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The Importance Of Temperature In Growing Cannabis Plants 

Added 31 May 2022

cannabis bud

Cannabis plants don't demand too much. All they need is some water, light, nutrients, and space to grow. But of all the essential factors required, the humidity and temperature are perhaps the most important.

You can create a fantastic environment for your plant, but it will still not perform if the temperature is not in the correct range.

In short, they require temperatures similar to us humans. They get uncomfortable if it is too warm or too cold. In addition, they need an environment that is neither too humid nor too dry — just a perfect balance like we humans need.

A straightforward way to check if the temperature and humidity are in the proper range is to get into the grow room and check how you feel. If the temperature is too hot for you, it's too hot for the plant. Similarly, if it is too cold in the grow room for you, the plants will also struggle.

If you have grown a few cannabis plants, you already know the importance of temperature and humidity. They are not only essential in the growing stages, but you can't ignore them during the drying and curing stages as well.

In a nutshell, the temperature and humidity can be your best friends if you want to grow stunning cannabis buds.

This article will discuss everything about temperature and humidity you need to know. We will also discuss how you can increase or decrease the temperature and humidity in your room.

Best temperatures to grow cannabis

best temps to grow cannabis

Cannabis plants require different temperatures in different stages of their growth. So, just like the temperature changes along with the season, you need to mimic them in your grow room.

Almost every cannabis plant will do very well if the temperatures are maintained between 20°C to 28°C or 68°F to 82°F. Of course, these temperatures are great when the lights are on in the grow room or when it is daytime for the plants. But, on the other hand, they prefer cooler temperatures when the lights are off or during the nighttime. It is similar to how we humans like cool air while sleeping at night.

Here's a breakdown of optimal temperatures for every growing phase so you know how to adjust the temperatures for the best results:

  1. Seedling stage

seedling stage

Seedlings prefer slightly warm temperatures ranging from 20°C to 28°C or 68°F to 82°F. In addition, make sure that humidity is not too low.

  1. Vegetative stage

vegetative stage

This is when the plants are still growing, whether they are seedlings or slightly mature plants. So basically, plants that are still in the 18/6 light/dark cycle are in the vegetative stage unless you are growing autoflowering plants because the light cycles don't matter much to them.

During the growing phase, cannabis plants will enjoy slightly warmer temperatures. Anything ranging from 20°C to 28°C or 68°F to 82°F will work perfectly.

As they progress in the vegetative stage, the plants will be able to withstand higher temperatures ranging from 20°C to 30°C or 68°F to 86°F during the day. 

However, be cautious when the temperature exceeds 28°C as the plants will struggle if the ventilation is insufficient. Poor ventilation can result in hot spots, and the plants will feel the heat faster than you can realize. 

The plants will love it if the temperature dips when the lights are off. 

This doesn't apply to plants growing outdoors as they are more resilient and handle temperature fluctuations better than most indoor plants.

  1. Flowering stage

flowering stage

This is when the plants start growing pistils that eventually turn into big buds. During this stage, plants prefer cooler temperatures ranging from 18°C to 26°C or 65°F to 80°F.

Cooler temperatures help the plants develop bigger buds with more terpene production. As a result, the buds tend to be dense, more colorful, and produce better fragrance. 

For example, many growers wonder how to make their buds turn purple. Although this depends on genetics, many purple strains remain green even during the late flowering stages, mainly due to the temperature. Therefore, reducing the temperature by at least 8°C to 10°C between the light and dark cycles can bring out vivid purple colors if you have a purple strain.

Even if the plants can survive hot temperatures during the vegetative stage, the temperatures must be as low as possible during the flowering stage. It is essential because all your hard work depends on this final stage. 

From the development and density of the buds to the trichomes and the fragrance, low temperatures have a vital role to play. High temperatures above 28°C will reduce the potency and cannabinoids and force the terpenes to evaporate. Try to maintain constant temperatures ranging from 18°C to 27°C during the day, but it should go down at night. However, make sure it doesn't dip below 15°C. 

In short, the plants will do well if the temperatures are consistently 25° C during the entire flowering stage. You don't have to obsess over the temperatures but try and maintain it below 26°C as much as possible. 

Why is temperature critical for cannabis plants

Many beginners buy lights, grow tents, carbon scrubbers, and whatnot but hesitate to invest in devices that control humidity and temperature. You can somehow wing it if you reside in a location perfect for cannabis growth; however, if you don't, you must spend some extra money to manipulate the humidity and temperature.

You're probably wondering what happens if the grow room gets too cold or too hot — valid questions. So, let's get into why temperature matters for growing good cannabis plants.

High temperatures

Hot temperatures

High temperatures will usually result in heat stress. Although the plants will not die a quick death, the growth will be exceptionally slow, making it hard to get a good yield.

