Controlling Indoor Cannabis Humidity

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Cannabis humidity and temperature
February 26, 2021
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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. 

If one takes the time to think about it, there are many parameters surrounding the process of growing cannabis. A cannabis plant is a living thing that patterned its growth in the wild to follow Earth’s seasons, light cycle, and temperature changes. Therefore, aside from light, water, and nutrients, temperature and humidity are often overlooked vital parameters that influence the final crop.

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 so Important?

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. 

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. 

Temperature and Humidity

Relative humidity (RH) is merely referring to the amount of water air can hold at a given temperature. When relative humidity levels reach above 100%, excess water can no longer be held by the air and condensate into water droplets, such as rain, fog, and morning dew. 

The best relative humidity level for each plant varies. For example, plants that are native to hot and tropical areas are well-suited at higher relative humidity levels than plants from colder climates. 

We also have vapor pressure density (VPD) as a unit of measurement for the air pressure.

Measured in units of pressure, VPD is; RH, air temperature and leaf temperature in a single value. Instead of controlling these factors separately, VPD looks at how one variable affects the others. It’s a more accurate measurement than both absolute and relative humidity, and is often utilized in greenhouses and large-scale operations.

VPD can be computed as follows as per the ASCE Standardized Reference Evapotranspiration Equation:

T is the temperature in degrees Celsius, and RH is relative humidity:

Saturation Vapor Pressure (es) = 0.6108 x exp (17.27 x T / (T + 237.3))

Actual Vapor Pressure (ea) = RH / 100 * es

Vapor Pressure Deficit = ea – es

A simpler way of understanding VPD is with the formula VPD = [(100-RH)/100)*SVP.

How Cannabis Humidity Affects Different Types of cannabis

Marijuana plants are native to different parts of the world. Sativa varieties of marijuana tend to grow in naturally warm, subtropical climates and prefer more humid and warmer conditions. Indica varieties, on the other hand, grow best in drier, mountainous regions, and in lower temperatures. 

Marijuana plant varieties have been heavily crossbred over the years. Some of the strains on the current market are Indica-Sativa hybrids which makes them more sensitive to humidity. Simply guessing the ideal humidity level will not work. 

How Cannabis Humidity Affects Different Plant Processes

Here’s a list of processes essential to plant life and potential effects due to changes in humidity:

Body Processes of PlantIncrease in VPD (Low RH)Decrease in VPD (High RH)
Transpiration rateThe transpiration rate increasesTranspiration rate decreases
Bulk flow of water and plant stressCauses water utilization to increase, inducing stressWater utilization decreases, reducing stress
Nutrient intake at the rootsIncreases nutrient uptakeNutrient uptake declines
CO2 uptake and stomata openingStomata closes, reducing CO2 absorptionOpens stomata, increasing CO2 uptake

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

Nutrient Burn in Veg
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. 

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 Cannabis Humidity at Every Stage

Achieving the best humidity for marijuana 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. 

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

Clones

Cannabis clones are basically copies of a specific cannabis plant. This is done by cutting a piece off from the parent plant and giving the cut piece the opportunity to grow roots of its own. 

A clone essentially has the same genes as the parent plant as it literally was once part of the original plant. The critical part of a clone’s life is the initial stages as it has yet to develop a root system to intake water. Therefore, it relies mostly on its leaves to acquire water which is possible only with high humidity and temperature.

  • Humidity – 65-70%
  • Temperature – 20-25°C with the lights on, reduce by 4-5°C with the lights off.

Seedlings

Like cannabis clones, seedlings have yet to develop a proper root system and may sometimes have its own foliage growth outpace it. Therefore, to take advantage of this, high humidity and temperature level are needed. Until it has developed its roots, low humidity and temperature levels may slow its growth.

  • Humidity – 65-70%
  • Temperature – 20-25 °C lights on, reduce 4-5°C with the lights off.

Vegetative Stage

Unlike seedlings, the cannabis plant will have developed its root system and will now actively use it. Cannabis plants in this stage will prefer a moderate humidity level.

Poor Cannabis Humidity in Veg
Vegging cannabis may start to wilt if the humidity levels are too high
  • Humidity – 40-70% with humidity levels lowered by 5% each week approaching the Flowering Stage.
  • Temperature – 22-28°C, reduce 4-5°C with the lights off.

Early Flowering Stage

The cannabis plant at this stage benefits from lower humidity levels around to 40-50% range. One can get away with 55% but do not exceed 60%.

  • Humidity 40-50%
  • Temperature – 20-26°C, avoid high temperatures.

Late Flowering Stage (1-2 weeks before harvesting)

You can continue with the humidity and temperature in the early flowering stage. However, decreasing the humidity levels below 40% can induce increased resin growth which improves flavor and appearance, as well as improves yield. 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.

  • Humidity – 30-40%
  • Temperature – 18-24°C, reduce 5-10°C with the lights off.

Curing Stage

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 – 18.3 – 21.1°C

If you’re merely storing a cannabis product on a mason jar to be used in future pot sessions, following these ideal humidity and temperature ranges will keep your weed fresh and the smell not too pronounced. Read on for further tips on how to get rid of weed smell.

  • Humidity – 59% to 63%
  • Temperature – 25-29.4°C

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
Clone20-25°C65-60%
Seedling20-25°C65-70%
Vegetative22-28°C40-70%
Early Flowering20-26°C40-50%
Late Flowering18-24°C30-40%
Storage (Curing Stage)18.3 – 21.1°C60-65%
Storage (Ready for consumption) 25-29.4°C59% to 63%

How to Adjust the Humidity for Growing Weed and Temperatures

Now that you have the parameters for the optimum temperature and humidity for growing weed. To control humidity, you will need a humidity measuring device called a hygrometer. Some hygrometers may not be precise, so either use one or estimate through averages when using many.

Cannabis Humidity Monitor

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)
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