The goal when growing cannabis indoors is to mimic the weather outside. To replicate the different seasons, ranging from long lighting hours down to 12 hours, it is essential that the temperature and humidity are realistic to what the plants demand.
In this article, we explain the importance of maintaining the correct temperature and humidity levels during the various stages of a cannabis plant’s life cycle, how to control the environment, and potential risks to be careful of when growing cannabis indoors.
What is the optimal temperature for indoor cannabis?
Ideally, to keep plants as comfortable and happy as possible, aiming for a temperature between 22 and 24 degrees Celsius when the lights are on is simply fine, and a reduction down to 18–20 degrees when the lights are switched off.
What is the optimal humidity level for indoor cannabis?
The optimal humidity level will actually vary based on what stage the cannabis plants are at. During the seedling and vegetative stages, cannabis prefers warm and wet conditions, meaning the relative humidity should be around 60–80%. During the transition and flowering stages, the goal is to reduce the humidity levels to around 50%, aiming for 35–50% during peak bud production.
How to control the temperature when growing cannabis indoors
Not every indoor grow room will immediately meet the criteria for an optimal growing environment. Below is a short list of the different ways that you can affect the temperature and humidity levels of a growing space.
Extraction
The extractor is designed to remove large volumes of air from the grow room and exhaust it away. How well your extraction kit is working can play a huge role in how easily you may be able to control and maintain the ideal temperature ranges.
Electric heater
A heater added to a cold grow room during the Winter months can really help balance out the ambient temperatures, keeping the grow room within the correct range. Heaters will typically be used during the nighttime, when the lights are off and there is no external heat source.
Temperature fan controllers
These brilliant devices can be connected to fans, heaters and extractors using a sensor that allows the grow room to maintain itself accordingly without the presence of a grower needed. Temperature fan controllers can be expensive, but they are well worth the investment and can save a grower lots of time.
How to control the humidity levels when growing cannabis indoors
Now that we have covered how to control the temperatures, we can now talk about humidity. Below, we explain the different methods that can be applied indoors to affect humidity levels.
Humidifiers
These are a real lifesaver and can turn a dry grow room into a wet, foggy tropical paradise. Humidifiers come in all shapes and sizes, making them highly practical for smaller grow spaces and grow rooms where cannabis plants are in a state of vegetation (18/6).
Dehumidifiers
If the humidity levels are too high due to wet seasons, you can use a dehumidifier to remove excess moisture from the grow room and deposit it away. These units are especially useful when battling high humidity during flowering and can make or break a crop in some cases.
Always use a hygrometer to check
The best way to check what the exact temperature and humidity levels are to use a digital hygrometer. These inexpensive devices come equipped with a sensor that will provide an accurate live reading. Some models of hygrometers will allow you to see the lowest and highest data readings over a 24-hour period.
The potential risks of high temperatures and humidity indoors
● Cannabis plants during the final weeks of flowering can become infected with mold.
● High temperatures can cause heat stress, which leads to a world of potential problems.
● Insects and spider mites especially thrive in warm and wet grow rooms.
● The quality of the buds will be below par, resulting in a dry, light and airy flower.
● Terpenes may become significantly affected if the grow room temperatures are too high.
What is heat stress, and how do I identify it?
Stress caused by high, rising temperatures inside a grow room can quickly become problematic. Being able to identify symptoms of heat stress can help prevent it before further issues prevail. The primary causes of heat stress can be attributed to a number of variables, which are covered below.
Hot grow lights and ballasts
The main culprit for a hot and clammy grow room will be hot grow lights. During the vegetative stage, when lights can be on for between 18 and 20 hours per day, bulbs and ballasts can begin to produce hot temperatures that can then be very difficult to cool down.
Hot weather outside
During the Summer months, the weather can take a turn for the worst, resulting in unexpected heat waves and temperatures soaring above 30 degrees Celsius. There is not much you can do apart from run your grow lights at night if you are already flowering.
Poor ventilation and extraction
Old exhaust fans, or equipment that is not up to par anymore after years of back-to-back harvests, can be one reason why grow rooms can become hot and humid. It is a good idea to invest in a new extractor and carbon filter every third harvest.
No oscillating fan
Oscillating fans are perfect for creating air vortexes around the plants, keeping the air fresh, and constantly recycling. A grow room with no oscillating fans will soon become warm and hot, so try and add at least one in your grow space, gently blowing on the plants.
What to look out when identifying heat stressed cannabis plants
1. Leaves curling inwards
Cannabis plants will respond to hot and dry conditions by curling their leaves inwards, in an attempt to reduce the amount of foliage exposed to light. The reason they do so is to drastically reduce the rate of transpiration the leaf will experience. Keep a lookout for the leaves of your plants to curl inward.
2. Small and stunted plants
Cannabis plants that were exposed to high temperatures and wind burn during flower production, will respond by producing stunted plants that produce small buds. The quality of the buds will showcase the negative side effects of heat stress.
3. Low yields
Low yielding plants can be one of the worst case scenarios for a home grower. It can be a shame when the reason your once great looking crop produced a low yield is down to heat stress. Temperatures above 26 degrees Celsius will affect yield and terpenes.
Atami’s conclusion
Dialing in your temperature and humidity levels accordingly to the vegetative and flowering periods will ensure you produce the best indoor crop possible. Spend your money wisely and buy a digital hygrometer, humidifier, dehumidifier, oscillating fans and small electric heater. This way, you will have all the bases covered, allowing you to adapt to times when the climate is cold and wet or hot and humid. Consistency is key, and keeping your plants happy and stress-free will produce top results!
