Brad Smith by Brad Smith
Co-Founder & Content Writer


One of the greatest advantages of farming in a grow tent is the ability to control the climatic conditions in there, and ensure your plants thrive throughout the year. As any experienced grower will tell you, the most important factor when growing top-quality plants inside a grow tent is humidity. Plants require a specific moisture content throughout their life cycle, and maintaining the optimum levels is crucial. If you can figure out how to lower humidity in grow tent when it gets high, you have nailed the number one secret to thriving plants.

What is Humidity?

Humidity is the amount of water content held in the air in form of vapor. In this article, you will notice the term relative humidity (RH) a lot, and it simply refers to a condition where the air is fully saturated with water vapor and therefore can’t hold anymore. For plants to thrive, relative humidity must be maintained at a specific level to ensure it’s not too high or too low.

What’s The Best Humidity Level in Grow Tent?

Plants need different levels of humidity at different phases. This is because of how much water the plant needs in each growth stage. For example, when plants are young and in the seedling stage, they require more humidity to survive than when they are older. Since young plants don’t have roots yet, their only source of moisture is through the leaves.

As plants grow and develop roots, their need for water through leaves becomes less because they get everything they need via the roots. During the vegetative, flowering, and late flowering stages, too much humidity will cause more harm than good because the excess moisture is not needed anymore.

tent humidity levels

Let’s look at the different stages;

#1 Seedling stage- 70-75% levels

As we said, plants are developing at this stage, and their roots are not working yet. This means they are only getting water from the atmosphere through their leaves. Having high humidity helps the plant to thrive and mature at this stage.

#2 Vegetation stage- 50-70% levels

At the vegetation stage, the plants have already started developing roots but they are not yet mature enough, so they still need water from the atmosphere for growth. If you lower humidity at this level, the development of the plants will become stunted.

#3 Flowering stage- 35-50% levels

The flowering stage is rather delicate because you want the plant to have the perfect environment for flowering. Too much humidity and the plant will start to rot and mold, while too low humidity may cause the plant to dry up. 40-50% humidity level is that perfect hit between too high and too low.

#4 Harvesting stage- 40-50% levels

The plant is fully grown at this stage and doesn’t require moisture for growth anymore. However, some humidity will ensure the yield doesn’t become brittle.

Why High Humidity in Grow Tent Is Bad and Causes Problems?

As a rule, the moisture content inside the grow room should not go beyond 70% even when plants are young.  If it does, you must find a way to lower it.

Why is this?

When relative humidity inside a grow tent is too high, the plants tend to absorb too much water into their system through the leaves. As a result, they absorb less through their roots. If the roots do not absorb much water, they are also not absorbing enough nutrients to keep the plants thriving. Consequently, the plants start suffering from mineral deficiencies, and you have yourself some weak, bad-looking plants.

Other reasons why high humidity is bad for your plants include;

  • High humidity is the perfect setting for bacteria and mold
  • The excess moisture will cause your flowering plants to develop bud rot and botrytis
  • It promotes unwanted microbiological growth
  • Too much water clogs the stomata and reduces the carbon dioxide intake, which in turn inhibits photosynthesis.
  • It minimizes your plant yield since most of the plants will either be rotten or weak.

Signs of High Humidity in a Grow Room

Ideally, you should have a hygrometer to measure the humidity levels inside your grow room. If you hang the hygrometer in a well-ventilated area above the plants, you can check the humidity levels regularly.

That said, some signs will make you know it’s time to lower the humidity in that grow tent;

  • White powdery mildew on leaves
  • Mold and bud rot
  • Poor leaf development and yellowing because of heating up
  • Growth and flowering are stunted

Why is the Humidity so High in my Grow Tent?

1. Increased transpiration

Did you know that plants only retain about 5% of the water they take in? The other 95% is then transpired back into the environment. That is a lot of water content to send back in the air, which is why you must ensure there is a good ventilation system to allow some of it to escape.

Unfortunately, the transpiration rate can sometimes overwhelm the ventilation system because either your plants’ leaves are too big or there are too many plants inside that grow tent.

2. Open water surfaces

Where does the air get its water content from? The answer is open water surfaces like rivers, oceans, lakes and man-made sources. If you have an open water surface inside the grow tent, it becomes an additional moisture source to the air and increases humidity. Such surfaces could be a mere container with water on the floor, a leaking irrigation system or any other water source inside the grow room.

3. Over-watering the plants

Again, plants only retain 5% of the water given to them and transpire the rest. If you over-water the plants, you are forcing them to transpire even more water into the air inside the grow tent. Unfortunately, there is only so much water content the air can hold, and this leads to high humidity.

4. The outside environment

While a grow tent helps to protect plants from external climatic conditions, it’s impossible to completely block out what is happening out there. If you live in hot, humid states like Florida, Texas and Hawaii, this humidity will definitely affect the humidity level inside your tent. The only solution to this to insulate the grow tent completely, but the slightest leak in the insulation will allow all that humidity to get in.

5. High plant density

Plants release carbon dioxide, which raises the temperature in the grow tent. As a result, the moisture content goes up. The way around this is to ensure you don’t have too many plants inside the grow tent, also known as plant density.

6. Grow lights

Grow lights can elevate the temperature in the grow tent if they are emitting too much heat. Choose lights that don’t emit much heat and turn them off when they are not needed.

