Temperature In Your Marijuana Grow Room

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 indoor cannabis grow setup. In this article I will explain what the ideal temperature is, and what you can do if the temperature is too high or too low.

The perfect temperature

Best temperature in cannabis grow room

The 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).

Download my free marijuana grow guide at this link for more about temperature

When the lights are off, the temperature should lie between 62ºF and 72ºF (17ºC and 22ºC). Another important rule is that the temperature differences between day and night cannot be too high, a maximum difference of 10ºC. So when it’s 82ºF (28ºC) during the day, it cannot go below 64ºF (18ºF) at night. A temperature difference of 5ºC is ideal.

Measuring the temperature

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.

Measuring temperature in cannabis grow room

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.

Lowering the temperature

Lowering the temperature in cannabis grow room

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 100cm x 100cm x 200 cm (3.2 ft x 3.2 ft x 6.5 ft).

For the extraction, use the following simple formula; number of watts divided by two = extractor in cubic meters (feet). So 2 x 600watt HPS is requires an extractor fan of 600m3 (22000 cubic ft.). 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.

Download my free marijuana grow guide at this link for more about temperature

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.

Increasing the temperature

Increasing temperature in cannabis grow room

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.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible

Robert Bergman

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Robert Bergman


Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations ... See profile

35 comments on “Temperature In Your Marijuana Grow Room”

  1. What’s going on guys, new to the site and like what I see… I’ve been growing for many years now and I’m having a problem with leaves getting a rusty orange color and then yellowing and crisping up, I have my ph And ppms and air temp and humidity right so I’m kinda stumped on what it could be, any suggestions I’d really appreciate

    • Tony,

      Yes. When temperatures reach below 50 f, cell damage occurs.. As the temps lower to 40 f, plant matter dies. There are ways to extend the length of your grow. Shelter being the 1st concern.

      Why don’t you join us at our support forum. We have friends all around the world who are always will to share information to you, in order to help you grow successfully from start to finish.. – I❤️GM

  2. I always enjoy reading special when I learn something new. I want to thank you, Robert, for giving for giving some real valuable in information. I am a going to read on and see what else I might learn. Thank you

  3. SHIT! Guess things have, not only gotten complicated as hell since I was a kid, from reading all your directions, I MURDERED the plant I was trying to cultivate indoors! GOOD and awsome descriptions, along with step by step details for people like ME!

  4. I have a standard closet space of 2’5″ in length by 8′ 5″ in width and 5′ ” 3 1/2″ from the 1000 watt switchable ballast (Metal H and HPS- running MH right now for 24 hours for 9 days). When the reach 18″ tall Ill switch the ballast. The medium is gardening soil with worm castings and has nutrients in the soil for three months. I am using Canna brand Terra Vega and Terra Flores twice a week in the water (no more than .6 oz per gallon of water.) I have a CO2 generator and controller/monitor to help with pace of production. I have a 14000 btu ac. Next is an ozone generator. My average temp is between 72 and 77 degrees F. My humidity level is 37%. I have 12 girls vegging. How long does the co2 generator take to get to 1500 ppm (It is a Titan Ares 2 burner series LP and a Titan controls Atlas 2 preset co2 monitor and controller…yes I already calibrated it outside? This is my first time. I’m a newb.

  5. Scott,

    82-83 is OK for veg cycle. What is “Phabtom” 315w lamp…LED?
    6″ is extremely close to plants and will scald them.

    I advise you to join our forum, where we can guide you to be a better grower. Despite being expert growers; We all do not hold the same knowledge. However; our membership is versatile and have a wide range of expertise.


  6. I have a 40x40x72 tent I installed a phantom 315w ,later I added another one ballasts out side tent temp inside is around 82 to 83f is that a good temperature or is it too hot,don’t have any way to pump in fresh air except for a window out side the closet about 10 feet away should I remove one of them .at night the temp is around 68 to 70 like you talked about,light is about 6 inches away from plants any advice on how to keep it cooler or is this ok thanks

  7. BeHigh,

    Of course you can. I suggest a portable AC unit. Not only do they come with a dehumidifier built in; They provide you with better temperature control. Some a mere dehumidifier won’t do. I spent about $200 on a nice LG model 🙂

  8. Can you use a dehumidifier to help reduce the humidity in the grow room? My room is 5×7.5 And I’m only using 400w hsp. I have a 12″ fan and a vent fan on the wall. Any suggestions welcome.

