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What Is The Best Temperature For A Marijuana Grow Room?

marijuana temperature grow room

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Temperature and marijuana plants

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. 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. Download my free marijuana grow bible for more tips about growing marijuana.

Temperature in your marijuana 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). 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. To help keep the temperature lower in an indoor grow room, consider using LED lights. To find out more about LED grow lights, click on the banner below:


Measuring temperature in your marijuana grow room

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 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. Download my free marijuana grow bible for more tips about growing marijuana.

Lowering the temperature in your marijuana 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.

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 temperature in your marijuana 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.

If you want to start growing, download my free grow guide and order some marijuana seeds. All top quality marijuana seeds are available in my marijuana seed shop. We ship seeds to the US, CA and many other countries. For any growing related question please visit the marijuana support page.

Source: ILoveGrowingMarijuana.Com


About Author

Robert Bergman is a master marijuana grower. Robert Bergman is the author of 'Marijuana Plant Care' and 'The Marijuana Grow Bible'.


  1. I have a similar set up. I went to lowes and bought 4 rolls of insulation and insulated the room and threw up 4 sheets of drywall. Then I unhooked my a/c duct from a bathroom and re routed it to my room. Works pretty decent. Still around 84 degrees Fahrenheit. Thinking of getting a portable a/c to help too.

  2. Amanda Darling on

    Hello me and my husband are new growers we have we have a 5x5x5 ft tent we have a 900w and a 150w led light are Temps when the lights are off is about 67 degrees Fahrenheit with a humidity level of 60 and when my lights come on my Temps get to 85 degree Fahrenheit at the highest but my humidity levels drop to about 30 when my lights are on… is this okay or normal for humidity to be higher when lighta are off and lower when on??? I also have a 250cf inline fan for my intake at the bottom of my grow tent and I have a 180 cfl
    inline fan on top for my exhaust I have a 4inch fan and a 10 inch fan for circulation along with a big box fan

  3. Is it a blower fan? You’re best bet is to have the intake fan blowing air from outside, or from the bottom of the tent to the light, then have an extracting fan blowing hot air out from the top. Since heat rises, extracting the top layer of air is crucial.

  4. Jesse Knapp on

    You want to keep your humidity levels in that room from forties through to 68 and no it’s not going to hurt the plans long as you have a lot of air moving in if you’re pushing a lot of heat in the room

  5. Kornelio Ruedas on

    I have a ? If I use a humidifier my humidity is in 50s 60s 70s depending the time by using a humidifier wouldn’t it get more humid

  6. Kornelio Ruedas on

    I have a 4x4x6.5 feet a 400w hps two small fans and one for the light but my temp is still like 79 80F how can I lower the temperature is my hps light fan not enough

  7. ScooterGirl on

    I use a wall thermostat with a small 250 watt MyHeat heater. The heater plugs into the thermostat and that in turn plugs into an outlet. Works very well and the grow room maintains an even 68F when the lights are out.

  8. ScooterGirl on

    The subject isn’t mathematics. It’s the affect temperature has on living things and since we’re talking about the affect of temperature on plants to say five degrees is five degrees is disingenuous at best. After all, a cannabis plant can easily survive a 20 degree drop from 80F to 60F but a 20 degree drop from 27C to 7C may very well kill them. It may be 20 degrees either way but there’s a big difference in how it affects a plant. Or a human for that matter. If my body temp goes up 5F I need an ambulance. If it goes up 5C I need an undertaker.

  9. Derek Rowland on

    Is true. Just say it out loud and rethink it. 25C (or 25 DEGREES Celsius) and 20C ( 20 DEGREES Celsius). That is a difference of 5 degrees on THE CELSIUS SCALE. Yes that translates to a greater difference if you are COMPARING it to the Fahrenheit scale. But it is still degrees, just a different scale.
    from WIKI
    The term degree is used in several scales of temperature. The symbol ° is usually used, followed by the initial letter of the unit, for example “°C” for degree(s) Celsius. A degree can be defined as a set change in temperature measured against a given scale, for example, one degree Celsius is one hundredth of the temperature change between the point at which water starts to change state from solid to liquid state and the point at which it starts to change from its gaseous state to liquid.

  10. There’s only one thing that you want to leave on 24/7 for two weeks and that is your cloning and then you pull them out into another room that you turn on for 18 hours on 6 hours off then when you go to your floor instead you should be at 12 hours off 12 hours on

  11. You never want your plans to go below 64 if they go below 64 you’re taking a risk of killing them 68 to 78 or 82 is perfect temperature to run again never below 64 even says it in a handbook

  12. Invest in a more powerful extractor fan? do you mean 2x4x5 in feet or meter? In feet it´s kinda small for two 600 watt but it´s good you don´t use HPS light or you would have a huge problem.
    How is your ventilation? Do you use air cooled reflectors like cool tube, spudnik etc? I would recommend Spudnik. It´s worth the money or you could get two cool tubes, they´re also good enough.
    Do you have any windows close to your grow? I would connect one fan to just the carbon filter and one smaller extractor for just the light, both with two speeds so you have more control over your temperature.

  13. Nonayer Effinbsns on

    If the glow is green it shouldn’t be a problem, however, if it is in the red spectrum (ie: orange) then I would suggest putting it under something that will block that glow. It is still a light leak and that can be a problem.

  14. I use a ceramic heater to keep my temp at about 67 degrees I noticed this am that the heater emits a glow when it’s on with this harm my bloom

  15. Brett Canfield on

    I have a 2x4x5 grow tent with 2 fans inside a 600watt led platinum led grow light a 125cfm extractor and my temps are still high 90 degrees. What can I do to lower temperature?