Things to Do to Avoid High Humidity in a Grow Room?

We have talked about common culprits that cause humidity to soar inside the grow tent. Avoiding these triggers is ultimately the best way to ensure humidity doesn’t go above the recommended levels.

  • Water adequately. As we mentioned, too much water only causes the plants to transpire more and increase humidity. The excess water in the soil also evaporates when it’s hot and becomes humidity. To avoid all this, water your plants sparingly and only when they need it.
  • Do not mix mature and young plants. Again, mature plants require less water, and they transpire more through the leaves. This means their environment requires a more active approach to lower the humidity. If you place them in the same tent as young plants, the young plants will suffer because they are not mature enough for that environment.
  • Get rid of unnecessary water points. If you have any containers holding water, leaking systems, or pods of water on the floor, make a plan to get rid of them. Empty the water reservoirs, seal the supply points and get a proper drainage system for the grow tent. The water evaporates, raises the tent’s humidity, and becomes a perfect breeding environment for fungi and bacteria.
  • Use absorbent soil. If you remember your science well, there are three types of soil-loam, sand, and clay. Each has a different water-holding capacity, with clay soil holding the most water and sand releasing the most. When farming, use loam soil for its relative drainage abilities and nutrients. If dealing with excess moisture, you can go for sand soil as long as you keep an eye on the nutrient levels.
  • Insulate your grow tent. While you can’t control the humidity coming from outside the grow house, you can prevent it from getting in using insulation. You can get easy foil insulation from amazon or a store near you.

How to Lower Humidity in Grow Tent?

control humidity

You have checked your hygrometer, and its reading says the humidity in your grow tent is too high. Or maybe you have started to notice some signs of high humidity, such as bud rots and mold, and you want to take immediate action.

Here are ten things you can do to lower the humidity in grow tents, assuming you already have a working ventilation system and the tent is insulated against external factors.

1. Invest in a dehumidifier

The most effective way of lowering the humidity in grow tent is to use a dehumidifier, which works by sucking in all the moisture in the air. However, a household humidifier may not be sufficient in this case so you may want to invest in a commercial dehumidifier. While it may be expensive to buy, a commercial-grade dehumidifier is cheaper in the long run because it uses less power to do more work.

Best Humidifiers for Grow Tent

Be sure to read our complete buying guide for best grow tent humidifiers.

When choosing a dehumidifier, go with one that can take off the exact amount of humidity your plants transpire. It will also help if it comes with onboard humidistats to maintain the optimum humidity level without your help. If you live somewhere where the external humidity is too high, connect the dehumidifier to your drainage system, so you don’t have to keep draining the tank manually.

2. Use an air conditioner

Since temperature and humidity go hand in hand, using an air conditioner to lower the temperature inside the grow tent will lower the humidity. That said, choosing the right size of air conditioning unit is crucial. Too small a size, and it won’t control the temperature, and too big, it will end up causing condensation.

You can pair up a good AC unit with a fan to keep the environment cool and reduce water vapor. Be sure to also clean or change the filter regularly, just like you do with the home AC because a clogged air filter will prevent good airflow.

3. Boost up the ventilation

ventilation method

A sound ventilation system works by allowing enough airflow through the grow tent. If it’s not working well enough, you can boost it by;

  • Widening the intake holes or creating new ones. This will allows more air in and increase the efficiency of the exhaust fan.
  • Use a couple of floor fans instead of oscillating wall fans. Floor fans pull air through the plant canopy and ensure optimum exchange of old air with a fresh one.
  • Invest in an atmospheric controller, which besides helping the dehumidifier will also control the speed of the inlet fans depending on how humid or hot it is inside the grow tent.

4. Consider defoliating

As plants grow in age, they need less water through the leaves, and they transpire more. One of the best and cheapest tricks to go around this is to cut down some of those leaves. Without so many leaves transpiring water into the air, humidity in grow tent is bound to stay low. Be sure though to leave some leaves on because of photosynthesis and flowering.

Conclusion

For optimum growth and development of plants, indoor growers must monitor both humidity and temperature levels inside the grow room. Understanding how these environmental factors affect the growth of your plants is the only way to stay on top of them. As a rule, you must ensure enough airflow inside the tent and keep away anything that could cause humidity to go up.

Frequently Asked Questions

How high is ‘Too High’ for Grow Room Humidity?

Humidity levels vary from one stage to another, so it’s impossible to have a fixed relative humidity. That said, the best humidity level should not go above 60-65% to avoid damaging the plants.

How can I lower the humidity in a grow room at night?

Plants grown in grow tents use light at night for photosynthesis. Unfortunately, the lights can also emit heat and cause temperature and humidity to rise. You can lower the humidity by using lights that don’t emit too much heat and keep the dehumidifier and fans on all night.

How do you lower humidity in a grow room quickly?

The fastest way to lower humidity inside a grow tent is to crank up the air conditioner and dehumidifier. After this, you can work on removing the factors causing high humidity and find a working formula for maintaining the best humidity levels.

About the author

brad smith sm
Brad Smith

Hello there, I am Brad “Omni” Smith, the proud founder and chief editor here at OmniHomeIdeas.com.
So, while they say, „Home is Where the Heart Is“, we believe that home is where every story begins. If you’re passionate about the nest that you call home, join us at Omni Home Ideas.

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