By now, you already know that it's crucial to take care of flowering plants. It's okay if the temperatures go up slightly during the vegetative stage but not during the flowering stage. 

Apart from heat stress, where the symptoms are burnt and cupped leaves, and slow growth, the plants will face several issues if the temperatures are too hot to handle.

For example, most pests like spider mites love high temperatures. And if the humidity increases along with the temperature, you will also face problems like white powdery mildew and other fungus issues, including root rot. Apart from wilting, the plants will stretch beyond their normal growth, leading to airy buds. Furthermore, the plant will be deprived of oxygen in the root zone, leading to many other issues.

If you're continuously struggling with high temperatures, you'll have to come up with a plan to reduce the heat. 

But, before you do anything, take a look at your grow lights. HID lights tend to produce a lot of heat that can be unbearable for the plants, especially if you have a tiny grow room. For instance, many growers use MH (Metal Halide) lights during the vegetative stage and then switch to HPS (High-Pressure Sodium) lights during the flowering stage. However, both types of lights produce immense heat. Comparatively, LED lights emit less heat, so you may consider switching to them.

Here are a few ways to combat high temperatures in the grow room:

  1. Get an Air conditioner

Agreed, it's an extra investment. But, think about this — is it better to get excellent yields with one harvest, or would you prefer multiple harvests with low yields? Without a doubt, you should pick the first choice because you'll save a lot of money, including electricity bills and time. 

In addition, think about what multiple harvests will do to your lights. They will lose their efficiency pretty quickly. Of course, the AC will make your bills more expensive, but you won't have to struggle with multiple harvests to get what you need.

You can also install coolers since they are a tad cheaper than ACs. However, they will also increase the humidity as they throw cool water. Therefore, use a cooler only if you live in an area with a dry climate. It would help if the humidity ranged below 35%.

  1. Increase air circulation

Install more fans so that the air circulates more. However, you need to ensure that the hot air is vented out of the grow room, so it doesn't recirculate around the plants. Installing more fans is great, but it's useless if the ventilation isn't designed correctly.

In addition, remember that fans don't necessarily cool down the temperatures. Instead, they prevent the air from collecting in small areas and aid in even distribution. Therefore, if you reside in a hot climate, the fans will simply throw in hot air in the grow room. On the other hand, if your grow room's temperature is higher than your room's overall temperature, the fans will cool down the grow room to a certain extent.

  1. Manipulate the light cycle

In most regions, the temperatures are hotter during the day and cooler at night. You can take advantage of this by adjusting the timer to switch on in the evening. For instance, if the temperature drops after 4 pm, the timer can switch on at 4 pm and go on till 10 am when it's not too hot yet. 

Of course, this applies if you are following the 18/6 cycle. If you're growing autoflowers, the timer can switch on at 6 pm and go on till 6 am. The plants will enjoy slightly cooler temperatures by following this technique as the difference between day and night can sometimes go above 10°C. In addition, it will prevent your grow room from becoming too hot during the day due to the bright lights.

  1. Genetics

There's a reason why some strains are resistant to cold while others are heat-resistant. Seedbanks typically give you detailed information about the strain.

So, if you live in a hot climate, it's best to go for heat-resistant strains that can survive the heat. Similarly, if you live in an area that is too cold, get cold-resistant strains. You can browse through all strains on our Grow diaries catalog to get more information on what suits you.

  1. Use CO2 generators and supplements

One popular way to deal with the heat is to increase the levels of co2 in the grow room. Carbon dioxide is an essential element in the process of photosynthesis. When the levels increase from about 400 ppm to at least 1200 to 1500 ppm, the rate of photosynthesis increases, thereby helping the plants to function more efficiently. In other words, the plants will perform even in hotter conditions.

Plants intake CO2 when there's bright light. For instance, your plants are more likely to intake CO2 if you use extremely bright HPS, MH, or expensive LEDs. However, it takes experience to work with CO2 generators. Some growers also use dry ice or ferment yeast to create CO2. But, you won't be able to produce enough CO2 to combat hot temperatures.

The presence of CO2 in the grow room enables the plants to withstand high temperatures. However, there is a caveat — the CO2 must be at least 1200-1500 ppm to work. In addition, too much CO2 can have adverse effects on the grower, including dizziness, nausea and even death. Thus, this is only for advanced growers who understand the science behind using CO2. Beginners should look for other options like supplements that reduce heat stress. Of course, you can also use such supplements to prevent heat stress in the first place.

  1. Raise the lights

One way to quickly reduce high temperatures is to raise the lights slightly away from the plants. You can do this to ensure the tops of the plants don't burn. However, remember that you'll waste light. This is only a temporary fix until you figure out a way to reduce the heat permanently. 

Now that you understand what happens when the temperatures are too high, let's look at the changes that occur when the temperatures are too low.

