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Cannabis plants like a temperature similar to humans, or slightly warmer, not too dry and not too humid. When growing cannabis temperature is an important factor in keeping your plants healthy. 

Why is Temperature Control Important?

Heat is vital to your cannabis plants health but unlike humans and animals, plants cannot create their own heat. The plant is entirely dependent on its environment. As an indoor grower this is extremely important because you are in control of its environment.

A plant's temperature develops from a combination of external light, external temperature, and the amount of evaporation. You cannot read the plant's exact temperature with a thermometer, but you can read the temperature of its environment to ensure it is at an optimal level for healthy growth.

Cannabis plants usually won't die from being too hot, but it can cause their growth to slow from it. Freezing temperatures on the other hand can cause strange symptoms and even death of your plant. High or low temperatures can cause different disorders in your plant.

  

Effects of Temperature on Cannabis Plants

As mentioned earlier, having too low or high temperatures can cause issues for your cannabis plants. When growing indoors it is very important to control the temperature in your grow room. Here are some effects temperature can have on your cannabis plants:

Too Low Temperatures

Cold temperatures tend to slow down plant growth. If temperatures decrease to below 15°C, it tends to effect plant growth, and freezing temperatures can shock or even kill a cannabis plant. Some plants will continue to grow in the cold without showing any signs of stress, but they often don't yield as well as they should given the amount of light they receive. Indoor plants tend to be more vulnerable to cold than outdoor grown plants

When it is cool, plants are also more susceptible to certain types of mould, especially if it is damp too. Mould will sometimes occur after frost melts and covers your plants with water.

Colder temperatures and significant temperature fluctuations have also been known to change the colour of leaves to purple and can also reduce photosynthesis.

Too High Temperatures

A cannabis plant will usually not die from the heat but if temperatures get too hot it can cause plants to grow at a slower rate.

It is especially important to keep temps below 26°C in the flowering stage. If the temperature goes higher than this, it can slow down bud growth and may cause them to become airy and loose. The heat can also reduce the potency and smell of your buds. 

In a high heat environment, your cannabis plant will be more vulnerable to other problems such as:

• Spider mites
• White powdery mildew (especially if humidity is high)
• Root rot
• Nutrient burn (from increased water transpiration)
• Increased stretching
• Wilting due to root oxygen deprivation
• Reduced bud smell (terpenes can burn away at high temperatures)

  

Temperature Guideline Table

Now that you know why it is important to control the temperature when growing cannabis, next you need to know what the optimal temperatures are for growing cannabis. This table gives a general guideline you can follow.

Growth Stage	Temperature
Vegetative	20-30°C
Flowering	18-26°C

 

Optimal Temperature for Each Growth Stage 

Clones

Clones have not developed a root system yet, so they need to get their water from transpiration of the leaves until their roots form. This is why clones prefer humid conditions with lots of water evaporated in the air. Many growers will use a humidity dome or "cloner" to artificially raise the humidity for their clones.

Humidity is great for clones, but you shouldn't let it get higher than 80% if possible because this can encourage the growth of mould or mildew and can cause plants to root slower. 

Clones seem to root and grow the fastest when the temperature is slightly warm, in the range of 20-30°C.

   

Seedling and Vegetative Stage

Young seedlings tend to grow faster in warm temperatures with mid to high humidity. As the plant gets older, its ability to withstand drier and cooler/warmer conditions tends to increase. Outdoor grown plants tend to be less affected by temperature fluctuations than indoor plants.

During the vegetative stage it is good to aim for a temperature between 20-30°C. This isn't always easy but try to give plants slightly cooler temps during the "lights out" period than during the "lights on" period but still stay between the recommended range.

Maintaining slightly cooler night temperatures, in the recommended range, can promote the fast and healthy vegetative growth in cannabis plants.

Seedlings and plants in vegetative stage prefer comfortable room temperatures or slightly warmer (20-30°C). It is also best to avoid very low humidity during this stage.

 

Flowering Stage

During the flowering stage you should strive to keep the temperature under 26°C, especially towards the second half of flowering when buds are in the midst of development. 

If the buds stay too hot during the flowering stage, these high temperatures may be accidentally burning away some of the terpenes and cannabinoids which can reduce the final taste, smell, and potency of your buds after harvest. This is especially important after week 6 or 7 of flowering, as this is when the terpene content in the buds start to increase rapidly.

Keeping the temperature low during the flowering stage will help your plants produce potent, dense, smelly and trichome covered buds. 

