Best temperature for growing marijuana plants

Ensuring your plants have the perfect temperature is not the easiest thing to do. Some would say it is as much art as it is science.

That doesn’t mean you can’t figure it out, though. You just need to learn a few things first; which I will teach you in this article.

I’ll explain why cannabis temperature involves more than degrees, and why it is even more complicated when growing indoors. I’ll also show you how to recognize temperature-related problems and make adjustments in your grow room

Keep reading to learn everything about temperature while growing marijuana or skip ahead to your favorite section

Key takeaway

The ideal temperature for growing marijuana plants varies depending on the growth stage, but generally, temperatures between 68-86°F (20-30°C) is a safe range for indoor cultivation.

However, a range between 68-83°F (20-28°C) is considered optimal. During the vegetative stage, temperatures at the higher end of the range are ideal, while lower temperatures are preferred during the flowering stage.

It’s essential to maintain consistent temperatures and avoid large fluctuations, as extreme changes in temperature can stress the plants and affect their growth, health, and overall yield. In addition, nighttime temperatures should not drop below 60°F (15°C), as this can also harm the plants.

Maintaining the right temperature for growing marijuana plants is crucial for a successful harvest. By keeping the temperature in the optimal range, you can ensure healthy and robust plant growth, as well as a high-quality yield. 

A quick overview of temperatures for all grow stages

How cannabis plant growth is impacted by temperatures

Most of us understand how plants make sugar right?

Well, one example of how temperature can affect the overall health of your plants is the process of photosynthesis. To a certain degree, photosynthesis is not affected by temperature – it can safely occur at 60ºF (15ºC) or 85ºF (30ºC). Regardless of the temperature, your plant will still be able to produce enough sugar.

Be sure to check my free Grow Bible for more tips on controlling temperature in your grow environment

However, in general, too much heat causes plants stress.

If your plant gets too warm, photosynthesis is impacted, enzymes activity decreases, and fewer proteins are produced. Some proteins even break down. If this continues long enough, your plant can (potentially) die.

Below are some ways temperature impacts the marijuana growing process.

My selection of indoor seeds are perfect for growing in a controlled environment, as long as you keep a close eye on the right temperature. Some work fine when grown outside as well, but these are the seeds you need if you’re growing indoors.

Buy indoor seeds

• Wide selection of indoor seeds
• High THC levels
• Consistent heavy yields
• Easy going high for everyday use

BUY INDOOR SEEDS

Photosynthesis

Temperature becomes a factor when your plant needs to send those sugars to the places they’re needed. Sugar doesn’t move as well when it’s less than 68ºF (20ºC). In fact, the sugars will get stuck, and your plant will suffer.

In other words, make sure the plant is warm enough to function.

When this happens in mature cannabis plants and only lasts a few days, it’s not that big of a problem. Once the temperature is resolved, the backed-up sugars will go where they should be. However, in immature plants, this situation will stunt the plant’s growth.

What is too cold for growing weed?

When the temperature drops too low for the cannabis plant, it can cause various problems that can negatively impact growth and development. 

Here are some of the most common effects of low temperatures on the cannabis plant:

• Slow growth: When temperatures fall below a certain threshold, the plant’s metabolic processes can slow down, leading to reduced growth. In extreme cases, growth may stop completely.
• Leaf damage: If the temperature drops too low, the leaves of the cannabis plant can become discolored and develop frost damage, which can cause permanent scarring or even death of the affected tissues.
• Root damage: Low temperatures can also harm the roots of the cannabis plant, which can reduce the plant’s ability to absorb water and nutrients. This can weaken the plant and make it more susceptible to disease.
• Increased susceptibility to disease: In cold and damp conditions, the cannabis plant is more susceptible to various diseases, such as root rot and powdery mildew, which can cause serious damage.

Another temperature-sensitive function is respiration. Respiration decreases as the temperature drops. This should be a good thing because it lowers the amount of energy a plant uses. However, it also creates a dangerous crutch.

