Marijuana humidity

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.

Cannabis plant and humidity
Marijuana & Humidity

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

Cannabis humidity and its 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%.

Measuring temperature and humidity inside grow room
Measuring temperature and humidity

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.

Plant Phase:Humidity levels (RH): Temperature during day:Temperature at night:
Vegetative 40–60%71–82°F64–75°F
Early flowering40–50%68–79 °F 68–79 °F 
Late flowering30–40%64–75°F60–68°F

Controlling humidity levels of your grow room can get you better results. Download my free Grow Bible for more growing tips!

Cannabis clones 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.

Cloning marijuana plant
Cloning cannabis

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.

Planning to grow marijuana plants indoors? Find the best indoor marijuana seeds here.

Indoor marijuana seeds

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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.

Cannabis seedlings humidity levels

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.

Read our comprehensive guide if you want to learn more about how best to germinate your cannabis seeds. And if you need to know what the best cannabis light schedule is, read our guide.

Cannabis seedlings
Marijuana seedlings

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

Humidity during the 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.

Levels of humidity in cannabis 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.

Marijuana flowering state
Humidity during the flowering phase of marijuana plants

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.

Marijuana clones humidity level
Growth Week 170%
Growth Week 270%
Flowering Week 165%
Flowering Week 260%
Flowering Week 355%
Flowering Week 450%
Flowering Week 550%
Flowering Week 645%
Flowering Week 745%
Flowering Week 840%
Flowering Week 940%

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

Marijuana seedlings humidity level
Growth Week 160%
Growth Week 260%
Flowering Week 155%
Flowering Week 250%
Flowering Week 350%
Flowering Week 450%
Flowering Week 550%
Flowering Week 645%
Flowering Week 745%
Flowering Week 840%
Flowering Week 940%

How to raise the humidity in your marijuana grow room

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.

Increasing humidity in grow room
Changing humidity in grow room

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 the humidity in your grow room

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.

Reduce humidity inside marijuana grow room
Reduce humidity inside 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.

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 humidity levels in your weed plants

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.

Measure humidity levels inside grow room
Measure humidity levels

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.

Watering your marijuana plants

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 indoor grow room, the temperature decreases and extracting a lot of hot air is unnecessary, which usually is the cause of humidity increases.

Watering marijuana seedling
Watering marijuana

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 marijuana plants optimal humidity levels

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.

Marijuana plant outdoors
Cannabis plant outdoors

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.

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.

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Robert Bergman


Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations ... See profile

46 comments on “Marijuana humidity”

  1. I have tried to raise the humidity in my grow tent several ways at NO AVAIL… I am in the flowering stage of growing strawberry cough in a 3 by 3′ indoor grow tent however my hygrometer will not raised higher than 20% humidity I can’t get it to increase I have one of the airflow vents open with a fan but there is heat in the Room that the tent is in due to no insulation in that room,, however it is cold Back here and need to have the space heater I know it says about growing strawberry cough that she likes to be warmer than other strains but I can’t get it to raise the humidity at all on my hygrometer right now I am trying a crockpot so if you have any suggestions please help me she is in her 2nd week of flower and she Gets fed 2 gallons of water with her nutrients 3 times a week.

  2. This Is Incomplete/Incorrect……Adjusting humidty alone will hurt your crops no matter what they are. You need to adjust your temperature to work with the humidity…

  3. Hi
    And thanks for your explanation!!

    My sativa plant outdoor, growed in warm and more less humidity, what can i do? To prevent of related problems?

  4. Great article! I learned alot. My skywalker OG is looking very well in its 3rd week of veg. My Black widow is in its 2nd week of veg and coming right along. My Bruce banner is in its 1st week of veg and is finally starting to show some good growth. Thank You guys for workung hard to keep us up to date on the best growing techniques .
    Already a customer and very satisfied. If anyone reading this is looking for quality product and great customer service look no further.
    Thanks again. #Jayfla850

  5. Dear site owner. I am from Ukraine and I can access your site when I use VPN, but it does not open and says “Access denied” if I try to open it directly. I cannot understand the reason why you hate Ukraine 🙂 be a good guy, please.

  6. Hi,
    I have mi first plant growing. It’s on her day 5 but in The morning when i go to check the plant, the humidity is always 97%. It see that this always happens overnight when the lights are off. Do you have any advice?

