The Right Temperature In A Weed Grow Room And How To Maintain It

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.

The temperature in your marijuana grow room is one of the most important factors you can control. It affects growth, health, yield and quality. Having the right temperature in your grow room is vital for your plant to photosynthesize. If the temperature is too low, it will lessen the evaporation through the leaves and cause the suction force to shrink, which affect how nutrients are consumed by the roots.

Required nutrients will haven’t been consumed will stay in the growing medium and over time dissolve in the root system. A growing medium that is highly acidic lessens the effort the roots put in which leads to the plant taking in less water and nutrients, and it may stunt the growth.

The right temperature in weed grow room

There are many things that determine the temperature in your grow room. Where you grow room is situated in the home is critical. For instance, it is much cooler in the basement than it is in the attic. Also, room size, air flow, how many lights you have and the extraction amount all have something to do with it. You will need to consider this when creating a grow room.

When the light is on, the best temperature for seedlings and cuttings is from 68- 77*F /20-25˙C. Once your plants mature, they can handle even more and the temperature could rise to as much as 82*F. If the light is off, the temperature needs to be around 62-72*F/17-22˙C.

Measuring the temperature in your weed room

The temperature in your room should be measured with a thermometer. You can find both digital and analog thermometers in several locations for only a few dollars. I usually prefer a digital thermos/hygrometer, which has a built-in memory,

Remember only to measure the temperature in a shaded area and different parts of the grow room. Put a few fans in the room for good air flow. Lamps will not change the air temperature, but it will usually be a little warmer under the lamp than it is in the shaded areas. This is nothing to worry about, just be sure there is enough space between the lamp and the plant.

How to lower the temperature of your weed grow

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.

Heat causes 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.

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To extract heat, use this easy method; the amount of watts divided by two = extractor in cubic meters (feet). So, 2 x 600 watt HPS needs an extractor fan of 600 m3. Extractor fans blow the hot air out of an area and bring in fresh air. You can even add a carbon filter to ensure the marijuana smell doesn’t go out of your room. Using ballasts placed outside of your grow room also creates plenty of heat, so check for that too.

Raising the temperature in your weed grow room

It can become cold in the room when the lights are off. To raise the temperature use a space heater with thermostat to heat up your grow room. Just keep in mind that they use a lot of electricity. Also, radiators with thermostats are good as well. You can even turn off the extractor fan when the lights turn off. You can do this with a fan controller that has a thermostat, or a timer.

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.

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.

Increase the temperature while growing by using swivel fans
Increase the temperature while growing by using swivel fans
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.

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. As we mentioned earlier, the ideal temperature for growing cannabis is somewhere around 25°C. But remember, this can vary slightly from one plant to another. Whenever you’re working with a new plant, feel free to play around with the temperature settings in your grow room a bit, and see how they affect your plant. You’d be surprised just how much variety there is in the world of growing cannabis. When you focus on providing the perfect temperature, you’ll notice that your plants can thrive.

[Updated, originally published 31.1.2017]

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