What is respiration, and how it affects your weed plant at low temperatures?

In the case of the cannabis plant, respiration is an essential process that allows the plant to grow and produce flowers. During respiration, the plant converts stored sugars into energy that it can use to grow.

Here’s why that’s a problem:

The rate of respiration in the cannabis plant is directly affected by temperature. As the temperature increases, so does the respiration rate, and vice versa. This is because the rate of metabolic processes, including respiration, is influenced by temperature-sensitive enzymes that control these processes.

That leads to an even bigger problem:

If temperatures fall below the lower threshold, the respiration rate will decline, and the plant will experience stress. At very low temperatures, the plant may enter a state of dormancy, in which its metabolic processes slow down or stop altogether.

By the way, in my Weed Grow Bible you can learn more about the plant’s biological processes.

What happens when a cannabis plant gets too warm?

High external temperatures (above 86°F/30°C) while flowering will not only slow down bud growth but also reduce their smell and potency. If you care about growing buds with plenty of cannabinoids, you need to be sure the external temperature is kept under control during the flowering stage.

“A plant’s temperature develops from a combination of external light, external temperature, and the amount of evaporation. A plant’s exact temperature is not something you can read on a thermometer, but it is a definite measure of health.”

Heat also causes other problems for cannabis. When it is too hot, your plant is more likely to suffer from:

• Reduced growth: High temperatures reduce the plant’s ability to carry out photosynthesis. This can lead to reduced growth and decreased yields. If this continues long enough, your might even plant can die.
  
• Leaf damage: Prolonged exposure to high temperatures can cause the leaves of the cannabis plant to become discolored and develop leaf scorch, which can weaken the plant and make it more susceptible to disease.
• Increased susceptibility to disease: In warm and humid conditions, the cannabis plant is more susceptible to various diseases, such as powdery mildew and root rot, which can cause serious damage.

These situations are even more likely if it is too humid or if there is increased water transpiration.

The ideal temperature for marijuana per grow stage

Okay, we’ve established that temperatures affect the marijuana plant and how that works, but what is the best temperature for cannabis plants?
Good question, but a loaded one too. You see, it all depends on what stage your cannabis plant is in. From seedling to flowering, all stages of the cannabis plant’s growth have different temperature requirements

Best temperatures for cannabis seedlings and clones

The ideal temperature for seedlings and clones is between 68-77°F (20-25°C) with high humidity (65-70%). At these temperatures, they should quickly form roots and become more self-sufficient. 

Within this range, the seedlings can carry out their metabolic processes efficiently, which supports healthy growth and development. The temperature should also be stable and not fluctuate too much. Sudden temperature changes can stress the seedlings and slow their growth.

If the temperature falls below the lower threshold, the seedlings may experience cold stress, slowing their growth or even causing damage. On the other hand, if the temperature rises above the upper threshold, the seedlings may experience heat stress, which can have similar effects.

Clones do not have their root system yet, so they depend on transpiration for their water. Therefore, they require high amounts of humidity, at least until they have fully formed roots. Many growers use a humidity dome to create an ideal amount of humidity for clones.

Temperatures for weed plants in the vegetative stage

The ideal temperature during the vegetative stage is between 68-77°F (20-25°C), combined with a preferred high humidity of 70% or more. However, as the plant gets older, a slightly lower humidity is okay. This is more of an issue for indoor-grown plants, as outdoor-grown plants can withstand more temperature fluctuations. I always get strong genetics in my seed store to ensure the survivability of the plants.

Buy sunny climate seeds

• Grow very tall monster plants
• High THC levels up to 30%
• Consistent heavy yields
• Mostly sativa and sativa hybrids
• Feminized seeds for guaranteed bud

BUY SUNNY CLIMATE SEEDS

During the later vegetative stage your cannabis plants can deal with less humidity and slightly cooler temperatures during dark periods.