Cold temperatures

Cold temperatures

Just like high temperatures, extremely cold temperatures will also force the plant to grow very slowly. Although cannabis plants tolerate low temperatures ranging from 16°C to 20°C, they will struggle if the temperature dips below 15°C or 59°F

Also, this depends on the strain. So, if you have strains resistant to the cold, they may be able to handle temperatures anywhere from 15°C to 18°C or 59°F to 64°F.

Sometimes the plants show no signs of stress and continue to grow well, but they don't yield well ultimately. If your grow room is cold with high humidity, there are a lot of chances of attracting mold and fungus issues.

The good news is that the strains will look beautiful with rich purple and pink colors, but the extreme cold will halt photosynthesis. Cannabis, like most other plants, cannot handle freezing temperatures. If you're growing outdoors, consider shifting the pots inside for some warmth. However, if it's freezing indoors, you will have to invest in a heater to increase the temperature. 

Here's how to combat cold temperatures and save your plant:

  1. Use HID lights

LED lights are perfect for hot grow rooms because they produce little heat and are very bright. On the other hand, HID lights are bright too, but they emit intense heat, making it challenging for plants to survive if the temperatures are already too hot. 

You can use this to your advantage by installing HID lights if the temperature is too cold where you live. The best part is that HPS performs very well, especially if the temperatures are lower than 25°C. It also produces ample infrared light making the plants slightly more resistant to the bitter cold.

  1. Manipulate the light cycle

Like you can manipulate the light cycles when the temperature is too hot, you can do the same when the temperature dips too low. Usually, the temperatures are okay during the mornings but plummet during the night. 

In such cases, you can automate your timer to turn on during the night when it is the coldest, so the lights are on and increase the temperature slightly. Again this technique depends on whether your plants are in the vegetative or flowering phase. If you cannot reduce the temperatures, you could try growing autoflowering plants as they need only about 12 hours of light and dark cycles.

  1. Genetics

Purchase strains that are meant to be grown in cold temperatures. Again, you will find many strains in our catalog. Make sure you go through the information posted by the breeder thoroughly to make an informed decision. 

  1. Insulation

You can insulate your entire grow room with building insulation to ensure it withstands chilly temperatures. Simply wrap it around the grow room. In addition, rather than placing the pots on the floor, you can keep them on a rug or a carpet so the roots don't get too cold.

  1. Heaters

If the temperatures are too extreme, you will need a heater. There are various types of heaters so you must choose something that isn't too expensive or doesn't pull a lot of electricity.

If you have a small grow room, tube heaters will work very well as they are small and heat the air consistently. They are also small, so you can dedicate a small space to place them. However, tube heaters are not controlled by a thermostat, making it difficult to adjust the temperature according to your liking.

Instead, you can go for oil-filled radiators that heat the air without damaging the plants. Another option is to use a fan heater, but you need to be careful as hot air tends to reduce humidity, which can be detrimental for your plants. You can, however, use them outside your grow room to heat the air.

  1. Pay attention to the intake

Many growers construct their grow tent in such a way that they draw the air from the outside into their tents to deliver some fresh air to the plants. Although this is a great idea in normal circumstances, it may not work if the temperature outside is freezing. 

For instance, if the temperature outside is less than 15°C, you're drawing the cold air inside, preventing your plants from staying warm.

To solve this problem, position your intake fan to draw fresh air from another area indoors. Even if it is placed just outside the grow tent, the addition of carbon dioxide will help the plants grow well and keep the tent somewhat warm. Rather than installing the heater just for the grow tent, you can install the heater in the room and place the intake fan in such a way that it sucks in the hot air.

Monitor the temperature frequently

Last but not least, try to monitor the temperatures as frequently as possible. Sometimes the plants may not show any symptoms of stress even if the temperatures are too hot or cold; however, as mentioned earlier, the yield may not be satisfactory. 

To prevent this, monitor the temperatures using a digital thermometer. Keep in mind that cannabis plants love temperatures ranging between 24°C to 28°C (75°F to 82°F) during the day and about 17°C to 21°C (62°F to 70°F) at night. In other words, when the lights are on, the temperatures should be slightly higher, whereas the plants need cooler temperatures when the lights are off.

Summary: The Importance of Temperature in Growing Cannabis Plants 

Cannabis plants aren't very demanding. They will grow on their own outdoors. However, when you grow them indoors, you need to mimic the environment outdoors, which means you should manipulate the light cycle and the temperature. 

Cannabis plants love it when the temperatures range between 20°C to 28°C or 68°F to 82°F. It can go slightly higher during the vegetative stage, but it's important to reduce the temperatures during the flowering phase. 

High temperatures during the blooming phase will reduce the potency, burn off the terpenes and also reduce the yield to a great extent. 

There are various ways to reduce and increase the temperatures, depending on where you live. But, most importantly, choose strains that adjust well to your climate and plan ahead.