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Controlling Humidity Indoors

Cannabis: Controlling Humidity Indoors

Optimal cannabis humidity levels chart


by Nebula Haze & Sirius Fourside


To grow the stickiest, most potent buds, your cannabis will demand attention to its many needs.

One of these needs of the growing cannabis plant that is commonly overlooked is humidity.

Believe it or not, humidity is actually very important to your plant, and it will help determine its resilience against mold/mildew in addition to how much your plants need to drink.

Luckily, there isn’t much to learn, so you can be the master of humidity in your grow with very little effort!

Today, we will give you the information to master humidity and thus, give your plants a boost in production!

Water droplets on a cannabis leaf is a sign of high humidity

Why Humidity Matters To YOU

In all stages of cannabis growth your plants will have a constant need to intake water, and the amount of water they need fluctuates with the humidity in your grow room. When the humidity is high, cannabis plants use their leaves to absorb moisture from the air which causes them to  drink less water from their roots. Conversely, when the humidity is low, they will pull more water in through their roots since less is available in the air.

Purple glittery cannabis bud - Picture by 2moreweeksSince humidity changes how much water your plants drink, being in control of humidity gives you increased control over nutrient intake.

But controlling humidity isn’t just about prevention...having the right humidity encourages strong, healthy, leafy plants with vigorous growth. In fact, growing with DWC (Deep Water Culture) / Bubbleponics during vegetation with the correct humidity can actually turn into a struggle to tame out-of-control plant growth!

Additionally, after your plants start making buds in the flowering stage and get close to harvest, you might be able to manipulate the humidity to get your plants to produce more resin (trichomes/glitter which contain THC and other cannabinoids) while preventing plants from being attacked by mold. High humidity can sometimes cause mold - a grower’s worst nightmare for those fat, dense main colas.



Effects of Poor Humidity Management

We’re not out to scare you, but you should know what happens to a grow room with poor humidity management. We’ve experienced every single one of these effects, so don’t feel bad if you’re just learning this stuff!

White Powdery Mildew

  • White Powdery Mildew is a fungus that will shows up high humidity environments. This can be tricky since young cannabis plants flourish in humid environments. Luckily, you can stave off WPM by making sure there is proper airflow in your grow area; a small oscillating fan on low works wonders.

White powdery mold growing on cannabi leaves like spots of flour

Bud Mold or Rot

  • Bud rot or mold is - in my opinion - one of the worst pains a grower can experience as a result of poor humidity management. Imagine harvesting a huge, dense main cola from your plant only to see the insides are totally white or brown with mold. Buds in this condition are unusable and 100% should be thrown away.

Bud rot / gray mold can happen as a result of too-high humidity in the flowering stage

Nutrient Problems

  • Humidity is a factor that partly determines how much your plants will drink. If the air is dry, your plants will tend to drink more.  If they’re already drinking a lot due to high temperatures, low humidity can cause them to drink a lot of water and uptake too-high levels of nutrients. If your plant takes in more nutrients than the plant can use, the leaves will begin to show yellow or burnt tips, which is the result of nutrient burn. Sometimes too-low humidity can cause other apparent nutrient problems.

This marijuana plant has the first signs of nutrient burn - brown "burns" or spots alone the edges and tips of the cannabis leaves

Slowed Growth

  • Cannabis plants love higher humidity when they’re young, and lower humidity when they’re mature. To be frank, if they’re not getting these conditions, they’re not growing nearly as fast or healthy as they could be.



Humidity Basics

Okay, you know why you need to manage humidity when growing cannabis, but you may not know what humidity you are aiming for.

Humidity is actually a measure of how much water vapor is being “held” in the air. There are different ways to measure humidity, including absolute, specific, or relative humidity.

Most growers are talking about Relative Humidity or RH when they talk about humidity in the grow room.

Relative humidity measures how much water is in their air compared to the maximum amount of water that can be held in the air at that temperature.



Optimal Humidity is 70-80% RH

A new cannabis clone showing her first rootsClones need time to develop a root system to intake water. Young clones are solely dependent on getting water through their leaves from the air. This is only possible with high humidity.

