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The ideal temperature for cannabis - all stages of growth

It is not always easy to keep your cannabis plants at the perfect temperature. Some would argue that there is a fine line between science and art.

But that doesn't mean you can't figure out how to get the ideal temperature for your plants. You just have to learn a few things first, which we have summarized for you in this guide.

We are going to explain to you why the temperature is more than just degrees and why it is a little more complicated indoors. We'll also show you how to identify temperature-related problems and make adjustments to your grow room. Read on to find out all about temperature when growing cannabis.

The influence of temperature on cannabis plants

Plants are pretty self-sufficient, but they have a disadvantage when it comes to temperature. While vital to their health, plants cannot generate heat of their own. Unlike animals and humans, a cannabis plant is completely dependent on its environment.

How does the plant get its heat?

The temperature of a plant is dependent and arises from a combination of light, outside temperature and the amount of evaporation. The exact temperature of a plant cannot be read on a thermometer, but it is a definitive measure of health.

Cannabis plants don't usually die from it if it's too hot, but it can slow their growth. High outside temperatures (over 26 degrees Celsius) during flowering not only slow down flower growth, but also reduce the aroma and effectiveness.

If you are interested in developing high-cannabinoid buds, you need to make sure that you are in control of the room temperature during the flowering phase.

The quality of the buds is reason enough to worry about the temperature, but there is a lot more to it than that. You should also take care of the health of your plants. In general, too much heat causes stress on the plants.

If your plant gets too warm, photosynthesis is impaired, enzyme activity decreases and fewer proteins are produced. Some proteins are even broken down. If this continues long enough, your plant may die.

Below are some of the ways temperature affects cannabis growing.

Most of us understand how plants make sugar, right?

An example of how temperature can affect the overall health of your plants is photosynthesis. To a certain extent, photosynthesis is not affected by temperature. This can occur without concern at 15 ° C to 30 ° C. Regardless of the temperature, your plant can still produce enough sugar.

Temperature becomes a factor in when your plant needs to send this sugar to the areas in the plant where it is needed. Sugar doesn't move as well when the plant is below 20 ° C. In fact, the sugar gets stuck and your plant suffers.

Make sure the plant is warm enough to function properly.

If this happens in mature plants and only lasts for a few days, this is not a major problem. Once the temperature returns to normal, the sugar will go where it should be. In immature plants, however, this situation slows the plant's growth.

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Another function that is affected by temperature is breathing. Breathing decreases as the temperature drops. This should be a good thing as it will lower the amount of energy a plant uses. However, it also creates a dangerous hack.

Why this is a problem:

When a heavier plant develops due to a lower temperature, the overall breathing of the plant increases. Now, when the temperature rises, more energy is needed to keep the plant alive.

This leads to an even bigger problem: the additional energy requirement leaves little for growth.

Heat also causes other problems with cannabis plants. If it's too hot, your plant is more likely to suffer from:

  • Spider mites
  • Root rot
  • mildew
  • Nutrient burning
  • Withering
  • Stretching the plant

These problems become even more likely when it is also humid or when water transpiration increases.

The ideal temperature for cannabis depending on the growth phase

Your cannabis plant reacts differently to temperature changes depending on the growth phase. As a grower, you need to make sure that your plants are at the best temperature for each stage of growth. Below are the ideal temperatures for each stage of the growth process.

Clones do not yet have a root system, so they depend on the transpiration of their water. They therefore need high humidity, at least until they are fully rooted.

Many growers use a mini greenhouse or similar with a dome to create an ideal amount of humidity for clones.

The ideal temperature for clones & seedlings is between 20 and 25 ° C with high humidity. At these temperatures, they should take root quickly and become more self-sufficient.

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During the vegetative phase, young plants prefer high humidity (70% or more) and temperatures between 20 and 25 ° C. However, as the plant ages, a slightly lower humidity is okay.

For plants grown indoors, fluctuations in temperature and humidity can be more problematic than for plants grown outdoors, as plants grown outdoors can withstand more temperature fluctuations.

The ideal temperature during the vegetative stage is between 20 and 25 ° C with moderate humidity and in dark periods somewhat cooler temperatures (17-22 ° C).