These cooler ‘night’ periods are perfect for encouraging growth. Just don’t let temperatures drop below 59ºF (15ºC).

Best temperature for cannabis plants in the flowering stage

Once your plant reaches the flowering stage, it can thrive in an ideal temperate range of 64-79°F (18-26°C) with a low humidity range of around 40-60%. Flowering plants can withstand higher temperatures in hot and dry climates, but it’s not recommended for optimal growth and terpene quality.  

Don’t go over 82°F (28°C) because these higher temperatures cause terpenes to evaporate, and they also slow bud growth. If your buds are too hot during this time, you may be literally burning away the good stuff as they grow, leaving very little taste or smell by the time of harvest.

Be especially careful to keep it comfortable for your plant after week 6 or 7. This is when terpene production is at its max, and you risk evaporating them due to high heat.

For optimal trichome production, make sure it is slightly cooler during the dark periods, 60-70°F (15-21°C). The change in temperature may trigger increased terpene content as it optimizes plant processes. Just don’t make it too cold. This process leads to what is commonly known as the dew point. The dew point occurs when the air can no longer hold any more water vapor, and it condenses into droplets called dew.

Conclusion: the ideal temperature for cannabis

Whether you are growing indoors or outdoors, you need to be concerned about your plant’s environment. Try and create a temperature between 68-77°F (20-25°C) and the right humidity levels for every growth stage.

Understanding how to provide the best temperature for your plants is one of the easiest ways to ensure a successful grow. You simply need to learn some best practices, measure regularly and adjust as needed. There are plenty of ways to do this, even if you grow outdoors.

Give your plants the best start, however, by purchasing seeds from my marijuana seed store. We ship to every state in the United States; If the order doesn’t get to you or the seeds don’t germinate, you can get new seeds free of charge.

Cited sources:

• Effects of low temperature on the respiratory metabolism of carbohydrates by plants. T ap Rees ¹, M M Burrell, T G Entwistle, J B Hammond, D Kirk, N J Kruger
• Rapid responses of plants to temperature changes. Catarina C. Nievola, Camila P. Carvalho, Victória Carvalho, and Edson Rodrigues

Additional information about temperature

Drying and curing

Harvesting is not the last step to top-quality bud. Even with the best genetics, you’ll need to properly dry and cure your buds. Perhaps half of what determines great marijuana is how it has been dried or cured.

Professional dried and cured marijuana is more potent, looks better and produces a smoother smoke.

It will have that ‘sticky-icky’ feeling that marijuana lovers crave.

If you want the best results from your harvest, focus on maintaining the correct temperature and humidity throughout the entire process. This will help prevent mold and over-drying as well as make it easier to produce the best weed possible.

One thing people always ask me is when they should harvest their plants. This free Harvesting ebook gives a perfect idea of when to cut your plants.

Keep temperatures around 64°F (18°C) and the humidity at 45%. The values in the picture above are not correct. I made these pictures just after harvesting, so temperature and humidity were still high.

Create a smooth airflow bud don’t blow directly on your buds. This will cause them to dry too quickly.

Creating the perfect environment

Whether you are growing indoors or outdoors, you need to be concerned about your plant’s environment. Try and create a temperature that is ideal so that they can thrive.

Outdoor growing

With outdoor growing, keeping your plants warm can often be a problem. The good news is that most outdoor varieties can endure temperatures as low as 50°F (10°C) without any problem. That being said, fifty degrees is still not an ideal temperature because it tends to slow down tissue growth and photosynthesis later in the day.

Anything below 40°F (4°C) can result in damage to the tissue. 

Results of the wrong temperature

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

• Gas patio heaters
• Polyethylene plastic covering

Both options can keep gardens warm on frigid nights. The covering also protects plants from other dangerous weather elements. The idea is to maintain a temperature of at least 60 degrees to promote plant growth.

