Controlling Indoor Cannabis Humidity

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Temperature and Humidity gage
Author avatar By Derek LaRose
February 26, 2021
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Cannabis humidity, airflow and temperature (HAT) make up the golden triangle of optimizable environmental conditions. They help with cell flexibility, turgidity and many other processes. Get the cannabis humidity part right and you’re on the way to amazing results. Get it wrong and you’re on the path to major problems. 

It is quite often for novice growers to jump straight on the latest trends on increasing yields for cannabis. However, veteran weed growers know that it takes a lot of little things done right to get optimum yields – controlling temperature and humidity for growing weed are just parts of the entire pie. 

PotCast: Creating the Perfect Climate for Growing Indoors and Controlling Cannabis Humidity

In this podcast episode titled “Creating the Perfect Climate for Growing Indoors and Controlling Humidity,” join Kronic for 15 minutes as he tries to weave through the business of perfecting the humidity for growing cannabis indoors. 

Why is Cannabis Humidity Important for Marijuana Plants?

Humidity is essential to marijuana plants since it helps to think about how it will absorb air from its environment. All varieties of plants respire carbon dioxide to the atmosphere through their leaves, and they naturally lose water retained in their foliage. 

Cannabis Indoor Controled Environment

Humidity refers to the amount of water vapor in the air. The amount of water the air can carry depends on temperature. Naturally, hot air can handle more moisture than cold air. 

How Cannabis Plants Absorb water

A growing cannabis plant will continuously need to intake water as it needs water to fuel its growth. One factor that affects the way cannabis takes in water is humidity. 

Cannabis plants will use their leaves to absorb water and moisture from the air, however, this costs them lessened water absorption from the roots.

Conversely, lower humidity will have drier air and cause the cannabis plant to lose more moisture to the air as it respires. Once the air humidity becomes too dry, the cannabis plant will tend to lose more water to the air than they can acquire through their roots. 

Cannabis Water System
A cannabis watering system

To lessen the loss of water to the air, the cannabis plant will close off its pores which entail less respiration – this causes the plant to become unhealthy and look limp.

Therefore, it is safe to say that humidity can influence how the cannabis plant regulates its nutrient intake. Controlling cannabis humidity and temperature is one way of increasing control over your crop’s intake of nutrients.

So, what happens when you subject cannabis plants to constant bad humidity levels? Some of the common illnesses that cannabis plants can acquire actually come from bad humidity management inside a grow room. Coupled with unreasonably high temperatures, cannabis heat stress might also happen.

Here are some illnesses that come from either too high or too low humidity: 

Bud Rot

A symptom of too much moisture in the air. High humidity allows moisture to stay on the leaves of the cannabis plant allowing fungus responsible for bud molds to thrive. Buds afflicted by bud rot are unusable and cannot be rescued.

White Powdery Mildew (WPM)

This is another fungal infection coming from high humidity. It presents as white patches or spots around the leaves similar in look to flour. If left unattended, WPM can spread across the leaves and into the buds and affect the entire crop. Although WPM can be easily fixed, preventing it through control humidity indoor grow room is better.

Nutrient Burn

Yellow leaf tips are a sign of nutrient burn. As mentioned earlier, dry air caused by low humidity forces the plant to use its roots for water intake. However, this forces the cannabis plant to uptake more nutrients from the soil. This results in a nutrient burn characterized by yellow or copper-colored leaf tips similar to a dry, burnt leaf.

Stunted or Slowed Growth

Overall, poor cannabis humidity control affects how the cannabis plant absorbs water and nutrients from its environment. This happens whether in the veg or flowering stage. 

Poor Cannabis Humidity in Veg
Vegging cannabis may start to wilt if the humidity levels are too high

This compromised state will mean that they cannot fully utilize the water and nutrients supplied by the cultivator no matter the effort put into it. When not apprehended, this can eventually lead to stunted growth. 

Ideal Temperature and Best Humidity for Marijuana at Every Stage

Achieving the best humidity for growing weed isn’t a one-and-done thing when growing cannabis. Like many aspects of cultivating weed, some things need to be understood between plants’ life stages and humidity levels. Not to worry, although it sounds complicated, the concept and method are simple enough even for beginners.

A note to remember is that there are different ways to measure humidity which includes “specific”, “absolute”, and “relative” humidity. Most growers and articles often refer to Relative Humidity or RH when talking about humidity control in the grow room.

A cannabis plant’s life stages can be split into 5 to which there are optimum humidity and temperature for growing weed that ensures healthy growth: Seedling, Vegetative, Early Flowering, and Late Flowering. The auxiliary stage which is “Storage,” is not part of the cannabis’s growing life but is equally as important. Ideal storage conditions dictate the quality of the end marijuana product. 