The cooler night periods are perfect for encouraging growth. Do not let temperatures drop below 15 ° C. Once the plant matures, it should be able to withstand cooler temperatures and drier air during the day.

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Once the plant has entered the flowering phase, it can thrive in a comfortable, low-humidity room.

Unless you add additional CO2 to your grow room, a temperature around 25 ° C is ideal.

These lower temperatures encourage vigorous, trichome-rich flowers with a wonderful aroma. Temperatures that are too high would literally burn them up.

Do not exceed 28 ° C, as terpenes evaporate at higher temperatures and flower growth is slowed down. If the buds get too hot during this time, you may be literally burning off the good ingredients as they grow, leaving very little taste or odor at harvest time.

Make sure that your plants are comfortable in weeks 6 and 7. During this time the terpene production is highest and you risk to let it evaporate if the temperature is too high.

For optimal trichome production (part of the plant where the cannabinoids are made) make sure it is a little cooler in the dark periods. The change in temperature can lead to an increased terpene content as the plant processes are optimized.

But don't let it get too cold (not below 15 ° C).

This process leads to a so-called dew point. The dew point occurs when the air can no longer absorb water vapor and precipitate as dew drops.

If you're unfamiliar with terpenes, this is about more than just taste and smell. They also affect the color of the flowers and the entire plant. Terpenes are responsible for the red color of tomatoes and may do something similar with certain cannabis strains.

With the right temperature in the dark periods, you can bring out interesting colors on your plants (e.g. blue, pink and purple).

This phenomenon naturally depends on the cannabis strain. Most cannabis plants only get green flowers. However, if you want to see what your plant can do, keep the temperature cool at night and find out.

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Harvesting the plants is not the final step towards high quality buds. Even with the best genetics, you need to properly dry and refine your buds. The quality of good cannabis flowers depends to a not insignificant extent on the drying and refinement of the harvest.

Professionally dried and refined marijuana is more effective, tastes better, looks better, and produces a smoother smoke.

If you want to get the best result from your harvest, focus on maintaining the right temperature and humidity throughout the process. This prevents mold and overdrying and makes it easier to produce the best possible dried cannabis flowers.

Keep temperatures around 18 ° C and humidity at 45%. Make sure that the room in which you are drying your cannabis has an air flow or that there is continuous air exchange. However, do not let the airflow blow directly onto the crop. As a result, they dry too quickly.

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Create the perfect environment

Whether you are growing indoors or outdoors, you need to take care of the environment around your plant. Try to create a temperature that is ideal for them to thrive as best they can.

Outdoor cultivation

When growing outdoors, keeping your plants warm can often be a problem. The good news is that most outdoor strains can easily withstand temperatures as high as 10 ° C.

That being said, 10 degrees Celsius is not an ideal temperature as it slows tissue growth and photosynthesis later, even when it is warm in the day.

Anything below 4 ° C can damage the plant's tissues.

There are ways to keep your plant warm outdoors. Try:

  • Gas patio heater
  • Polyethylene plastic covers

Both ways can keep your plants warm outdoors on cold nights. The cover also protects plants from other potential weather hazards.

The idea is to have a temperature of at least 15 degrees Celsius adhere to to encourage growth.

Indoor cultivation

The temperatures in indoor cultivation are controlled directly by the grower, so there is room for failure as well as success. All you have to do is make sure your grow room is maintaining the correct temperature.

Here's the problem:

The ideal temperature in your grow room depends on several factors. For example, the location of your grow room in the building is important. A grow room or grow tent in the basement is much cooler than in an attic.

The size of the room, the airflow, the number of grow lights and the fan used also play an important role. Keep this in mind when building your grow room.

With active lamps, the ideal temperature for cuttings and seedlings is between 20 -25 ° C.

As the plants get older, the plants can withstand more evaporation and the temperature can rise to a maximum of 28 ° C.

When the lamp is off, the temperature should be between 15 ° C and 22 ° C. Another important rule is that the temperature differences between day and night must not be too great. Pay attention to a maximum difference of 10 ° C. If it is 28 ° C during the day, the temperature should not drop below 18 ° C at night.

In general, indoor cannabis growing works best in moderate temperatures:

  • 20 and 25 ° C during the lighting phase
  • With a temperature drop when the light is switched off of a maximum of 10 ° C.