Indoor growing

Indoor growing temperatures are directly controlled by the grower, leaving as much room for error as there is for success. All you must do is make sure your grow room maintains the correct temperature.

Here’s the problem:

The ideal temperature in your grow room depends on several factors. The location of your room in the building is an important one for example. A basement grow room is a lot cooler than in an attic with a flat roof.

Aside from that, the size of your room, the airflow, the number of lights and the extraction rates play an important role. Keep this in mind when building your grow room.

“Install a maximum / minimum thermometer and hygrometer in all grow rooms.”

Jorge Cervantes

When the light is turned on, an ideal temperature for the cuttings and seedlings is between 68ºF and 77ºF (20ºC and 25ºC). As the plants get older, they can evaporate a little bit more, and the temperature may increase to a maximum of 82ºF (28ºC).

When the lights are off, the temperature should lie between 59ºF and 72ºF (15ºC and 22ºC). Another important rule is that the temperature differences between day and night cannot be too high, with a maximum difference of 18ºF (10ºC). So, when it’s 82ºF (28ºC) during the day, it cannot go below 64ºF (18ºF) at night.

In general, marijuana grown indoors functions best at moderate temperatures between 68 and 77°F (20-25°C) during the light period and a drop of no more than 18°F (10°C) to 60°F (15°C) during the dark period. CO2-enriched plants will produce more at a marginally higher temperature of just under 82°F (28°C).

Here’s my selection of indoor seeds which all thrive when grown in a controlled environment.

Buy Bruce Banner seeds

• 40% Indica 60% Sativa
• Extreme THC levels up to 25%
• Fast flowering for quick harvests
• A hulky punch of happy relaxation
• Feminized seeds for guaranteed bud

BUY BRUCE BANNER SEEDS

If the floor can have a steady temperature at around 80°F (27°C), the roots will be warmed, and the stems and leaves will withstand influxes of colder air. A heating mat is ideal if you’ve only got a few plants to worry about.

Larger gardens might require the use of a recirculating hot water heater to ensure optimal temperatures.

Temperature and humidity

In a grow room, you need to think about humidity as well.

First, here’s a bit of information on how humidity works. The air our plants ‘breathe’ contains water vapor. The amount of water vapor in the air can vary. This is the humidity.

In a grow room, the humidity is always a bit higher because the plant’s leaves evaporate water. Marijuana plants only use 10% of the water they absorb for growing and evaporate the other 90%.

Also read Humidity and marijuana plants

However, the air can only hold a certain amount as well. When this number is reached, condensation begins. You’ll notice condensation as little droplets of water in the colder areas of your grow room or on your plant.

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

How do you know you are getting close to having too much moisture?

Here’s what experts do:

You measure the relative humidity.

The relative humidity (RH) is a measurement of the percentage of moisture already in the air. For example, an RH reading of 70% indicates that more water vapor can be absorbed into the air. However, 70% at 77 degrees is different than 70% at 68 degrees, because warmer air can hold more moisture.

Specifically, at the higher temperature, 2 pounds of air could have as much as .45 ounces of water vapor in it. On the other hand, 2 pounds of 68-degree air only holds .32 ounces. So, if you allow the air to cool down, the air can hold less water vapor, leading to condensation while also raising the relative humidity.

Example: 77 Degree air cooled to 73 degrees – RH rise from 70% to 80%

This process leads to what is commonly known as the dew point. The dew point occurs when the air can no longer hold any more water vapor, and it condenses into droplets called dew.

Dew point is important because:

It helps you find the ideal temperature for your grow room.

In the example above, we know that the dew point of 77 degrees with 70% RH is 66 degrees. Therefore, we must keep the temperature above 66 degrees to prevent excessive moisture.

Maintaining proper moisture in your grow room is important because the moisture levels can frequently change.

