In nature, cannabis ruderalis plants live in areas where there are short summers and little rainfall so we can assume that these plants are very strong and don’t love high humidity.

This statement is absolutely true and you should be more careful with autoflowering cannabis plants and humidity.

What is humidity?

Humidity basically is the amount of water vapor that is in the air.

Absolute humidity is the maximum amount of water vapor that the air can hold at a specific temperature.

While the real or the relative humidity is measured in percentage from this absolute humidity value.

How do you measure humidity?

The only way to measure the humidity of the air is to buy a hygrometer.

There are various hygrometers out there and some are more accurate than others but almost all of them will help you understand what the situation in your growing area is.

I personally use a hygrometer that has a high/low feature to see how much the humidity and temperature dip at night and rise in the daytime. Acurite humidity meter

Best humidity for growing autoflowers

The optimal humidity ratio varies at different parts of the plant’s life and there are also other factors that may change the optimal humidity ratio.

Humidity in the pre-flower stage

At the early stages of autoflowering plants life (pre-flowering), the humidity must be somewhere between 40% and 70%, and it should not surpass the 90% mark or else the plant will suffer and will not be able to evaporate the absorbed water through the leafage.

Lower than 20% humidity can also stress the plant and it will not be able to grow and reach its full potential.

Humidity during flowering

When the plants start flowering the humidity must be lowered below 55%.

The ideal humidity for autoflowers during the flowering stage is somewhere around 20% to 40%.

While the autoflowers are flowering, high humidity can increase the chances of infections and mold.

If the humidity is too high, dense buds can be a perfect spot for the mold spores to land and start spreading which will ruin the entire crop in a matter of days. This mold (typically gray mold) starts spreading from the buds inside and you can’t detect it before it is already too late to save the plant.

To sum up, at the pre-flowering stage, you must keep your humidity from 40% to 70% but when your autoflower plant starts flowering lower the humidity to below 50%.

How to lower the autoflower humidity?

The first step to lowering the humidity for autoflowers is to increase the air circulation in the grow area. Only when that doesn’t work you should start thinking of other solutions.

If the increase in air circulation doesn’t help, one way to reduce the humidity is to increase the temperature in your grow room. However, autoflowering cannabis plants don’t like temperatures that are above 32°C (89°F) which is why this solution won’t work for many growers.

Another thing you can try is to put a bowl of rock salt in the grow room and that will also draw the humidity down.

If none of the previously mentioned solutions work then your only other option is to buy an electronic dehumidifier that will draw the water from the air lowering the humidity.

In the autumn months when the humidity rises I personally use a small, portable dehumidifier that can be placed even in a small closet grow room.

 How to increase humidity?

Since humidity basically is the water vapor that is in the air, introducing water to the room will quickly increase humidity.

You can mist your air with a hand mister or put a bowl of water in the room and let the water evaporate in the air increasing the relative humidity.

However, you probably shouldn’t use the hand mister in the flowering stage of the plant’s life because if the water gets on the dense buds mold or other infections can start to grow.

In extreme cases of low humidity, you can use an electric humidifier that will increase the humidity by constantly releasing water vapor into the air. But can quickly turn your grow room into a humidity dome, which is very bad for flowering plants, so using an electric humidifier for autoflower growing probably isn’t the best option.

Now that you know more about the best humidity levels for autoflowering cannabis plants, learn about what should the ideal temperature be for autoflowers!


  1. Many thanks for the humidity info for auto flowering plants…. Much appreciated information with do and donts.
    Great read, well conceived and well received. yah man

  2. Kneehigh85 on

    Keep in mind when adding a humidifier might I suggest a cool mist instead of a warm mist humidifier. The warm mist humidifier might be good in the beginning stages of vegetation but you will want to replace it with a cool mist humidifier. I recently learned this with my grow, the warm mist can make the room a little too warm and if left unattended (running out of water) it can make the warm air dry causing the plant to drink more than usual due to the low temp to Rh ratio and this can cause heat stress and also be a problem when you’re feeding bottle nutrients because the plant will take in the nutrients much faster than it should potentially causing nutrient burn which can be resolved by flushing the soil with ph’d water feeding a small dose at quarter strength and placing in temps lower than 80 with an Rh of 55%. This is why I recommend a cool mist humidifier it makes for temp and Rh control much easier and most cool mist humidifiers come with adjustable settings for output. Please keep in mind I’m speaking out of my general experience and in no way speak on behalf of the site or the author of this information.

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