Create The Best Climate For Marijuana Grow Room

Once you’ve decided to grow your marijuana plants indoors, your next step is to figure out how exactly you are going to do it. There are a number of factors to consider when setting up your growing environment indoors.

Whether it’s the temperature or humidity of the room, the carbon dioxide level, the ventilation or the smell, you are going to have to arrange everything perfectly so you create the ideal environment for your marijuana plants.

Although this may seem like a huge amount of effort at first, it has a huge payoff once you harvest. The only thing you might regret is not researching or preparing enough ahead of time; if you put in the work ahead of time, your result will be well worth it. This article will cover the following key elements of creating and maintaining a proper growing environment for your Cannabis.


Although it can vary somewhat between varieties and strains, the best temperature for Cannabis to grow in is usually somewhere between 68-77 degrees (20-25 Celsius).

If the ambient temperature around the plant drops much below this, the growth of the plant will slow and its potential yield will be inhibited or possibly even stopped entirely if the plant never matures. It’s good to note that this temperature is most important during a “day” cycle when you are letting the plant get light.

Marijuana temperature
Marijuana growing temperature

That’s when photosynthesis and the potential for growth occur. Still, you don’t want large temperature swings between day and night if you can avoid it.

If the temperature of your plant rises up above 77 degrees (25º Celsius), the metabolism of the plant will accelerate, and it will require additional inputs: more light, more water, more carbon dioxide, and more fertilizer.

Read our article Marijuana & Temperature for more info

Make sure you plan accordingly for changes in temperature, whether intentional or otherwise. It’s wise to invest not just in a thermometer, but a thermometer that is attached to a ventilation and heating system, so that it can automatically manage the temperature of your grow room. A working automatic system can also provide you with excellent ventilation for fresh air and avoid carbon dioxide shortages.


The ideal humidity in the environment of the Cannabis plant lies somewhere between 40-70%. To measure the humidity, you need a hygrometer.

marijuana humidity
Check your humidity while growing

Although any tool that measures humidity will do, an electric hygrometer is probably a better choice for most growers. It is more precise and often has some automatic functioning that gives you some measure of control over the humidity, which is always good for indoor growing.

If the humidity of your plant drops below 40%, then the plant will experience a faster rate of transpiration. There should be no huge concern. It will simply cause your plant to use up water at a higher rate. As long as there is plenty of water in reserve, you won’t have any issues.

Read our article Marijuana & Humidity for more info

On the opposite end of the spectrum, if the humidity is too high, you put the plant at risk for fungus, especially during the flowering period, where things can get rotten really quickly. You’ll almost certainly want a dehumidifier on hand if you don’t have one automatically set up so that you can fix moisture issues as they arise.

Carbon dioxide

Carbon dioxide as a foundational piece of the photosynthesis process, and one of the major building blocks for how plants work. Out in nature, the air will contain a lot of carbon dioxide naturally, and plants will automatically and constantly be supplied by a reserve of between 300-400 ppm CO2 present in the atmosphere.

Carbon dioxide
Carbon dioxide

Your plants use up carbon dioxide very quickly, however, something you need to be mindful of when working in a closed indoor system. If there is too little carbon dioxide for a plant to use, photosynthesis will not occur at all, and a plant will not grow.

The volume and ratio of carbon dioxide in the environment of your grow room is also something that benefits from having a well-controlled exhaust and ventilation system. CO2 is denser and heavier than oxygen, so a fan will mix it up and make sure that it reaches your plants.

Remember that if you aren’t adding extra carbon dioxide directly into the environment, you need to make sure you let the exhaust and ventilation run the whole time you have lights on. Photosynthesis uses up carbon dioxide very quickly!

Raise CO2 levels

You’ll want to keep a close watch on all the factors described above. Missing the right levels of water, fertilizer, carbon dioxide or heat could severely inhibit the growth and eventual yield of your plant, in worst-case scenarios ruining a crop entirely.

1200 ppm of CO2 can double your yield!
1200 ppm of CO2 can double your yield!