A properly ventilated grow room always experiences regular drops in temperature. The heat from the lamps as well as the naturally humid air is regularly sucked out through fans and other ventilation methods, changing the moisture levels.  Even though it regularly changes, you must make sure the humidity levels are optimal for your plants.

Measuring temperature

Outdoors it’s easy to measure temperature. Indoors isn’t that difficult either.

First, you should measure the temperature in your marijuana grow room with a thermometer. There are analog and digital thermometers, and they’re for sale everywhere for a few bucks. I always use a digital thermo/hygrometer (something like this one) with built-in memory so I can see what the maximum and minimum temperature was.

It’s also a good idea to get one with a temperature sensor on a wire, so you can hang the display outside your grow room, and can see the temperature when the lights are off.

Here are some tips for getting a good temperature reading:

• Always measure the temperature in the shade
• Measure at various places in your growing room.
• Provide good air flow by placing several fans. Lamps emit radiant heat; however, it does not affect the air temperature.

It will always be a few degrees warmer right under the lamp than in a shaded spot.

The amount of temperature change that comes from lamps isn’t really an issue; just make sure there’s enough distance between the lamp and the plant.

Do not misunderstand this, however, as radiant heat does affect the leaf temperature. Therefore, you do not want the lamps to be too close to the plants as this could cause heat stress.

Maintaining correct temperatures

Maintaining the perfect temperature for your plants can be tricky, but it is not impossible.  When growing indoors, you can control the environment. Outdoor growing is a little less certain.

Temperature is important for the photosynthesis of your marijuana plants; low temperatures reduce evaporation through the leaves. This causes a suction force, which takes up nutrients via the roots, to become smaller.

What happens next is:

The nutrients that are necessary, but aren’t absorbed, remain in the growing medium and eventually disintegrate in the root environment. High acidity in the growing medium reduces the working of the roots which makes the plant absorbs less water and nutrient, and……

Growth comes to a halt.

This is why maintaining the correct temperatures is crucial to the health of your plants.

In a grow room, if the temperature drops below 60°F (15°C) during the dark period, plants will grow more slowly, and yields will not be as abundant. This won’t be readily apparent if you aren’t particularly familiar with the garden’s normal output.

Grow rooms use lamps, so there’s also the possibility of heat stress.  It is caused by the improper placement of lights. In this situation, the leaves don’t actually burn, but they are damaged.

This rarely happens to outdoor grown plants because we are referring to temperatures greater than 105 degrees. However, this can occur in indoor setups if you are not paying attention.

You’ll recognize heat stress on the top leaves of your plant. They will start to turn yellow and curl inward. This happens as the plant tries to protect itself from the heat. To see it in action, put a lamp too close to your plant. It’s a simple mistake that can quickly cost your plant its life.

What may seem like a small external imperfection at first, can have serious consequences in an entirely different part of the plant. Therefore, it’s essential to create a good climate in your grow room.

Correcting temperature problems

If you’ve already noticed temperature-related problems with your plants, it is not too late to save them. Here are three different problems that you may experience in your grow rooms as well as ways to correct them.

Heat stress

Prevent heat stress from happening to your plants by keeping your lamps a bit higher than your plants and/or by using a fan to blow away some of that excessive heat.

If you want something more substantial, try installing air-cooled lights with reflectors that will reduce the heat near the light. Water-cooled lights are actually more effective at diminishing light-generated heat.

For the most part, HPS lights should maintain a distance of around 3 to 4 inches per 100 watts from the tops of the plants. Air-cooled lights make the acceptable distance range between 2 and 3 inches per 100 watts.

Also read Heat stress and marijuana plants

Water-cooled lights make the acceptable distance about 2 inches or even less per 100 watts from the plant tops. With light movers, you can move the lights closer or farther away depending on your preference.

Check this table with distances for all grow lights. Including LED and CFL.

Temperatures that are too low

A few nights of cool temperatures won’t significantly damage your crop, but if it continues to occur throughout the flowering period, it can definitely be cause for concern. A CO2 generator or electric heater can heat the room adequately.

