The Importance of Controlled Humidity Levels for Growing Cannabis


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Ever wondered how cannabis growers get the perfect aroma? From growing cannabis to consuming it, it all comes down to humidity.

The marijuana industry is booming – generating ~$34 billion every year. As more legal growers take on the challenge, Americans are on the hunt for the best indoor grow rooms (under strict governmental rules & regulations) to obtain the best yield and improve plant growth!

Many factors need to be controlled in a grow room, such as CO2, temperature, and humidity. Controlling humidity and temperature levels is a precise science and is essential for growing cannabis, from planting the first seed to harvesting the end product. 

As you probably know, all plants carry out photosynthesis, respiring CO2 from the surrounding environment via their leaves. During this process, the cannabis plant loses some water. If the air is not humid enough, the moisture content within the plant will be depleted. If this continues, the plants will lose more water than they can hold within the foliage, closing the pores in the leaves resulting in cell death. 

Some people think that watering their cannabis plants will help, however, this will decrease the amount of air in the soil, causing asphyxiation to the plant and likely root rot. But, with the correct humidity levels, they will thrive!

Controlling humidity levels is not difficult, but getting it right will boost your production levels. This article will tell you everything you need to know regarding humidity, from controlling it, to what equipment you can buy. 

Temperature & Relative Humidity (RH)

Humidity and temperature are closely related and play a vital role in harvesting cannabis. Humidity measures how humid the air is in a given area, in your case, this will be the grow room. Obtaining the ‘perfect’ amount of humidity (relative humidity, or RH) is crucial. 

When we talk about RH, it refers to how much water air holds at a certain temperature over time. When the RH reaches over 100%, surplus water is no longer detained by air, forming water droplets as condensation occurs on the leaves. The water droplets are usually in 3 forms: fog, dew, or rain.

NOTE: During the growing process, RH should fall slowly. 

Temperature is just as important as RH when growing cannabis so controlling both is essential for the best yield. It is important to note that hotter air holds more humidity than colder air. So what happens if the growing area conditions become too hot or too cold?

Too Hot 

If temperatures are too high, especially in the flowering stage, the plant’s buds will appear fluffy. This is likely to reduce potency while the growth rate will slow down. 

Too Cold 

If the conditions inside the growing area are too cold, the cannabis plant will grow much slower. If the temperature goes near freezing (32°F) the plant will be in extreme shock, potentially fatal. 

Ideal Humidity Levels for Growing Cannabis

The million-dollar question asked by many grow enthusiasts is: “What should my humidity levels be to optimize cannabis growth”. 

Well, the RH level depends on the plant. Plants that originate from warmer/tropical regions thrive with high RH levels as they are well-adapted to these conditions. However, plants from colder regions prefer a less intense RH level. 

Cannabis is thought to be native all around the world with Indica cannabis originating from dry mountain regions where temperatures are lower. However, with Sativa cannabis, this strain grows best and is thought to originate from much hotter, humid, subtropical regions. 

Nowadays, the seeds you will find are a Sativa-Indica hybrid plant. These hybrids have emerged from heavy cross-breeding which is why most growers will assess the plants’ morphology, and run trials using different RH levels to find the best yield or end product. 

Humidity Levels & Growth Stage

Depending on what stage your cannabis plants are at, will depend on how much humidity they require to grow. 

There are 4 stages to cannabis growth that every grower should familiarise themselves with: seedling/clones, vegetation, early flowering, and late flowering. 

  1. Seedling/Clones
  • This stage involves clones or seeds from the parent cannabis plant
  • Relative humidity level = high (65-70%)
  • High humidity is required to establish root growth & for water uptake in the leaves
  • Day temperature = 77°F 
  • Night temperature = ~70°F 
  1. Vegetation
  • This stage involves the plant growing well-structured leaves
  • Relative humidity level = moderate to high (40-70% depending on the strain of cannabis) – if the leaves look too wet during this stage, the humidity is too high
  • During the vegetation period, the plants develop strong root systems that increase the uptake of water. You may need to reduce the humidity levels during this period
  • Every week during this period, it is recommended to reduce the humidity levels by 5% and increase the temperature because, at this stage, the cannabis plant’s leaves will be absorbing and evaporating more water.
  • Day temperature = 71-82°F 
  • Night temperature = 64-75°F 
  1. Early Flowering
  • At this stage, the cannabis plant has established a strong root system, meeting its water requirements, however, it still uses its leaves to absorb water
  • Relative humidity level = low (40-50%)
  • Low humidity is required to prevent mold and mildew (see below for more details on this issue)
  • Day & night temperatures = 68-78°F 
  1. Late Flowering
  • This stage usually lasts 6 to 12 weeks as the cannabis plant fully matures
  • Relative humidity level = low (30-40%)
  • Low humidity improves the plant’s yield, taste, and overall appearance
  • Day temperature = 64-75°F 
  • Night temperature = 60-68°F 