Plants that are enriched with CO2 can produce the most biomass and thus flowers at a temperature of almost 28 ° C.

When the soil is at a constant temperature of 27 ° C, the roots are warmed and the stems and leaves can withstand colder air. A heating mat is ideal when you only have a few plants to worry about. For larger gardens, a water heater may be required to maintain optimal temperatures.

Temperature and humidity

In a grow room, you also need to think about the humidity.

Here is some information about how humidity works. The air that our plants breathe contains water vapor. The amount of water vapor in the air can vary. That is the humidity.

The humidity in a grow room is always a little higher, as the leaves of the plant allow water to evaporate. Cannabis plants only use 10% of the water that they absorb through the roots and leaves and evaporate the other 90%.

However, the air can only absorb a certain amount of water (steam). When this maximum amount (saturation) is reached, condensation begins. You will notice condensation as water droplets in the colder areas of the grow room or on your plant.

Temperature plays a big role in humidity as it affects the amount of water vapor that the air can hold. Warm air can hold much more water vapor than cold air.

How do we know we are about to have too much moisture?

Experts do the following: They measure the relative humidity.

Relative humidity (RH) is a measure of the moisture content of the air. For example, a relative humidity of 70% means that more water vapor can be absorbed into the air. However, 70% at 25 degrees Celsius differs from 70% at 20 degrees Celsius, as warmer air can absorb more moisture.

Particularly at the higher temperature, 1 kg of air can contain up to 12 grams of water vapor. On the other hand, 1 kg of air holds only 9 grams at 20 degrees, so if you let the air cool down, the air can contain less water vapor, which leads to condensation and at the same time increases the relative humidity.

Example: air cooled at 25 degrees to 22 degrees - RH increase from 70% to 80%

This process leads to the so-called dew point. The dew point occurs when the air can no longer absorb water vapor and precipitates into dew drops.

The dew point is important because:

It will help you find the ideal temperature for your grow room. In the example above, we know that the dew point of 25 degrees at 70% relative humidity is 19 degrees. Hence, we need to keep the temperature above 19 degrees to avoid excessive humidity.

The ideal humidity for cannabis
ClonesSeedlingsGrowth phaseFlowering phaseDrying

Maintaining the correct humidity in your grow room is important as the humidity level can change frequently.

In a well-ventilated grow room, the temperature drops regularly. The heat from the lamps and the natural humidity are regularly extracted by fans and other ventilation methods, which changes the moisture content. Even if this changes regularly, you need to make sure that the humidity is optimal for your plants.

It is easy to measure the temperature outdoors. But it's not really difficult indoors either.

First of all, you should measure the temperature in your grow room with a thermometer. There are analog and digital thermometers that are widely available for a few euros.

We always use a digital thermometer / hygrometer (something like this) with built-in memory so we can see the maximum and minimum temperature.

It is also very convenient to attach one with a temperature sensor to a cable in the grow room so that you can have the display outside of your grow room and read the temperature without opening the grow room.

Here are some tips for taking a good temperature reading:

  • Always measure the temperature in the shade
  • Measure at different points in your grow room.
  • Provide good air circulation by setting up multiple fans. Lamps give off radiant heat; However, this has no effect on the air temperature.

It is always a few degrees warmer directly under the lamp than somewhere in the shade. The amount of temperature change caused by lamps is not really a problem. Just make sure there is enough space between the lamp and the plant.

Don't get this wrong, however, as radiant heat affects leaf temperature. Therefore, the lamps should not be too close to the plants, as this can lead to heat stress.

Maintaining the ideal temperature

Maintaining the ideal temperature for your plants can be a bit tricky, but it's not impossible. When you grow indoors, you have control over the environment.

The temperature is important for the photosynthesis of your cannabis plants. Low temperatures reduce evaporation through the leaves. This reduces the plant's suction power to absorb nutrients via the roots.

What happened next:

The nutrients that are necessary but not absorbed remain in the growing medium (soil, etc.) and eventually disintegrate in the root environment, which changes the pH of the growing medium towards acidity. A high acid content in the growth medium reduces the functionality of the roots, which means that the plant can absorb less water and nutrients.

Growth comes to a standstill.

