5 Top Tips On Controlling Humidity During The Growing Period

Read on to discover 5 top tips to control humidity levels in your grow room, how to set your optimum conditions and what to avoid in the vegetative phase of your cannabis plant.
10 March 2020
6 min read
5 Top Tips On Controlling Humidity During The Growing Period

  • 1. The optimum conditions
  • 2. Using hygrometers
  • 3. Humidity control
  • 3. a. Humidifier
  • 3. b. Dehumidifier
  • 4. Keep fans on 24/7
  • 5. Supplementing with co2
  • 6. Running lights at night-time
  • 7. What to avoid when growing in the vegetative phase?
  • 8. The top 3 fast buds strains for challenging conditions
  • 9. In conclusion

The growing period describes the stage of a Cannabis plant’s life cycle, where the amount of lighting received is around 18 hours. As we attempt to replicate Spring indoors, we must also do our very best to replicate the optimum temperature and humidity levels. Below Fast Buds give 5 top tips on what to consider and how to create the ideal environment inside your garden.

1. The Optimum Conditions

During the vegetative phase, the optimum conditions for Cannabis plants are high humidity levels and a warm temperature between 22-25 degrees Celsius. During this stage, plants will focus all of their energy on producing thick stems, bushy foliage and as many new growth sites as possible. 


5 Top Tips On Controlling Humidity During The Growing Period: Optimum Conditions For Cannabis Growth


  • Light Cycle - 18 hours light / 6 hours darkness
  • Day Time Temperature - 22-25 degrees Celsius
  • Night Time Temperature - 16-20 degrees Celsius 
  • Humidity Levels - 75% 


2. Using Hygrometers

Having the ability to accurately measure the indoor temperature and relative humidity levels your plants are receiving is one way to safely monitor your garden. A hygrometer is a gadget that will give a data reading of the temperature and humidity present, with most displaying the lowest and highest temperature change so you can see the difference within a 24-hour period.

Our top tip here is to invest in a good hygrometer and to even keep a spare one around in case of emergencies. After you have set one up in your garden, you are at least in a position where you can then dictate the optimum conditions using a humidifier or dehumidifier depending on the situation. They are not expensive and will be the difference between a conscious grower or one who is hitting and hoping.  


5 Top Tips On Controlling Humidity During The Growing Period: Optimal Conditions for Vegetative Stage 

3. Humidity Control

As we know that during the growing phase, Cannabis plants prefer high humidity levels around 70%, with a comfortable temperature of 24 degrees Celsius. If you are choosing to supplement with Carbon Dioxide, then consideration must be taken regarding an increase in temperature above 25 degrees Celsius. 

Our top tip here is to increase or decrease the relative humidity present, by using either a humidifier or dehumidifier. Both will allow you to control the environment until you meet the ideal setting, with many growers opting to connect the device to a temperature controller that will kick in and off when required. 


A humidifier is an electrical device that sprays mist into the air to increase humidity. Normally it's placed by intake fans or left inside a garden during the 18/6 period. It is ideal for when the fresh air from outside is too dry, or the humidity levels are below the desired 60-75% mark.


An electrical device that sucks air through a vacuum and removes the moisture from the air. Usually used during the flowering period of 12/12 in order to reduce moisture levels to the safe zone of 30-40%.

4. Keep Fans on 24/7

One bad habit that some beginner growers can develop is switching everything off at night. Once the lights go out, everything else should remain consistent and for the capacity and airflow inside to stay as it was when the lights were on. By encouraging a grow room that houses stagnant air is only asking for problems with airborne pathogens such as powdery mildew.

Our top tip here is to keep only the grow lights on a set timer. Your intake fan, exhaust fan, carbon filter, oscillating fans, and wall fans should be kept on permanently. By doing this, the threat of pathogens is removed and the airflow around the bottoms and tops of plants remains consistent.