You can bring up the CO2 levels in the air to raise the metabolism of the plant, up to 1500 ppm CO2. Then you want to make the temperature is stable between 77-86 (25-30 Celsius). This also means you want to bring the humidity up from 40% to ~60% (keep a close eye here, that’s quite humid).

You’ll also need to provide extra fertilizer. If you manage all of these factors extremely carefully, you will end up with a significantly higher yield within the exact same grow time, but you have to put in a little bit of extra work.

If you want a strong cannabis plant with very tight internodes and buds and a short turnover for harvesting, chances are good that you’ll want to add CO2. Beware, however, this technique is very difficult so only experienced growers should try it.

Also, if you have a heat problem and the exhaust system doesn’t work properly you could try using artificial CO2 as an ameliorative option. Things happen faster when you do this, so if you don’t need to, skip it (especially if you are a new grower).

Tip: make sure to download our free Grow Bible for more information about growing with CO2

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    Once you know what you are doing, you can try it to save a little time. Remember to be careful with adding CO2 though, because although plants benefit from it, it can make the air unbreathable for humans. Plus, it’s unfortunately fairly difficult to measure.

    Check this video by Mycoheadgrower about integrating supplemental CO2.

    You can measure the CO2- levels, but the necessary tools are very expensive. While it does involve a chemical process, it is a one time test. Measuring involves a glass tube that has been divided into degrees and also a syringe.

    The tube will include a reactive substance for CO2, it is then filled with the sample air. Activate the test by breaking the tube at both ends and attaching it to the syringe. Once the air flows through it, the reactive substance will change color. If it does, it indicates the CO2 level.

    Cannabis absorbs carbon dioxide and exhales oxygen through stoma in the leaves. Without enough fresh air, those stoma will eventually close, and the plant itself will die. Not so dissimilar from what happens if there’s no fresh air for humans to breath. Make sure you have a good ventilation and exhaust system, whatever it is and however you set it up. It helps you manage a lot of different facets of growth.

    Ventilation and air flow

    Proper aeration and movement of air in a room can be difficult to control completely, but it’s just as important as getting the right nutrients, light, and water. Proper air flow will give you a stable humidity, temperature, CO2 level and also control the aromatic nature of an indoor grow room.

    Proper ventilation and air flow
    Proper ventilation and air flow

    If you’re growing indoors, you want the best possible environment for your plant. Out in a natural setting, you’d have wind as a natural source to strengthen the structural integrity of your cannabis plant, and rain to help keep it free from dust and troublesome parasites. Try to recreate this sort of environment to the best of your ability. Read on to learn how!

    Make sure that your ventilators are spaced out to different locations around your grow room if possible because you want to move all of the air around, not just the air in one section of the room. You’ll get the best results if you maximize the efficiency of your ventilation units.

    Generally, you want two ventilators facing each other in a room, and the exhaust system installed on another side entirely. This sort of setup gives you a more stable atmosphere in the room, helping to balance carbon dioxide ratios, humidity, and temperature.

    If it seems like you’re having a lot of trouble with too much humidity in your grow room, consider having the ventilators face the ceiling. That way you will limit any inhibiting factors caused by humidity or moisture. Another option is hanging ventilators right at the same height as your grow lamps. This will help blow the hot air around the lamps out of the room and reduce the temperature in your grow room.

    Control temperature

    In case you haven’t got it yet, it’s best for your cannabis plant to get lots of fresh air, and especially good to get lots of CO2. The easiest way to make this happen is just to let the exhaust system run all day long. However, it’s very easy to run into problems with this sort of method.

    Control temperature equipment
    Control temperature equipment

    Outside air will not necessarily be the right temperature for your plants, especially if you are growing at a latitude that experiences winter (which is a lot of indoor growers, frankly), or if you just happen to have cold nights. Although a marijuana plant is hardy, it’s still a warm-weather plant. It thrives in relative heat.

