The temperature in your grow room is very important for the photosynthesis of your marijuana plants. For example, low temperatures reduces evaporation through the leaves. The result is that the suction force, which takes up nutrients via the roots, becomes smaller.

The nutrients that are necessary, but aren’t absorbed, remain in the growing medium and eventually disintegrates in the root environment. A high acidity in the growing medium reduces the working of the roots which makes the plant absorbs less water and nutrients, and the growth can even come to a halt.

So what posed as a small external imperfection at first, can have serious consequences in an entirely different part of the plant. Therefore it’s very important to create a good climate in your grow room. Read this article and you know everything there is to know about temperature & marijuana.

And please share your knowledge or ask questions in the comments below.

Effects on the plant

A plant’s temperature is vital to their health, but unfortunately, unlike animals and humans, plants cannot create their own heat. Instead, your marijuana plant will be entirely dependent on its environment.

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.

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.

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.

Tip: make sure to download my free Grow Bible  and learn how a marijuana plant grows

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.

When a heavier crop develops because of a lowered temperature, total crop respiration increases . Because of this, when the temperature increases, more energy is needed to keep the plant alive. This leaves little room for the plant to grow.

Maintaining the perfect temperature for your plants can be tricky. Lucky for growers, there’s already plenty of research around the ideal temperatures for your plants. So keep on reading…

Grow room

The ideal temperature in your grow room depends on several factors. The location of your room in the building is an important one; in a basement it’s 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.

Relation to humidity

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? 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.

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 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. Regardless, you must make sure the humidity levels are optimal for your plants.

We discuss ways to increase humidity here.

How to measure temperatues

You 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 a 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.

Always measure the temperature in the shade, and at various places in your growing room. Provide good air flow by placing several fans. The lamps emit radiant heat which does not affect the air temperature.

Therefore it will always be a few degrees warmer right under the lamp than a shaded spot. That is not really an issue, but make sure there’s enough distance between the lamp and the plant, so the tips of the plants won’t literally burn. Radiant heat does affect the leaf temperature.

How to lower temperatures

Heat often becomes a problem in your grow room. This has to do with the fact that the lamps we use 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.

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

You can put a carbon filter on it, 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 at night. Your lamps will turn on for a few hours after the sun has set, and turn off a couple of hours after sun rise. 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 professional growers who use an air conditioning unit so they can run it during the day and at night.

How to increase temperatures

When the lights are on, it’s not 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 temperature can get below freezing.

When the lights are off, it can get cold in your grow room. Luckily there are plenty of things you can do to increase the temperature. A simple space heater with thermostat is usually sufficient enough to heat your space. However, they do consume a lot of electricity. A radiator with a thermostat works fine too. You can also turn off the extractor fan (that provides the fresh air) on the moment the lamps turn off. This can be done by a so-called fan controller with thermostat, or with a timer.

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.

Tip: make sure to download my free Grow Bible for more information about the marijuana plant life cycle

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.

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.

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 on the picture above are not correct. I made this pictures just after harvesting so temperature and humidity were still high.

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

Too low

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).

What if the temperature in your marijuana grow room is too cold? 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.

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.

Too high

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.

Tip: make sure to download my free Grow Bible for more information about the influence of temperature on marijuana plants

Marijuana plants won’t usually die from heat, but they can cause the plant to grow slower. High heat (above 80 degrees) while its flowering will not only slow down the bud growth, but it will also reduce the smell and potency. If you care about growing buds with plenty of cannabinoids, you need to be sure to keep the temperature under control during the flowering stage.

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 and increased stretching. These situations are even more likely if it is also humid, or if there is increased water transpiration.

Outdoor

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.

Result of wrong temperature

Anything below 40°F (4°C) can result in damage to the tissue. Gas patio heaters can keep gardens warm on frigid nights. Maintaining a temperature of 60 degrees will promote plant growth substantially.

These outdoor plants can also benefit from a polyethylene plastic covering that keeps things nice and toasty while also protecting the garden from the elements. Heaters can increase protection even more.

Cooltubes

For a quick fix, just keep the lights farther away from the plants. 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 watt from the tops of the plants. Air-cooled lights make the acceptable distance range between 2 and 3 inches per 100 watt.

Water-cooled lights make the acceptable distance about 2 inches or even less per 100 watt 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.

Heatstress

Don’t stress out your plants. 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.

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.

Prevent this 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.

Check out my marijuana seedshop  for a wide selection of top-quality seeds. 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