When the lights are on, it’s not normally necessary to increase the temperature. The lamps themselves will take care of that. However, it is important to distribute the air over your room evenly so that you’ll get the same temperature everywhere. Use swivel fans for this and aim them between the lamps and the plant.

Fresh air from outside should also be well distributed over the growing area so that there won’t be any cold spots. Especially in winter when the temperature can get below freezing.

When the lights are off, however, it can get cold in your grow room. Luckily there are plenty of things you can do to increase the temperature.

Try these options:

• A simple space heater with a thermostat (keep in mind, they consume a lot of electricity)
• A radiator with a thermostat
• A CO2 generator

You can also turn off the extractor fan (that provides the fresh air) the moment the lamps turn off. This can be done by a so-called fan controller with thermostat, or with a timer.

These strains are perfect if you have to deal with cooler climates. Both outdoors or during your indoor winter grow these strains are sturdy survivors.

Buy cool climate seeds

• Very quick Autoflowering seeds
• High THC levels up to 26%
• Strong mold resistant
• Need less sunlight
• Feminized seeds for guaranteed bud

BUY COOL CLIMATE SEEDS

Temperatures that are too high

Heat often becomes a problem in a grow room. This has to do with the fact that the lamps produce a lot of heat. Fluorescent lighting is not really a big deal, but HPS lamps can heat up your room to soaring temperatures of 122ºF (50ºC), which is fatal to your marijuana plants.

How do you not burn up your plants?

First of all, the dimension of the room is important. For a 600-watt HPS lamp always use a minimum space of 3 ft x 3 ft x 6.5 ft (100cm x 100cm x 200 cm).

For the extraction, use the following simple formula:

Number of watts divided by two = extractor in cubic feet (meters).

Using this formula, 2 x 600watt HPS requires an extractor fan of 22000 cubic ft. (600m3). The extractor fan blows the hot air outside and sucks fresh air inside.

That’s not all you can do, however:

You can put a carbon filter on your extractor, so it doesn’t blow marijuana smelling air out. You can also put ballasts etc. outside your grow room because they generate a lot of heat.

Perhaps the easiest way to keep the temperature low is by running your lights at night. Your lamps will turn on for a few hours after the sun has set and turn off a couple of hours after sunrise. This way you’ll have your lights on at the coolest period of the day.

You can imagine that if it’s hot weather outside, you’ll also suck this hot air into your grow room with your extractor. So, the room temperature never gets below the hot temperature outside. There are also professional growers who use an air conditioning unit so they can run it during the day and at night.

Understanding how to provide the best temperature for your plants is one of the easiest ways to ensure a successful grow. You simply need to learn some best practices, measure regularly and adjust as needed. There are plenty of ways to do this, even if you grow outdoors.

When you focus on providing the perfect temperature, you’ll notice that your plants can thrive.

Give your plants the best start, however, by purchasing seeds from my marijuana seed store. We ship to every state in the United States, If the order doesn’t get to you or the seeds don’t germinate, then you can get new seeds free of charge.

Happy growing,

Robert

Frequently asked questions

Is it a good idea to grow seedlings outside?

Seedlings require a warm environment and high humidity, typically around 80 degrees. Most outside environments cannot reliably provide these temperatures. Read more about seedlings here.

Can LED lights help reduce the heat in a grow room?

LED lights typically use less energy, but they still produce heat. While you can use them for your grow, keep in mind they may not provide the optimal result. Learn more about LEDs in this article.

Can marijuana survive with incorrect temperatures?

The numbers in this article are the ideal temperatures for the best results. Some hearty strains, especially those designed for outdoor growing, can grow under more extreme temperatures

How does a heat extractor work?

An extractor removes both warm air and CO2 out of your grow room, helping to lower temperatures on your grow room. Use it with fans and cooled lights to create the optimal environment n your grow room.