Maintaining high levels of humidity will dramatically increase the chances of clones rooting successfully, and will increase young clone growth.

This is why many growers use a humidity dome for new clones.

Here’s a step-by-step picture guide to cloning cannabis:



Young plants (seedling to end of vegetative stage)

Optimal Humidity is 40-70% RH

40-70% Relative Humidity is ideal for vegetative cannabis plants - This pictures shows a humidity monitor (called a hygrometer) in the correct rangeYoung cannabis plants in the vegetative stage grow a lot of leaf mass in a great growing environment, and plant can sometimes grow foliage at a faster pace than the roots.

The roots aren't the only way your plant gets water. Leaves can actually pull water vapor directly out of the air, and higher humidity in the air allows the leaves to get more water to the plant if needed. Until your plant has fully developed roots, higher levels of humidity in the air will tend to promote faster growth.

That’s why younger plants tend to grow better with high relative humidity, especially in a comfortably warm environment that mimics springtime or summer.

Warning Signs:

  • The RH is too high if the plants are forming wet spots on their leaves
  • If the RH drops below 25%, the dry air will tend to limit growth and cause nutrient problems to appear on leaves, especially for young seedlings



Humidity for flowering plants

Optimal Humidity is 40-50% RH (or less)

Ideally, the relative humidity of your grow room should be lowered to around 40-50% at the beginning of the flowering stage. As plants approach harvest, some growers lower humidity down to 30% or even less to force cannabis buds to produce more resin.

By the beginning of the flowering stage, your cannabis plants have created a large root system to fulfill their water needs. They will still be able to take in water through their leaves, but a relatively lower humidity than the vegetative stage helps prevent molds or mildews from forming.

Mold is especially dangerous in the flowering stage because it can form on or inside your buds or colas, without you knowing. If you find that you have mold growing in any of your buds, or buds are rotting from the inside, you should immediately throw all contaminated buds away. Bud rot can ruin entire harvests, so if one bud is infected, thoroughly search your grow room for more. If you can't lower the humidity, cut your losses and harvest now, to avoid any other buds getting bud rot.

Even if the humidity is low, it is still important to have air moving over and around your plants. Small fans blowing over and under the plant canopy will help keep air moving so that wet spots don't form around any parts of the plant.

Extra resin production

  • During the last 2-3 weeks before harvest, some growers will use a dehumidifier to drop the humidity of their grow area as low as possible. This not only prevents bud rot during the last few weeks, the dramatic drop in humidity may stress the plant in just the right way to increase resin production, as a result, you harvest extra-potent buds with more sparkly trichomes. While this technique has not yet been proven to increase resin, many growers swear by it.

Critical Hog cola - trichome monster



Humidity for drying buds (early stage cure)

Optimal Humidity is 55% RH (for beginners)

Glittery bud in hand - Picture by DVDAmoogKeeping the humidity around the 45% - 55% range will allow your buds to dry, but keep them from drying too quickly.

This range is also optimal for preventing mold, so it doesn’t get any better for drying if you're a beginner!

As an advanced drying and curing technique, some growers like to keep the humidity a little higher so buds dry slower (which tend to increase the quality of your cured buds). The reason this is considered advanced is higher humidity during drying must be done with care to prevent mold! 

It's important you have good air circulation in the drying area, but it's recommnded that no air is being blown directly over the buds. More information in the link below!


Temperature and How It Relates to Humidity

Temperature and relative humidity are closely related to each other.

Temperature and Humidity Monitor - Digital - Available from Amazon.comRelative humidity is measuring how much water is “being held” in the air compared to the maximum amount of water that can be held at that temperature.

Warm air can “hold onto” more water than cool air. Plants will tend to thrive at different relative humidities depending on the temperature of the air. Once the air becomes too saturated with water, it will tend to form dew or films of water over leaves, which leads to mold.

For those interested in learning more about the science behind this, there is a term used by greenhouse growers known at VPD, or Vapour Pressure Deficit, that roughly measures the temperature and relative humidity.

According to Wikipedia: “the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 kPa.”

When growing plants, temperature is just as important as the Relative Humidity (RH) of the air, if not even more so! You need to control both temperature and RH at the same time to get the best results when growing cannabis.

