Humidity And Marijuana Plants

We are going to take some time to delve into some much-needed information to further your understanding of the importance of humidity for seedlings when growing marijuana plants.

What is humidity

Humidity is expressed as the quantity of water vapor in the air. Humidity plays a necessary role in influencing how much evaporation occurs in cannabis plants. Your plant will take in more nutrition and water when the humidity is low. If for any reason the evaporation strain gets to be too much, marijuana plants will guard themselves against scarcity by closing their stomata. So naturally, this will stunt the growth of your plant due to the lack of water absorption.

Protecting marijuana plants from humidity problems

This is why you must have the proper humidity levels in your marijuana factory. When the plant is in the growth stage, it will need high humidity, unlike the blooming phase. This is mainly because the roots of baby plants are much smaller. Measure the humidity using a hygrometer. In the first stages of growing your plant, the humidity can be about 70%, and you can reduce it by 5 every week until it gets to 40%. Included in this article is a helpful schedule to assist you with the right information on humidity levels for both indoor and outdoor growing.

Also read Structure and Function of Stomata for more information about evaporation

Relation to temperature

Before we go any further, we have to discuss the connection between temperature and humidity. First you must understand that the percentage of absorbable water is determined by the temperature. At the temperature of 68 degrees Fahrenheit, air can take in up to 7.2 ml of water. In this case, air has a corresponding humidity of 100%.

Humidity and temperature in your marijuana grow room
Humidity and temperature in your marijuana grow room

With temperatures and humidity such as that you are guaranteed to see absolutely nothing because the water vapor is very high. At 32 degrees Fahrenheit, the air will take in around 5 ml of water. For this reason, air is much drier during the winter time than the summer months.

‘The hotter it is, the more moisture the air could hold at the same temperature.’

Jorge Cervantes

Since you will be airing out your grow room quite frequently, you will end up releasing the humidity as well. So you have to be sure that your marijuana factory is always moist so the humidity level goes up. Later in the article you will be informed of options for adjusting the humidity levels.


At first, your marijuana cuttings will have very small roots so they are not able to take in much water yet. At this stage, you want them to evaporate a very small amount of water. Cutting will only evaporate a small amount of water when the humidity is high, and require fewer roots.

Another way to reduce the amount of evaporation is chopping the bigger leaves halfway from the cutting. A humidity of 70% is exactly what you want when putting the cuttings in your marijuana factory.

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Your cuttings will begin growing roots in a clone box when the temperature is around 71.5 degrees Fahrenheit in a very humid environment and fluorescent lighting. I suggest the cuttings go beneath a 600-watt HPS lamp in an environment with 30% humidity for good results. For as long as the 2nd or 3rd week of blooming the roots are going to grow the size of the actual plant. The roots must be durable for proper water and nutrient absorption.

Also read How to Make Marijuana Clones

If you’ve produced a bountiful harvest with your desired attributes, cloning these marijuana plants is the next best thing to do so you can recreate the same results.


Seedlings require a separate set of rules since they have a taproot at germination that takes in moisture and grows fairly quickly. Never clip the leaves of a seedling. The leaves are necessary for absorbing light and evaporating water.

To help the seedlings take in water and nutrition through its leaves, have the humidity at high levels. Begin at 60% and slowly work your way down to 40%. Click this link to buy marijuana seeds

Flowering stage

As your plant start flowering, you can gradually decrease the humidity level. At this stage the roots are very mature, so the plant is able to take in the most nutrients and water. You will want to reduce the humidity level in the flowering stage since mold has a tendency to flourish in high humidity environments.

Also read Flowering Stage for Marijuana Plants

With age, there is a higher chance of your marijuana plant getting mold. As a result of the high humidity, water gathers around the tops and that when mold has the opportunity to develop. Bud rot is the most frequent type of mold.


Humidity during the flowering phase of marijuana plants
Humidity during the flowering phase of marijuana plants

Once the plant is fully grown you will need to start thinking about harvest time. Our free little Harvest Guide will help you determine the best moment to cut your plants.

Harvest Guide

harvest guide
  • Time your harvest for Perfect Taste
  • Get THC levels for a Perfect High
  • Don’t waste any Precious Bud

    Follow this schedule to ensure you give your plants the right amount of humidity for optimum growth. Notice there is a difference in clones and seedlings.

    Clones humidity level

    Growth Week 170%
    Growth Week 2 70%
    Flowering Week 165%
    Flowering Week 2 60%
    Flowering Week 3 55%
    Flowering Week 4 50%
    Flowering Week 5 50%
    Flowering Week 6 45%
    Flowering Week 7 45%
    Flowering Week 8 40%
    Flowering Week 9 40%

    Check this complete Marijuana Grow Schedule with temperatures, humidity, watering and nutrients tips.