    Additionally, you can run into trouble when the humidity of the outside air you are circulating through your grow room is too low. Dry air can damage a plant and inhibit development. All of these issues, fortunately, are fixed by setting up an automated system which gives you greater and more precise control over the environment of your grow room. Buy the best fan controllers at this link

    The thermostat is a device that regulates the temperature by automatically turning the heat on and off. You probably are already familiar with this device. What you will need to do is set the right temperature for your grow room.

    If you connect a controller with a thermostat to the exhaust system, it can turn the exhaust on when it gets too hot. It can also switch it off when it is too cold. The device can also be connected to a central heating, radiator or climate control.

    You have options, but at the end of the day, you’re just looking for something that can sense heat and activate/deactivate heating accordingly. More about temperature in my free marijuana grow guide.

    Control humidity

    To control humidity, use an automatic moisture meter. It works the same as the thermostat. You can link it to the exhaust system and turn it on or off based on the amount of humidity present.

    Optimal humidity growing marijuana
    Optimal humidity growing marijuana

    One problem with a setup like this is that if the exhaust system gets messed up at all, the carbon dioxide will not be refreshed in the room. Then again, that’s not so different from where you’d be without it, right? The automatic moisture meter can even be connected to a humidifier or dehumidifier, which will really fine-tune the humidity levels in your grow room.

    Earlier in this grow guide, we offered a couple of hypothetical spots in which you might place your personal indoor grow room. Now we’ll offer a couple of extra hints for those small spaces (some of which are still applicable for larger locations).

    For example, if your grow room is a cupboard space, you’ll want to be using a cupboard that is close to a window or some sort of exhaust/vent system. That way you’ll ensure that you have a way to cycle air through your grow room.

    Measure twice and cut once – when it comes to sawing holes for tubes and pipes, especially in cupboards or rooms of a house. You can fill up any leaks you develop around the edges with silicon, but if you make a lot of holes, you’re just making a lot more work for yourself. Buy the best grow gear at this link

    Add air circulation into your marijuana grow room
    Add air circulation into your marijuana grow room

    For most indoor growers, you’ll want to take discretion into consideration. One of the biggest ways to help keep your grow room private is to manage the odor, which we’ll discuss just ahead. In addition to the smell, however, you’ll want to take some simple steps to camouflage your set-up.

    First of all, you’ll want something that hides the exhaust vent you have outside somewhat, without interfering with its ability to put out all that air. You can use plants, or bits of plastic, or those funny fake rocks. Use your imagination!

    Another tip is to find any rattling surfaces and apply silicon or plastic to those, too. That way you’ll minimize any loud vibrations or clattering you could have from all those fans and motors.

    Also, ask around when you’re checking out various ventilation systems. Some are noticeably louder than others and getting some help from a shop-owner or someone with personal experience can be helpful.

    Tip: Looking to buy seeds? Check out our marijuana seed shop

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    Control smell

    There’s a reason people refer to the marijuana plant as “skunky”. It has an extremely powerful odor, especially during the flowering stage. It gets intense.

    Your exhaust system will obviously take care of this, whisking the smell away from the grow room for you, but that isn’t the end of it. That smell is going somewhere, but you don’t always want to be pumping the air around your house with the scent of rich flowering cannabis.

    Carbon filter
    Carbon filter, great to get rid off the smell

    For this, you can use an odor filter, usually some sort of carbon binding contraption that traps molecules in it. There are a number of different options for this sort of device, and they work far better than trying to spray the air with some other powerful scent in an attempt to mask the odor. Buy the best carbon filters at this link

    Most odor filters are cylinders made out of metal with carbon in them. Filters are placed inside outlet propellers so that the air that leaves the growing area is clean. This works because the carbon sucks up the marijuana scent as it passes over it.

    Add air circulation and carbon filtering into your marijuana grow room
    Add air circulation and carbon filtering into your marijuana grow room

    To help the carbon last longer, use cotton. Wrap the cotton around the cylinder – it will filter out extra dust particles. This will extend the life of your filter, but it’s not a permanent solution. You will need to replace the carbon every 25 weeks, otherwise it may start to fail.

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


    Avatar for Robert Bergman

    Robert Bergman

    Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations. His passion for growing led him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]


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