  • If the air is too hot and dry (high VPD), plants will tend to have stretched growth.
  • If the air is too cool and humid (low VPD), plants are more prone to problems with mold or fungus.

Here’s what that means to you, the grower:

Cannabis likes a comfortable room temperature, or a little warmer - not too dry, not too humid. For most growers, that is all you need to worry about.

If your grow room feels warm or cold, humid or dry, that is a sign that you may want to look into changing the temperature and/or humidity of your grow area.

Generally, cannabis plants prefer temperatures in the 70-80 °F (21-27 °C) range during the day when lights are on. When lights are off (or at night), cannabis plants are happy with slightly cooler temps.

What if it gets too cold?

Colder temps will tend to slow down growth. Temps lower than 60°F (15°C) tend to upset plant growth and freezing temperatures will shock or even kill a cannabis plant.

Can cannabis stand higher temps? What if my grow room is too hot?

Please note that too-high temps in the flowering stage will not only slow down bud growth and cause fluffier buds, but it will also tend to reduce the potency of your buds - keeping the grow room temperature under control is important in the flowering stage!

Advanced Tips

  • With bright grow lights (such as 600W HPS and larger), cannabis plants can thrive at temps up to 85°F (30°C) even with no CO2 enrichment, as long as there is low relative humidity and plenty of air movement.
  • Cannabis plants can thrive at higher temperatures, up to an ultimate max of about 95°F (35°C), in an environment with low humidity, extremely bright grow lights and enriched CO2 (above 1500 PPM) - this is not your average grow!
  • Cannabis prefers slightly cooler temperature at night (warmer night temperatures will tend to slow down growth - so you can use this knowledge and raise night temperatures to slow down stretching plants that are growing too tall too quickly)



How to Control Humidity in the Grow Room


Step 1: Get a Humidity & Temperature Monitor to see if you need to adjust anything

I like the Ambient Weather WS-07 Wireless Thermo-Hygrometer because it lets you remotely check the temperature and humidity from another room (don't forget to get 6 AAA batteries).

Step 2: Refer to Chart

Ideal temps are 70-80 °F (21-27 °C) during the day (lights on) and slightly cooler at night (lights off).

Refer to the following chart to determine if relative humidity is in the ideal relative humidity range:

  • Young plants (seedling to end of vegetation)

    • 40-70% RH (younger plants prefer higher humidity - if you see wet spots forming on the leaves, humidity is too high or you need to increase air circulation)

  • Flowering plants (making buds)

    • 40-50% or less RH (a little less humidity helps protect plants from mold - during the last few weeks of flowering, a lower RH can possibly increase resin production as buds approach harvest)


Step 3: Correct humidity and/or temperature that is too high or too low

Many indoor growers tend to deal with too much heat and high temperatures - this is because grow lights can get very hot, and cannabis grows are often confined to very tight spaces due to stealth or privacy concerns.

The relative humidity of your grow area depends mostly on where you live, and how you’ve chosen to construct your grow area. Many growers struggle with lowering their humidity, because the mass of leaves and vegetation in a small grow area tends to cause the humidity to go up.

Here’s how you can correct common cannabis problems with heat and/or humidity.


  • Humidity is too high - How to lower Humidity
  • Humidity is too low - How to increase humidity


  • Too hot - How to lower temperature safely
  • Too cold - How to safely warm things up

Humidity is too high - how to lower humidity

  • Dehumidifier -  will pull moisture from the air in a small area like a grow room or tent - a good one can be hooked directly to a drain for very humid areas like the UK and other humid parts of the world, otherwise dehumidifiers will automatically shut off when full and you will have to manually remove the water that it collects from the air.

  • Increase ventilation (add a more powerful exhaust fan or otherwise improve your exhaust system) - this increases the total amount of airflow in the grow area. This strategy will only work to lower humidity if the intake air has a lower RH than the air in the grow room.

    Advanced: If you do get a high-powered exhaust fan to lower humidity, you may want to consider getting atmospheric controller with a humidity setting to automatically adjust the fan speed to achieve the humidity you want.

