking grouting sponges and leaving them in the tent to raise humidity. Rather than a bowl of water.

    February 04, 2022
  • David WILLIAMS
    David WILLIAMS

    I run an AC inline fan with temp/Humidity control in a 5×5 tent. You have to find that sweet spot especially when your lights go out for the dark period and often its a day to day adjustment on fan speed and heater temp. My tent is in the garage and I’m currently in veg stage so those garage temps also affect the adjustment.

    January 25, 2022
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