    • Do you have a fan ? If not you need a fan to pump cool air in your grow area to help keep it dryer. Make sure you only water when the lights come on so the plants can absorb the water much quicker. Make sure your not leaving any open water bottles or even a wet rag in the room because that will cause an increase in humidity also. Every little bit helps bro.

  7. Kind of hard to move already established hugh plants out of rain or high humidity. So what else can you do? I put fans outside, just like citrus growers do! Hope it works!

  8. You have a interesting grow that I’m going to follow. I’ve grower 35+ yts and do good but going to try this in one tent and see if I produce a good harvest versus mine . Really nice grow you are talking about . Thanks man grow on Johnny James

    • Only problem with tents is when u get mites n bugs they stay with plants. Introduce safers insecticidal soap or home brew. But does affect/taste no matter what. Keep your open grow room flat white and clean is better. I know it’s hard to control humidity and temperature but trust me tents constrict also

      • Humidity needed for proper growth changes slowly. Start 70 and reduce 10% every 5 weeks til 40%. Then theres your bloom humidity. Higher temps for veggie then slowly reduce when its blooming.

      • Temperatures are in relation to where original ma and pa were from. Start by finding what original strain came from and adjust temp but not humidity levels

      • Humidity level too low in growth slows transpiration. Which is greatl y needed in growth stage. In moderation. Bloom slowly drop temp and humidity for mold/mites and proper transperation needed for harvest

      • The humidity% listed is for clones. Adjust to 60% for seeds. Adjusting humidity about same as clones

  9. You guys are absolutely awesome thanks for all the information and I’ll just free stuff you offer. Information wise. Could you lower your prices a little bit kinda High otherwise A+ with everything else from start to finish. Glad I am customer IL GM recommend very highly to anybody even first timers, use Autoflower seeds in you will not fail in the hundred percent money back guarantee makes me want to stay with you for that reason alone. Big company. America loves you

  10. erroneous information. Humidity should never drop below 60%. Growth rates drop by 1% for every 5% below 60. at 40% that’s 8% lower growth rates.

    Put clones under HPS at 30% humidity. Wow. Just. Wow….

    he’s probably never even heard of VPD.

  11. Bob,

    FYI… When viewing your site with a Mobil device, plastered across the top of the web page is a large view of your logo. Not a big deal unless one is attempting to read your comprehensive compilation of info. in public places I.e on a 2 hour flight, ect. ..
    Thank you,

  12. […] If you install all of your ventilation and air flow equipment properly, it will also be significantly easier for you to manage the humidity and temperature of the environment in your grow room. Keep a weather eye on the temperature and humidity: you’ll want a high humidity (50-70%). Air in the room should be between 68 and 77 degrees Fahrenheit (20 and 25 Celsius) while the substrate should be close to 64 degrees fahrenheit (24 celsius). If you are using a hydroponics system or growing your plants in water, you’ll want the water to be a lukewarm 68 degrees fahrenheit (20 degrees Celsius). Read more about Temperature and Humidity. […]

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  14. […] Temperature and humidity need to be at the right level for your plants to reach their highest budding potential. If the temperature is too high, your buds won’t have as potent of an aroma. Controlling your temperature can mean controlling the amount of potency and smell of your buds – when done properly. Make sure you pay special attention to your grow room’s temperature throughout its entire life cycle. If you do this, the results will definitely be to your liking. Read more about temperature or humidity […]

  15. Lori. I suggest you join our support forum, and post us a picture in order for us to see what your issue is. I am sorry, but I did not really understand what you are asking. 🙂

    Mike. I suggest you invest in a shade cloth. 50% should do.

  16. I am starting my seedlings with 19 hrs. Of light 8-9 hrs outside the plants seem to love it but it’s 90-98 degrees everyday. I use fluorescent the rest of the time. Isn’t the high temps bad for them even though the sun is Nat.

    • Im in Nevada its the best because i dont have to take water out the air. Its easier to setup your own environment

  17. thanks robert was wondering why my plants seemed to stop growing. I have amnesia haze in 7th week of flower and it looks like they just stopped growing. I am going to alter the humidity like you said and see what happens. I will let you know how it works out . Thanks from a newbie keep it green !!:]

  18. Robert, are you certain the growth aspect is only two weeks? My plants take at least 4-6 weeks of growth.