Avoid hot temps during the flowering stage. Keep the temperature in the range of 18-26°C with mid to low humidity. You can also have slightly lower temps at night than during the day.

 

Temperature and Humidity

Temperature and humidity are closely related. Temperature has a significant effect on the relative humidity of your grow area.

Relative humidity measures how much water is "being held" in the air compared to the maximum amount of water that can be held at that temperature.

Temperature plays a big role in humidity because it affects the amount of water vapour that the air can hold. Warm air can hold onto more water than cool air. Plants will thrive at different relative humidities depending on the temperature of the air. Once the air becomes too saturated with water, it tends to form a dew or film of water on the plant, which can lead to mould. 

Temperature is just as important as humidity when it comes to growing plants. You will need to control both temperature and relative humidity at the same time to get the best results from your grow.

Cannabis likes a comfortable room temperature, or a little warmer, not too dry, and not too humid. Generally, cannabis prefers temperatures in the 21-27°C range during the day when lights are on and when lights are off, they are happy with slightly cooler temps.

However, if the temperature drops lower than 15°C, plant growth tends to slow down, and freezing temperatures can shock or even kill a cannabis plant.

 

How to Control Temperature

Whether you are growing your cannabis indoors or outdoors, you need to be concerned about your plant's environment and try create a temperature that is ideal for them to thrive in.

 

Outdoor Growing

Controlling the temperature outdoors is not possible, which means keeping your plants warm can often be a problem. The good news is that most outdoor varieties can endure temperatures of a slow as 10°C without problems. keep in mind thought that 10°C is still not am ideal temperature because it tends to slow down tissue growth and photosynthesis later in the day. 

Temperatures below 4°C can result in tissue damage.

These are some ways to keep your plants warm:

• Gas patio heaters
• Polyethylene plastic covering 

Both these options can keep your plants warn on chilly nights. the plastic covering can also protect plants from the other weather elements that could damage your plants.

 

Indoor Growing

Indoor growing temperatures are directly controlled by the grower. It’s all up to you to make sure your grow room maintains the correct temperature. To help you with this, here are some steps you can follow:

Step 1: Invest in a way to monitor temperature

It is essential to purchase a thermometer to help keep track of and monitor the temperature in your grow room. There are also monitors called Thermo-Hygrometers which measure both temperature and humidity.

Step 2: Refer to the temperature chart

You can always refer back to the guideline chart above to determine if the temperature in in the optimal range for the growth stage your plant is in.

Step 3: Make adjustments if the temperature is too high or too low 

  

How to lower too hot temperatures:

Exhaust system - Make sure you have an adequate exhaust system to pull out hot air and increase air circulation to your plants. If you don’t have an exhaust fan pulling out hot air created by your lights, then fixing this is your first step. An exhaust system not only keeps temps down but also improves air circulation which helps your plants thrive.

Air circulation - Hot spots tend to be created when there is a lack of proper air circulation. Installing fans in the grow room will work wonders to help circulate the air inside the tent. Remember not to point the fans directly at the plants. Leaves can be gently rustling but stems should not be waving around like crazy. Rather direct the fan to blow over or under the canopy.  

Change the light schedule - If you find that your temperature is climbing too high at a specific part of the day, then you can change your light schedule. Consider having "lights off during the day and "lights on" at night. For example, some growers have their lights turned on at night when it is cooler and keep the lights off during the hottest parts of the day.

Consider getting a different or smaller grow light - Some grow lights give off a lot more heat than others. 

Air conditioner - If you have good air exchange and have tried the steps above, but the temperature is still too hot, you can use this to help cool the air.

Get a heat resistant strain - Some strains can naturally handle the heat without any problems compared to other strains.

 

How to increase too cold temperatures:

Insulation - Consider insulating your grow room or tent with something that holds in heat. 

Bigger light - You can consider upgrading to a bigger grow light. This will mean more heat for your plants. 

Heating mat - Placing a heating mat under the plants can help make things warmer. 

Change the light schedule - Not only does changing your light schedule help with too high temps but it will also help with too low temps. Keeping your grow lights on at night will keep plants warmer when it is coldest. 

Get a cold resistant strain - Some strains are hardier than others when it comes to the cold.

  

Some Final Notes on Controlling Temperature in the Grow Space

Invest in a hygrometer and thermometer to monitor and keep track of the temperature and humidity in the grow room. Cannabis likes a comfortable room temperature, or a little warmer, not too dry and not too humid so as a grower you will need to create that perfect balance in their environment. If your grow room feels too warm or cold, humid, or dry, it is a sign that you may want to look into changing the temperature or humidity of your grow area.