Cannabis On A Bucket Containers
Indoors cannabis plants in vegetative stage

The following ideal conditions (for both humidity and temperature) are specific to indoor cannabis growing:

Seedling

Seedlings and young clones have weak, underdeveloped root systems. To help them absorb moisture, relative humidity should be kept high: between 65% and 80%. During this stage, you should aim for a temperature of 77°F during the day and 69.8°F at night.

Vegetative

As young plants continue to develop their root system, their ability to absorb water improves. You can now lower Relative Humidity to 40% to 70%, with humidity levels lowered by 5% each week approaching the Flowering Stage. Temperatures of 69.8 – 82.4°F during the day lowered to 64.4 – 75.2°F at night.

Hygrometer showing 43% of relative humidity and temperature of 72ºF
Hygrometer at 43% of relative humidity and temperature of 72ºF

Early Flowering

By the time the plants enter the flowering stage, their root system should be fully developed. You can now drop RH to 40 – 55%. This can also help keep your buds free from mold and mildew. Keeping temperatures between 68 – 78.8°F helps keep air moisture at ideal levels.

Temperature and Humidity
Hygrometer to measure temperature and humidity

Late Flowering

Tweaking humidity levels at this stage isn’t absolutely necessary, but it can help improve the yield, flavor and appearance of the plants. Lowering RH to 30 – 40% and temperatures to 64.4 – 75.2°F slightly stresses the plants, encouraging more resin production. This can lead to more potent buds, crusted with sparkly trichomes. However, this step can also induce more stress to the plant and should be done slowly ~1% at a time and observing the crop’s reaction.

fan in a growing tent
Fans at a higher velocity help lower temperature by pushing more hot air out of the room

Curing Stage: The Do’s & Don’ts Of Cannabis Storage

Tweaking the humidity and temperature doesn’t stop as soon as you gather your cannabis harvest for the season. After drying the cannabis materials, you would have to store or cure your cannabis to prepare it for the production process of whatever type of product you would like to produce. 

The storing part of the curing process needs to have the exact levels of humidity and temperature. If the perfect range of humidity and temperature levels aren’t met, the cannabis materials might be ridden with diseases and premature rotting. 

  • Humidity – 60-65%
  • Temperature – 64.9ºF – 69.9°F

The implications of improper humidity levels are enormous and can be devastating. Fortunately, these can be averted by starting with small steps, such as knowledge of the proper cannabis storage practices. 

Jar of Cannabis

The Do’s

  • Store the cannabis in a cool, dry, and dark place. Always keep the containers away from direct sunlight as the UV rays in it may induce the breakdown of cannabinoids.
  • Store the jars in places with temperatures below 21°C (70°F). Extreme temperatures may trigger the onset of molds, as well as dry out the coveted terpenes and cannabinoids.  
  • Monitoring humidity is vital for inhibiting the growth of mildew and other contaminants. Make use of tools such as hygrometer to keep the RH levels in check. In case of excessive humidity, use food-grade desiccants such as those found in dried seaweed packages to help reduce moisture from the surrounding air.
  • Use inert containers such as glass jars or metal canisters as containers like these will not affect the flavor and aroma of the buds. Silicone containers are also recommended.
  • Ensure that the jars are airtight to reduce oxygen exposure. Excessive air exposure may accelerate the decomposition of plant constituents, while air deficit may increase RH.
  • Use the correct container size relative to the amount of stash being stored. Keeping small amounts of buds in oversized jars means more oxygen could get in. Meanwhile, storing too many buds in a small container could crush it and cause the trichomes to break off. Moreover, it could increase the RH levels inside the jar and invite molds.

The Don’ts

  • Do not refrigerate or store the buds in the freezer. The changes in temperature and humidity during refrigeration may attract mildew and mold. Moreover, the buds will become brittle. This may cause the trichomes to break off from the buds.
  • Do not store the cannabis in plastic bags, as its static charge may cause the loss of trichomes. In addition, plastic containers can elicit sweating.
  • Refrain from using a tobacco humidor for cannabis. A humidor is a humidity-controlled box designed for storing cigarettes or pipe tobacco. It is fashioned from cedar wood, which contains oils that can be transmitted to the cannabis, ultimately affecting its flavor. Moreover, the sponges inside may overly drench the plant.
  • Keep the cannabis away from any electrical appliances that emit heat.

Optimal levels for indoors cannabis growing

What are the optimum humidity levels for indoor cannabis growing? Here’s a quick rehash of the ideal humidity and temperature for indoor growing:

StageTemperatureHumidity
Seedling69.8 – 77ºF65-70%
Vegetative62.4 – 82.4°F40-70%
Early Flowering68 – 78.8°F40-50%
Late Flowering64.4 – 75.2°F30-40%
Storage (Curing Stage)64.9 – 69.9ªF60-65%

How do you monitor humidity levels?

