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 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
Temperature’s effect on plants
Plants are fairly self-sufficient, but when it comes to temperature they are at a disadvantage. Even though it is vital to their health, plants can’t create their own heat. Unlike animals and humans, a marijuana plant is entirely dependent on its environment.
How does a plant get warm?
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.
Marijuana plants won’t usually die from being too hot, but their growth can slow from it. High external temperatures (above 80 degrees) 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.
The quality of your buds is reason enough to care about temperature, but there are many more.
You also should care about their health.
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 die.
Below are some ways temperature impacts the marijuana growing process.
My selection of indoor seeds are perfect for growing in a controlled environment. Although some work fine when grown outside as well, these are the seeds you need if you’re growing indoors.
Photosynthesis
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.
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 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.
Respiration
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.
Here’s why that’s a problem:
When a heavier crop develops because of a lowered temperature, total crop respiration increases. Now, when the temperature increases, more energy is needed to keep the plant alive.
That leads to an even bigger problem:
The extra energy requirement leaves little left over for growth.
Heat also causes other problems for cannabis. When it is too hot, your plant is more likely to suffer from:
- spider mites
- root rot
- white powdery milder
- nutrient burn
- wilting
- increased stretching.
These situations are even more likely if it is also humid, or if there is increased water transpiration.
The ideal temperature for marijuana per grow stage
Your marijuana plant will respond to temperature changes differently based on their growing stage. As a grower, you need to make sure your plants have the best temperature for the growing stage they are in. Below are some ideal temperatures for every stage of the growing process.
Seedlings and clones
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.
The ideal temperature for clones is between 68-77°F (20-25°C) with high humidity. At these temperatures, they should quickly form roots and become more self-sufficient. These temperatures are similar for seedlings.
Vegetative stage
During the vegetative stage, young plants prefer a high humidity (70% or more) and temperatures between 68-77°F (20-25°C). 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 are able to withstand more temperature fluctuations.
The ideal temperature during the vegetative stage is between 68-77°F (20-25°C) with moderate humidity while also providing slightly cooler temperatures during dark periods.
The cooler ‘night’ periods are perfect for encouraging growth. Don’t let temperatures drop below 59ºF (15ºC). Once the plant matures, it should be able to withstand cooler temperatures during the day and drier air as well.
White Widow has been developed through the years and is a very , very sturdy plant.
Flowering stage
Once your plant reaches the flowering stage, it can thrive at a comfortable room temperate with low humidity. Unless you are adding additional CO2 to your grow room, a temperature under 82°F (28°C) is ideal. These lower temperatures encourage potent, trichome rich buds that you can smell and enjoy before you burn them.
Don’t go over 82°F (28°C) because 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. 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.
If you aren’t familiar with terpenes, they involve much more than taste and smell. They also impact the color of both the buds and the entire plant. Terpenes are responsible for the tomatoes red color, and they possibly may do something similar in certain cannabis strains. With the proper dark-period temperatures, you can bring out interesting colors in your plants (such as blue, pink and purple).
This phenomenon depends on the strain, of course. Most marijuana plants only grow green buds, but if you want to see what your plant can do, keep it cool for it ‘night’ temperatures and find out.
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.
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
Curled – too hot 
Small plants – too cold 
Burned – too close to bulb
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.
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, 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).
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.
Here’s my selection of indoor seeds which all thrive when grown in a controlled environment.
Bruce Banner is not just a guy that turns into a big green monster… It packs quite the punch as a marijuana strain as well! Be sure to check out Bruce Banner
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%.
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.
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 disintegrates 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.
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.
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 seedstore. We ship to the United States, Australia and a number of other countries. 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. For more on seedlings read this article.
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.
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.
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