Controlling Humidity 

Giving plants the correct amount of humidity, allows their pores to open up inside the leaves, respire more CO2, and grow faster. However, exposing them to too much humidity can cause diseases such as bud rot due to water build-up encouraging bacteria and fungi to overrule the foliage. 

Benefits of controlling humidity include:

  • Reduces bud rot from happening
  • Ensures the cannabis plant will survive
  • Improves potency
  • Improves taste

How to Increase Humidity

  • Reduce the power of your exhaust fan – this traps warm air inside the growing area
  • Increase the temperature in the room 
  • Gently spray/mist your plants and soil to keep them moist – avoid flowering plants as this can cause the buds to rot
  • Surround your plants with water (in open containers)
  • Use a humidifier to trap moisture in the growing area

How to Reduce Humidity

  • Increase the power of your exhaust fan – this pushed warm air outside the growing area
  • Plug in a dehumidifier
  • Water the plants at the start of their light period/when lights are turned on
  • Increase the amount of cool air in the growing area

Keeping the humidity at the right level throughout your cannabis plant’s life cycle can be difficult, however, it is vital to monitor the plant’s growth during its 4-stage lifecycle. 

At Atlas Scientific, we have a humidity sensor that not only reads relative humidity for accurate humidity monitoring, it also measures the dew point and air temperature. 

Signs of Poor Humidity Levels

You hear some horror stories from cannabis growers where things have gone wrong, and this is generally from poor humidity management. 

Below are a few issues that can occur, and what you can do to prevent them from happening to your stock.

White Powdery Mildew (WPM)

This fungal infection occurs in high humidity environments, which can be difficult as high humidity levels are required for the early stages of cannabis growing. When the cannabis plant is infected, it will be covered by white powder or mildew, which can be prevented if caught early on. 

If you notice this taking over your plants, ensure there is ample airflow in the growing area – you can use a swinging fan.

Bud Mold/Rot 

Another fungal infection, which affects the dense cores (inner part) of the plant, making it turn white or brown with mold/rot. If you notice this on the plant’s buds, you will have to throw them away as they are now unusable. 

Ensure there is enough air circulation, plenty of space between plants, and inspect the plants daily to prevent it from occurring.  

Nutrient Issues

Plants need nutrients to grow but give your cannabis plants too much, they will get nutrient burn, also known as nute burn. This is when the leaves turn yellow and the edges and tips turn brown looking burnt. 

This process occurs when the plants take up too much water due to high temperatures and low humidity in the growing area. As they drink more water, the uptake of nutrients via their roots is excessively increased causing a nutrient burn. 

Reduced Growth

When cannabis plants are young, they require higher humidity levels than mature plants (these grow best in lower humidity levels). If these requirements are not met, your cannabis plants will not reach their potential growth and optimum health. 

Summing Up Humidity, Testing Equipment, and Advice

Whether it is personal or commercial use, humidity levels play a vital role in growing cannabis. By controlling relative humidity (RH) levels, you will maximize your yield and reduce issues such as white powdery mildew and bud rot. 

At Atlas Scientific, we have the perfect solution to control RH levels and make growing cannabis stress-free! We understand finding the perfect humidity sensor is a mission itself, that’s why we have built our own.

Remember to check your plants daily and keep a close eye on both temperature and humidity throughout the growing process.

Hopefully, after reading this, you will be well on your way to growing healthy cannabis plants, but should you have any questions, please do not hesitate to reach out to one of our staff at Atlas Scientific, part of our world-class team. 

Thank you for reading, we hope to see you again soon, happy growing!

Humidity Circuits & Probes

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