Sustaining proper moisture levels becomes all the more critical when growing indoors, where simulating perfect conditions is a must. Needless to say, consistent monitoring and control is essential. Buy a good hygrometer and thermometer and record your numbers twice a day (light and dark). Be attentive and observant and you’ll be richly rewarded.

Cannabis Humidity Monitor
Hygrometer (cannabis humidity monitor)

A good way to measure the moisture content of a grow room is with a hygrometer, but there are other gadgets that can help.

  • Hair Hygrometer. These devices use human or animal hair to measure humidity, as they tend to contract or expand in response to moisture. While they’re quite cheap, they’re only reliable when measuring mid-range humidity (30 – 80%).
  • Dew-Point Hygrometer. These determine humidity by measuring the temperature at which vapor starts to condense. They’re very accurate, but not cheap. Some dew-point sensors can also measure VPD.
  • Psychrometer. Also known as the Wet-and-Dry-Bulb Thermometer, these measure air moisture by comparing the evaporation rates of wet and dry surfaces. They’re less pricey than dew point devices but can be tricky to operate.
  • Humidistat. Able to monitor and maintain favorable moisture range, these are humidifier/dehumidifier units with built-in hygrometers. When they sense a change in humidity, they automatically restore appropriate levels by increasing or decreasing moisture in the grow area.
  • Humidity Controller. Similar to a humidistat but a lot cheaper, humidity controllers also adjust moisture levels. However, they need to be plugged into an external humidifier or dehumidifier, and multiple sets may be necessary to maintain a large space.

Monitoring the amount of moisture in the grow room requires day-to-day logging and adjusting. Humidity is sensitive to fluctuations in temperature, so checking moisture levels during the light and dark cycles is vital. Any drastic changes to RH should be immediately noted and acted upon.

How do you control humidity levels?

When controlling humidity, it’s important to remember that factors inside and outside the grow room can contribute to the amount of vapor in the air. The following measures can be followed to maintain humidity levels within an optimal range.

Grow Room Exhaust System

Install Insulation

Insulation helps maintain a constant temperature inside the grow room. One of the most common ways to seal a grow room is by lining the walls with rigid foam boards, creating an additional barrier against the outside environment. It helps prevent external elements causing fluctuations in humidity.

Manage Air Circulation

A ventilation system prevents stagnant, humid air from attracting molds and fungi. However, it must be set at the right levels. Fresh air can lower the temperature of the room, causing transpiration to slow down. Gentle airflow can help keep vapor in the air, allowing plants to absorb the moisture, while a well-pruned plant will help a plant transpire efficiently and correctly.

Use Humidifiers And/Or Dehumidifiers

Humidity can be manipulated with humidifiers and dehumidifiers. Smaller humidifiers work just as fine, but larger ones mean you won’t need to refill the tank several times a day. Sizing a dehumidifier, on the other hand, is a little bit trickier. An important thing to remember is that plants transpire 97% of absorbed water. Estimating the total water input in the garden will help identify the right device.

dehumidifier
Humidifier in a growing tent

For instance, 30 plants that receive 2 liters of water each per day means a water input of 60 liters. The dehumidifier should be capable of pulling this amount of moisture from the grow room. 

Never Overwater

Overwatering can lead to stagnant water and drowned roots. This adds to the humidity of the room as the plants release more vapor without absorbing any. To prevent this, ensure proper drainage and provide just the right amount of water.

Reservoirs in the grow room may also increase humidity levels. Make sure they’re covered.

Water At Appropriate Times

Watering the plants when the temperature in the room is still low causes transpiration to slow down, resulting in lower humidity levels within the grow room. It’s essential to consider the timing of watering to make sure it will not cause sudden spikes or drops in humidity.

Have large Companion Plants

If your growing environment is too dry, try adding large plants to the grow room. These will transpire and gently raise humidity levels. The water vapor released will help feed the smaller plants, too. Companions should only be kept until the early vegetative stage of cannabis, as excess moisture in the later phases may encourage pathogens.

Ways to Lower Humidity

  • Increase fresh/cool air supply. Small fans or regular fans at the lowest setting can work
  • Running your exhaust vent at a higher velocity to pull more hot air out of the room
  • Watering your plants at the beginning of their light period
  • Use an appropriately sized dehumidifier
  1. Best for Large Grow Rooms: hOmeLabs 50pint 2500 sq.ft. Dehumidifier
  2. Best for Medium Grow Rooms: Vremi 22pint 1500 sq. ft. Dehumidifier
  3. Best for Small Grow Tents or Boxes: Eva-dry Wireless Mini Dehumidifier

Ways to Increase Humidity

  • Try to increase the temperature in your grow space
  • Running your exhaust vent at a lower velocity to keep more hot air in the room
  • Mist your plants (avoid this during the flowering stage)