The following should be left ON during the darkness period:

  • Exhaust fan + carbon filter

  • Intake fan

  • Oscillating Fans

  • Wall Fans

 5 Top Tips On Controlling Humidity During The Growing Period: Keep Fans ON


5. Supplementing With CO2

Plants require intense light, a food source and Carbon Dioxide, in order to produce sugars for the roots and exchange for Oxygen. Despite being told that the safe zone for Cannabis plants is 24 degrees Celsius, this is highly debatable when considering some of the tropical climates cultivars can handle outdoors. 

When supplementing indoor Cannabis plants with CO2, it is necessary to work in an environment that exceeds 25 degrees Celsius and in some cases up to 30 degrees Celsius. The reason is that stomata on a leaf that is responsible for the uptake and transfer of CO2, function best when temperatures are close to 30 degrees Celsius.

Introducing CO2 into the garden is better suited for those with a thorough understanding of what PPM is required at what rates, according to the nutrient and amount of lighting. It is good practice to know that Cannabis plants do not utilize CO2 during darkness and will begin one hour after lights are on.

6. Running Lights At Night-Time

It is common practice for a grower to run their lights at night time, thanks to advantages associated such as lower temperatures, higher humidity, and a cheaper electricity tariff. Choosing to have the 6-hour darkness period during the hottest points of the day, will allow the fresh air to be pulled in to be much cooler, allowing for better temperature control management. 

Our top tip here is to use this technique especially if you are growing plants indoors under 18/6 in the Summertime. The colder the air you are bringing into the grow room, will allow your plants to transpire water.

7. What To Avoid When Growing In The Vegetative Phase?

- Overwatering your plants can cause humidity levels to spike, due to the buildup of excess water. As the Cannabis plants struggle to transpire properly, not only will the rate of growth cease, but the relative humidity will remain constantly high.

- Try not to water plants late at night prior to lights going out. It is better to feed your plants on a consistent basis such as one hour after lights are on every day. This is also the time when stomata begin to function and uptake available CO2.


5 Top Tips On Controlling Humidity During The Growing Period: Lights


- Never create an environment where there is no fresh air being circulated around the plants. This is important during when lights are on and especially lights off. When flowering Cannabis, creating such an environment will guarantee mold and other plant diseases.

- Powdery mildew will appear overnight in some cases and what looks like a harmless white powder is, in fact, a pathogen that has attached itself to the leaf tissue. When you see signs of mildew, this is a clear indication the air in the grow room has become stagnant encouraging anaerobic bacteria and airborne pathogens to thrive. 

- Avoid not using equipment such as hygrometers and hoping the outside fresh air is perfect. During dry seasons, indoor-grown Cannabis plants can suffer tremendously causing a bad start. 

8. The Top 3 Fast Buds Strains For Challenging Conditions

Working with highly resistant cultivars can be the difference between losing a crop, and growing some top-shelf flowers. Below are our 3 best strains for those facing challenging conditions.

Amnesia Haze Auto
5 out of 5
I really liked this one a lot! Very nice dense flowers. I wish she had yielded more, but overall I am definitely happy with this girl!
Verified customer
Reviewed 16 June 2020

An excellent choice for indoor growing, where high humidity can influence the flowering room. This strain is highly resistant to mold and is a foolproof cultivar.

Mexican Airlines Auto
5 out of 5
Really,really one of the Best sativa autoflower that i ever had.Great high,great taste,great yeld.Thanks a lot <br>
F italian.
Verified customer
Reviewed 12 January 2016

For flowering in a naturally humid climate, a wise choice would be this sativa dominant strain. She can take the heat and humidity and will thrive.

Lemon AK Auto
5 out of 5
She was very easy. She grew quite chunky considering she's a sativa, but the effects did feel sativa. She has a lovely taste.
Verified customer
Reviewed 19 January 2020

Growers who experience dry climates will find comfort with this Lemon AK Auto. Highly resistant and a rock solid performer.

9. In Conclusion

It is important to provide a warm and wet environment for Cannabis seedlings or clones. As plants mature in size and progress closer to the flowering stage, the level of relative humidity can be slowly reduced toward the 30% mark. If you are working with CO2, remember stomata function better in high humidity and temperatures above 27 degrees Celsius and above, and won't open until the first 60 minutes of the lights being turned on.


Good luck with growing the healthiest and best plants possible!

10 March 2020