    Seedlings humidity level

    Growth Week 160%
    Growth Week 2 60%
    Flowering Week 155%
    Flowering Week 2 50%
    Flowering Week 3 50%
    Flowering Week 4 50%
    Flowering Week 5 50%
    Flowering Week 6 45%
    Flowering Week 7 45%
    Flowering Week 8 40%
    Flowering Week 9 40%

    How to raise

    So, you have a few options for raising the humidity in your grow room. For starters, you can try spraying water on the walls and flower.

    Raise cannabis humidity

    Also, try putting the lights further up so the temperature will go down a little near the plants. This way you won’t have to turn on the extractor fan as often. If you put bottles or buckets of water around the grow room, or even wet towels, this will raise the humidity level.

    How to decrease

    Dehumidifiers are most certainly your best option. It has the capability of taking moisture from the air and draining it or holding it in a reservoir. Make sure to get a larger humidifier because the smaller ones fill up quickly.

    Decreasing humidity in marijuana grow room
    Decreasing humidity in marijuana grow room

    Once your marijuana plant starts flowering the humidity will need to be decreased, so it is necessary to dehumidify. Try using the extractor fan on a higher setting than usual or even you can even shoot cold air into the room.

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      Keep in mind that the humidity outside may influence the humidity in your marijuana factory. For instance, if on a rainy day the humidity increases quickly, you can shut off the extractor fan or turn it to a much lower setting. Make sure the temperature doesn’t rise too quickly because you’re getting less cold air from outside.

      How to measure

      Hygrometers are used to measure humidity levels (wiki on hygrometer at this link). All you have to do is put it over the plants, so long as it is in an area that easily aired out. The analog hygrometers cost $5 and you can buy a digital one for $10.

      The higher priced hygrometers are usually of better quality. For ease of use, get the one that has a wire attached to it. This makes it easier to see the humidity without having to turn the lights on in the room. These devices have a built-in storage bank that tracks the highest and lowest values, to help you determine how well you stayed within the preferred range. Buy the best hygrometers at this link.


      Watering your plants causes the humidity to go up quite a bit, which is fine when in the growth stage because you just spray the floor and walls to raise the humidity. During the flowering stage, humidity levels often go up way too much after you water your plants.

      Once the lights are no longer on in the grow room, the temperature decreases and extracting a lot of hot air is unnecessary, which usually is the cause of humidity increases.

      Watering marijuana and humidity
      Watering marijuana and humidity

      Nourish your plant with water once the lights are on, that way they will evaporate most of the water throughout the day. Understand that spraying water on top of the buds is pointless because you will raise the possibilities of the humidity rising and leading to mold.

      Outdoor plants

      Humidity is much less of an issue when growing your cannabis plants outdoors. The springtime and the start of summer offer higher humidity which works just fine since there are no buds present on the plant for moisture to gather. Any moisture on the plant from morning dew will easily evaporate throughout the day.

      Once Summertime ends, the flowering stage starts and there is a shift in the climate, resulting in cooler days and more rainfall. This usually makes the humidity higher. In the cooler months, the morning dew could be an issue since the sun is not guaranteed to come out and the temperatures are sometimes too low to evaporate it.

      Luckily a tad bit of rain will not cause the buds to rot, but just in case you may want to be on the lookout for this. In the last of the blooming stage, it might be a good idea to brush the dew off your marijuana plants in the mornings. If you notice that rain is expected, go ahead on move your plants to a location where they will be kept dry, away from the rain. It is always best to take the necessary precautions.

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      Thanks for reading. Please leave comments or questions below.

      Robert Bergman

      FAQ about humidity and marijuana plants

      What is the optimum humidity for a marijuana plant?

      The optimum humidity can vary based on factors such as the strain you’re growing and the stage of growth your plant happens to be in. As a rule of thumb, you can start new plants in the earliest phases of growth at around 70% humidity. Every five weeks, reduce the humidity by 10% until reaching a minimum of 40% humidity when the plant is in the later stages of its growth.

      How can I raise or lower humidity in my growing environment?

      Humidity is all about the amount of water in the air. The easiest way to control humidity is to use humidifiers and dehumidifiers, which are devices that add or extract water from the air. If you don’t want to invest in a humidifier and/or dehumidifier, you can try spraying water to raise humidity or using extractor fans to lower humidity.

      How can I monitor humidity?

      Monitoring humidity is simple. You can get an inexpensive device called a hygrometer which will tell you the humidity in the room.

      Why is humidity important for marijuana plants?

      Humidity is an important factor for all plants. Just like people, plants respire, exchanging gas with the atmosphere around them. When the air is dry (low humidity,) plants can lose moisture too rapidly. When the air is very wet (high humidity,) plants can be prone to problems like mold and mildew.

      Avatar for Robert Bergman

      Robert Bergman

      Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations. His passion for growing led him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]


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