This is why maintaining the right temperatures is vital to the health of your plants.

If the temperature in a grow room falls below 15 ° C during the dark phase, the plants will grow more slowly and the yields will not be as high. This is not readily apparent if you are not particularly familiar with the normal performance of the grow room or the productivity of your cannabis strains.

Lamps are used in the grow room, so there is also the possibility of overheating or heat stress. This is caused by the incorrect positioning of the lights. In this situation, the leaves won't actually burn, but they will be damaged.

This is rarely the case with plants grown outdoors, as these are temperatures of over 40 degrees. However, this can occur with plants grown indoors if you are not careful.

You will recognize heat stress on the top leaves of your plant. They begin to turn yellow and curl inward. This happens when the plant tries to protect itself from the heat. It is a simple mistake that can quickly damage your plant or cost your life.

Fix temperature problems

If you've already noticed temperature related issues with your plants, it's not too late to save them. Here are three different problems you may encounter in your grow room and ways to fix them.

Too hot Too cold Burned by a lamp

Avoid exposing your plants to excessive heat by hanging your lamps a little higher and / or by using a fan to blow some of that excessive heat away.

You can also use air-cooled lamps with reflectors that reduce the heat near the light. Water-cooled lamps are actually more effective at reducing the heat generated by light.

In most cases, HPS and MHL lamps should be kept a distance of about 8 to 10 cm per 100 watts from the tops of the plants. Air-cooled lamps make the acceptable distance between 5 and 8 cm per 100 watts possible.

Water-cooled lamps provide an acceptable distance of about 5 cm or even less per 100 watts from the tops of the plants. With EasyRolls lamp jojos you can move the lamps closer or farther away as you wish.

A few nights of cool temperatures won't do any significant damage to your plant, but if these continue during the flowering period this can definitely be a cause for concern. A CO2 generator or an electric heater can heat the room sufficiently.

When the lights are on, there is usually no need to raise the temperature. The lamps take care of that. However, it is important to distribute the air evenly in your room so that you get the same temperature everywhere. To do this, use pivoting fans and align them between the lamps and the plant.

The fresh air from outside should also be well distributed over the cultivation areaso that there are no cold spots. Especially in winter when the temperature can be below freezing.

However, when the lights are off, your grow room can get cold. Fortunately, there are a ton of things you can do to increase the temperature.

Try these following out:

  • A simple room heater with a thermostat (remember, these use a lot of electricity)
  • A radiator with a thermostat
  • A CO2 generator

You can also turn off the exhaust fan (which provides the fresh air) as soon as the lights are turned off. This can be done by a so-called fan control with a thermostat or with a timer.

Heat often becomes a problem in a grow room. This has to do with the fact that the lamps give off a lot of heat. Fluorescent lights don't give off much heat, but HPS and MHL lights can heat your room to temperatures as low as 50 ° C, which is deadly to your cannabis plants. LED grow lights also emit less heat.

How can you avoid burning your plants?

First of all, the dimension of the room is important. Always use a size of at least 100 cm x 100 cm x 200 cm for a 600 watt HPS lamp.

Use the following simple formula for ventilation:

Number of watts divided by two = extraction capacity in m³.

Using this formula, a 600 watt HPS lamp requires an exhaust fan with an extraction capacity of 300 m3. The exhaust fan sucks the hot air out and pulls the fresh air in.

You can also add a carbon filter to your exhaust system to neutralize the smell of the plants. You can also put the ballast for the lamps etc. outside of your grow room as they generate a lot of heat.

Perhaps the easiest way to keep the temperature low is to leave your lights on at night and give the plants the dark phase during the day. Your lamps turn on a few hours after sunset and turn off a few hours after sunrise. This way you turn on your lights at the coolest time of the day.

You can imagine that in hot weather you will also suck this hot air into your grow room with your extractor. The room temperature is therefore never lowered below the outside temperature. There are also professional growers who use air conditioning to keep them running during the day and night.

Understanding how to get the best temperature for your plants is one of the easiest ways to ensure successful growth. All you need to do is learn some best practices, measure regularly, and adjust as needed. There are many ways to do this, including when growing outdoors.

If you focus on providing the perfect temperature, you will find your plants can thrive. Give your plants the best start by buying seeds from

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