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How to Increase Humidity in Your Grow Tent

Last updated: June 30, 2020

Increase Humidity in Your Grow Tent

It’s not very difficult to grow a cannabis plant. But if you want to grow that primo sticky icky that’s covered in crystals and produces a beautiful and strong high, you’ll need to get certain factors right to keep your plants growing at their healthiest and happiest. Temperature, nutrients, light cycles and more are easy to remember, but there is another consideration to keep in mind; humidity.

Plants need very specific humidity levels to thrive, and they can range from strain to strain and even through the life cycle of a single plant. But knowing what humidity your plants desire is only a small part of the battle. The real challenge is matching the humidity in their environment to your plants’ preferences. In this article, we’re going to discuss how you can raise the humidity in your grow tent to keep your plants comfortable and ensure they reach their full potential.

Understanding Relative Humidity

Humidity seems like a fairly simple concept on the surface. How much moisture is in the air? But there are different ways to measure humidity. When it comes to horticulture, we’re going to be mainly focusing on relative humidity. So, what is relative humidity?

Relative humidity is the amount of moisture in the air relative to the ambient temperature. Air can hold different levels of moisture depending on how hot or cold it is. Hot air can hold considerably more moisture than cold air. Relative humidity is expressed as a percentage of the total volume of water that the air can hold at any given time, up to 100%. Once relative humidity reaches and surpasses 100%, it means there is more moisture than the air can hold. This results in condensation, dew, fog, and other forms of moisture.

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When the temperature drops but the amount of moisture in the air stays the same, the relative humidity increases because the lower temperature air is able to hold less total water than the warmer air, so that same volume of water represents a higher percentage of its total capacity. Likewise, if the temperature raises but the total moisture in the air stays the same, the relative humidity percentage will go down since the air is capable of holding a larger volume of water now.

Factors That Affect Relative Humidity

There’s a lot going on in your grow tent, and all of it can affect the relative humidity inside. Each of these factors affects it independently, but they also interact with each other.


As we’ve mentioned, changing the temperature of an environment will also change the relative humidity. In a grow tent, you have fans and lights that are helping to adjust the temperature. Lights produce heat, which can lower the relative humidity of the grow room. Not all lights create the same heat, though. Modern LED lights create much less heat than metal halide or high-pressure sodium lights. This means they’ll have less of an effect on the relative humidity of your grow tent.


Ventilation and airflow will have a large effect on the relative humidity of your grow tent. If the air is being exchanged too quickly then it can dry out and lower the humidity of the environment. But too little airflow means the air will stagnate and the humidity will rise.


Since humidity is a measure of moisture in the air, it makes sense that having more water in the environment would lead to a higher level of humidity. The larger your water sources and the more water present in your grow tent, the higher relative humidity levels you will experience.

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How Much Humidity Do Cannabis Plants Need?

Now that we understand relative humidity and some of the factors that can affect the humidity levels in your grow tent, let’s talk about how much humidity your cannabis plants really need. Your plants will intake water through their roots mainly, but they’ll also absorb some of the moisture in the air through their leaves. This leaves their roots freer to absorb the vital nutrients they need to grow and thrive. But your plants’ humidity needs will change through their life cycle.


When your plants are first starting and they’re very small, their root systems are not developed enough to uptake loads of water. Hence, they’ll be absorbing a lot more through their leaves, so the humidity levels will need to be high enough for them to properly hydrate. For these young plants, humidity levels should be somewhere in the range of 65%-80%.

Vegetating Plants

Once your plants are out of their initial growth stage and they’re ready to start vegetating, you’ll need to decrease the humidity in your grow tent. At this stage, humidity levels of 55%-70% should suffice. Keep in mind, each strain has its preferences, so these numbers are just a range to get you started.