  16. I have 3 plants in a 5 foot grow tent under 4 23w cfl’s, and I’m averaging about 88°c and 40% humidity, Is that to high of a temperature for them? They’re growing well I just don’t known if should really trip on it that much.

  17. robert mccully on

    Outside– the hotter the better,, I have grown in the backyard on a asphalt driveway in pots which gets 20 to 30 degrees hotter. On a 100 degree day. It grows like a weed…. had to water every day….. so keep it watered, and hot.

  18. If you are on a budget, get some CFLs. The buds won’t be as dense, but you can get more than enough light for a grow tent for under $20. I went to walmart and got 18,000 lumens for $15.

    Then just buy some outlet-to-lightbulb adapters and make a hanging row of lights with a power strip.

  19. I build wine cellars for a living and control temperature with special refrigeration units any idea how I could get into the market —-like where to advertise fot building vapor barrier rooms

  20. Halegen lights are not grow lights. They are highly inefficient. They produce way too much heat. Either switch lights or stop growing until you can. Get some t5 high output lights. Your plants will thank you. Your energy bill will thank you.

  21. I am a beginner at growing weed but my grow tent settles at 32.5 degrees Celsius I can’t bring it below that but I have a low budget I have halegen lights 4 fans and ducting but it is still to hot I have opened all the ports I don’t know what else to do

  22. You do not need to leave the lights on 24/7 it’s completely unnecessary, in the begging when your plants are in vegetative growth you should maintain a 18/6 hour light cycle and once you want your plants to flower (aka start producing buds) you should change to a 12/12 light cycle this will signal your plants to begin flowering running the lights 24/7 in the vegetative stage is a waste of money and unnecessary in my opinion, there is zero difference between 18/6 and running them 24/7 it’s all down to your personal preference.

  23. I wish to grow in attic, in south CA, so it will get hot…aside from using a carbon filter and 1-2 600watt lights i did not plan for anything else. Now, i assumed i needed to leave the lights on 24/7 but from reading your article seems like i was wrong. could you inform me on the topic? first time grower so it’s a bit overwhelming

  24. Well Valentino, where is your grow room located in the house? Is it a enclosed and separated environment that you are willing to alter the walls with exhaust and intake fan, maybe even an air conditioner for humidity and additional heat control? I’d suggest at least an 800 cfm exhaust and 400 for your intake, both would be available online or at your nearest growstore.

  25. Here’s the problem: we heat our house with wood and i cant control heat all the time sooo what could i do?

  26. Not true. A 5 degree difference in Celsius covers a greater swing in temperature than a 5 degree difference in Fahrenheit. For example, 25C is 77F but 20C is not 72F but rather 68F. A 10 degree difference covers an even greater range; 15C is not 67F but 59F, nearly 20F degrees colder than the baseline 77F.

  27. suggest getting a solar attic fan or two that comes on with the sunlight. cools things down well. you’ll need to allow air to come up from the house to replace that being exited.

  28. i have a 10 by 6 by 7 foot room an i live in Florida built a room in my attic its awesome! ;) but i can control the temp…. very hott.. 100 degree im not a new grower i just need help with the problem im haveing i no that 100 is way to high an that between 68 to 77 is were it should be… help if any ideas… i got 10 babys melting oh ps i got ac runing to the room..

  29. I have an interesting scenario..5×5 grow tent in a basement, winters are well below freezing here. Fresh air intake and outlet to the room. 1000 Watt HPS. started first week of flower. When I switched to 12 -12 cycle I was concerned about night time temps dropping too low. Daytime temp is steady at (24C or 75F) I put a far infrared electric heater in the room on a thermostat to maintain night time temps at (18C or 66F)
    Here is the kicker…Far infrared light is invisible to the human eye but it is a light spectrum the plants can see. I did some research after I realized this could be a problem and articles I found said that Far Infrared light helps plants produce more and bigger flowers. I can not find any other information out. In the mean time I unplugged he heater and have my fresh air intake turned off at night with a timer. Checked the new night time temp and it was 17C or 62.6F without the heater.

  30. urbanfarmer209 on

    I’m at week five you think I did to much damage to my plants already by not controlling the temperature?

  31. Leaves might be curling if lamp/ heat source is too close to the plant. I believe 10″ from plant should work. Happy growing!

  32. Organic Grower on

    Actually, it is not a 1 to 1 scale so 10 degrees on one scale is not the same on the other. The formula is 9/5 x C+32= F so 20C= 68F, but 30C= 86, which is 18 degrees higher on the C scale compared to the 10 degree jump in F. Hope this clears this up a bit. Peace.

  33. Hey Robert, Ive bought a couple of your strains and theyve popped about 2 days ago.6 for 6.Pretty color, strong stem, but on the 3 GL****, the leaves are curling.Did I over water a little? Plus I have a small space heater and have been able to keep the temp right.Holla back at ya boy.

  34. Not sure about the math but 5-10 degrees F or C is still 5-10 degrees, just on a different scale. Either way, you dont want 40-50 degree swings for sure.

  35. Colorado native on

    I believe the temp swing from night to day should be 5-10 degrees Fahrenheit not Celsius. Trust me for a good crop you do not want a temp swing of 41-50 degrees from night to day.

  36. temp and humidity should be managed together for best plant vigor, consider the heat index, a number that determines how they “feel” together to us but also good to understand with plants. I have found a heat index 4 points above ambient to be the best way to adjust things

  37. Ajalvarado39@yahoo.com i need to ask a few questions and want to show u whats going on with mine b4 its to late i need help improving the look and the way its growing plz email me and i send u a pic of my plant

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