  9. You can certainly see your siklls within the paintings you write. The arena hopes for more passionate writers like you who aren’t afraid to say how they believe. All the time follow your heart.

  10. Oh yeah; It is not going to be cheap. There is no such thing as an inexpensive LED setup at this time. IMO. Even if you could find quality components and build it yourself; Be prepared to make an investment. Peace

  11. Well Tim,

    As you can see by the pictures we use HID lamp predominantly. You will need to do research ON LED lamp system in order to find yourself the best deal. As for advise on what size to buy in order to take care of the closet space; I suggest you join our “Support Forum”, and post this topic their. We have an expert or 2 that do use LED lamps, and you can get more information, that way. Happy Growing 🙂

  12. Hello and THANKS! I appreciate all your time and hard work in putting this grow guide together. I lease my indoor space and have a history of energy usage. As a result I have to go to an LED lighting system for my plants, as LED consumes much less energy than even fluorescents and they give a superior range of light spectrum, IMHOP. The plants will be grown in a 8’L x 3’W x 8’H closet, wrapped interior with a mylar space blanket, with sliding doors. I have the possibility of a ductwork thru the floor into a 1200 sq.ft. crawl space beneath the house if I need it. No HEAT FOOTPRINT is my goal. My question is: do you know of the most productive LED light size for this space, AT A REASONABLE PRICE? Any hints at what I MUST do to this space? Thanks for your input. I do appreciate all that you do for “us”.

  13. If my ladies are exposed to excessive heat, will it cause them to hermie? Asking because had 2 plants with seeds, only shock was 88+temps for a couple days before I got fans

  14. Unfortunately, some of us can’t set up an air exchange system to draw in outside air and vent it out, due to security reasons, lack of funds, or electric load. I’d like to see someone address this issue and suggest reasonable ways of cooling under these conditions. (OK, this article did suggest things like running lights at night or growing in a basement, but these are not always possible either.) Due to these limitations, I’ve had to raise many crops in rooms where the air was over 90 degrees F for much of the cultivation period. I’ve still had satisfactory results, but I wish there were some way around this problem, because these temperatures are not optimal. Any suggestions would be appreciated.

    • I don’t know how big your grow room is, but I have a 3.5 x 6 closet with 400 HPS/MH grow 4 plants plus clones. I’ve no trouble with a 3-speed fan keeping the temp at 80 f or below on low speed. You can quickly lower temp by evaporation.
      Build a 18″ square by 2″ deep frame covered with screen or 1/4″ mesh above a reservoir to catch melt. Fill with ice cubes or cover with wet burlap and place between fan and plants. Adjustment as needed. Ideal to bring air from outside grow room. I live in the SW so AC nearly always on.
      Another way to cool is to rob AC by splitting a vent so half feeds the grow room.
      Necessity is the mother of invention

    • Buy and air cooler perhaps?? Or open the window and use lots of oscilating fans. I’m new to growing, but I know a good air cooler that has water/ice in the bottom should bring the temperature down

    • get a small box freezer/ turn it up full-(store in the coolest place- outside grow room ) cut the desired size of your intake ducting in the lid – insert ducting- seal edges and away u go- cheap aircon – leave the lid open about 5- 10mm to draw in air if room drops below desired temp reduce the air flow –

    • I’m using a new style LED grow light which doesn’t produce the heat. I’m not using a fan at all! Just letting natural air flow in through the gaps in the cupboard (broom storage cupboard)

      • On doing the same, I purchased 2 mylar rolls of 4ft x 50ft, and lined part of the walls and floors. And hung my LED from the ceiling. Full spectrum exposure, crack the windows at night for flow. If it’s too cold. I set the mini heater to 70°f and let the darkness in

  15. Will excess heat cause my plant to go to early and heavy blossom?
    I built a new grow room a year ago and almost all plants go to early and heavy blossom.
    I need help! Thanks in advance