Flowering Plants

Now that your plants have made it through the vegetative stage and they’re ready to begin flowering, they’re going to be asking for even less humidity. When they first start flowering, you’ll be dropping the humidity to somewhere in the 40%-50% range. As they get later into the flowering stage, you’ll want to keep reducing the humidity in their environment until your about 30%-40% relative humidity by the time they’re ready for harvest.

Young plants, seedlings, and clones Vegetative stage Early flowering stage Late flowering stage
65-80% 55-70% 40-50% 30-40%

10 Ways to Increase the Humidity in Your Grow Tent

Now that we know what factors affect humidity and approximately how much humidity your plants will need at any stage of their growth, let’s discuss specific ways to increase the humidity inside of your grow tent.

1. Add More Plants

Plants “breathe” through a process called transpiration. Basically, they’re excreting water vapor through their leaves after absorbing it through their roots. Naturally, this transpiration results in rising humidity. Adding more plants to your grow tent will increase transpiration, and thus, increase the humidity of your tent. These plants don’t have to be cannabis, though. Any type of large plants will help increase the humidity.

2. Reduce Ventilation

Don’t remove your ventilator fans entirely, but try setting them on low. Your plants need fresh air and they need circulation, but too much airflow can dry out the air and your plants. Try reducing the airflow and let it raise your humidity.

3. How to Increase Humidity in Your Grow Tent

It may seem simple, but hanging some wet towels near the air inlet of your grow tent is a great way to add moisture to the air. As the air passes over and through the wet towels, it will pick up the water in them and hold it, increasing the humidity level of your grow tent.

4. How to Increase Humidity in Your Grow Tent

Since plants in the first stage of their lifecycle require the most humidity to thrive, you need a way to keep them wet and happy. Luckily, propagators make this very easy. It’s like a small plastic dome that keeps all the moisture inside. You’ll see the water droplets forming on the lid as condensation; proof that your seedlings/clones are getting plenty of humidity.

5. Add a Humidifier

When we need more humidity in our homes, what do we do? We add a humidifier. Well, what works for us will also work for our plants. Adding a humidifier is a great way to instantly increase the amount of humidity in your grow tent. If your humidifier has different levels to choose from, you can even keep adjusting your humidifier to match your plants’ changing needs throughout their growth cycles.

6. Reduce the Heat

Hot air can hold more total water than cool air so, with the same amount of moisture in the air, a hotter tent will have lower relative humidity. By removing some of the heat you can raise the relative humidity. If you’re using several lights, try removing one or more of them to reduce the heat output in your tent and see how much it affects the humidity of your grow.

7. Spray the Walls of Your Tent

Using a simple spray bottle, spray the walls of your grow tent each day. When these water droplets evaporate, they will increase the humidity level of the tent. You can raise the humidity more by spraying more water or applying it multiple times throughout the day.

8. Use a Swamp Cooler

Swamp coolers add moisture to the air while they reduce temperatures. This results in more water in the air, plus a higher relative humidity from the decrease in temperature.


9. Add Water Sources

More water sources mean more places for moisture to come from. If you add a bucket of water or several small containers of water to your grow tent, there will be more water to evaporate, adding moisture to the air.

10. Cool the Tent Down

If you add an air conditioner to your grow tent, the decrease in temperature will result in an increase in relative humidity. This is a simple way to raise the humidity without adding any water. But be careful as plants may not like it if you make their environment too cold!


Throughout the different stages of growth, your cannabis plants are going to have different humidity needs. You’ll need to adjust the humidity levels in your grow tent to accommodate these changing needs if you want to produce the best buds and largest harvests. We’ve discussed how your plants’ needs will change through their life cycle, from needing high humidity as seedlings and clones, to their much lower needs during the flowering stages. We talked about relative humidity and what factors affect it. Finally, we covered ten different ways that you can increase the humidity in your grow tent right now. You could add a humidifier, use an air conditioner to decrease the temperature and raise relative humidity, or you can simply spray the sides of your tent with a water bottle each day.

Featured Image Credit: TinaKru, Pixabay