Diagram of an exhaust system

  • Avoid over-watering plants - soggy soil (or leaving extra water in plant trays sitting in the grow area) will tend to raise the humidity of the grow room

  • Consider slight defoliation - very leafy plants will tend to raise the humidity of the grow room, especially when there isn’t good ventilation or air flow.

  • Air Conditioner (some are portable, some ACs fit in your window) - tends to lower humidity (in addition to cooling the air)

Humidity is too low - how to increase humidity

  • Humidifier - will add additional moisture to the air, I like the evaporative type of humidifier because it also works to cool down temps slightly. Make sure you get a humidifier with a large holding tank (holds at least 6 gallons of water at a time). Those "one-room" humdifiers with a 1.5 gallon tank often need to be refilled several times a day.

Get an evaporative cooler on Amazon.com
This humidifier is an evaporative swamp cooler that holds 6 gallons of water at a time - great for hot, dry climates.

  • Swamp cooler - will cool down air while increasing the relative humidity; works best when humidity is too low and temperature is too high.

Temperature is too high (too hot) - how to lower temperature & cool things down

  • Increase ventilation (add a more powerful exhaust fan) - this increases the total amount of airflow in the grow area. This strategy will only work to lower temperature if the intake air has a lower temperature than the air in the grow room (if the air outside is hot, too, adding more ventilation won’t do much to lower the temperature)

Diagram of an exhaust system

  • Air Conditioner (some are portable, some ACs fit in your window) - in addition to cooling the air, an AC will also tend to lower the relative humidity of the air


  • Light Schedule - Consider having your lights turn on during the cooler parts of the day (for example some growers have their lights on at night, and keep lights off during the hottest parts of the day)

Temperature is too low (too cold) - how to raise temperature & heat things up

  • Insulation - Consider insulating your grow box or grow tent with something that holds in the heat

  • Bigger grow light - Get a bigger (and hotter) grow light, such as a 600W or 1000W HID grow light

  • Heating mat - placed under plants



Check It Out!

Monitor temperature & humidity in your grow room!

I like the Ambient Weather WS-07 Wireless Thermo-Hygrometer because it lets you remotely check the temperature and humidity from another room (don't forget to get 6 AAA batteries).

Monitor temperature & humidity even while you're away!

Here's a really simple solution that lets you check your temperature and humidity of your grow room in real-time, even if you're not at home!

  1. Buy an indoor/outdoor temperature+humidity sensor and display, such as the model  listed above

  2. Put the "outdoor" module in your grow tent or box.

  3. Put the "indoor" display in front of a webcam (you may have one built into your laptop, or buy a cheap and easy-to-use one like the Logitech HD Webcam C310)

  4. Install LogMeIn (it's free)

  5. You can now log into your computer from any other computer, open the program for the webcam, and see what your grow room stats are in real-time.

Thanks to Tokiehardo for this awesome tip!


Beautiful cannabis plants - Trophy picture by thatsnotmybike

Too Hot, Too Dry - Humidity Story from a Grower in Denver

by thatsnotmybike

I'm preparing things for my first grow and trying to make sure that I plan everything 'right' and I'm not surprised by too many things, and hopefully none too late to fix.

Humidity has me confused. I've seen sources saying the greater the humidity the better, some that say humidity makes no difference as long as the plant is hydrated, and some claiming nothing will grow if you're not nailed at 45%.

The ambient humidity near Denver is pretty low - the room I plan to grow in sits at 20% on average. Is this too low for my plants to be happy?


This humidity is too low for healthy growth of cannabis plants. This grower decied to build something known as a swamp cooler: http://burningman.rengeekcentral.com/swampcooler.html

It moistens the air while keeping the temperature down, which can also be a problem in Denver.

Update from the grower: For anyone stumbling onto this in the future, the swamp cooler in the link above is friggin awesome. I have it connected to my intake ducting and it's keeping the cabinet at 73F with 80F outside temps! I cannot downplay how great it works, but I recommend creating a way bigger reservoir, so you don't have to refill it every 6-8 hours.

Beautiful macro pic of trichomes from cannabis bud - Trophy picture by thatsnotmybike

View current grow by thatsnotmybike here: http://www.reddit.com/r/thatsnotmygrow/