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How to Use CO2 in Cannabis Grows

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How to use CO2 in cannabis grows is one of the many questions that we’re frequently asked. CO2 is essential for cannabis plants and every other plant, as to them CO2 is like oxygen and they need it to survive. Cannabis plants can deal with CO2 levels of up to 600% the amount that there naturally is in the air around us. Basically, it makes their cells multiply much faster, so if you use extra CO2 during the flowering period you’ll get buds that are much thicker than usual which, if done correctly, makes for a much bigger yield.

If you don’t use CO2 in the right way you could end up with yellowing plants, or long stretched out plants with hardly any buds. You’re going to need to know what you’re doing to implement CO2 correctly. There are many systems that can be used to get more CO2 into your crop; beginner systems that are used as a little extra boost and don’t require much care, and then professional systems that measure the PPM of CO2 that there is in the atmosphere. Professional systems are obviously much more effective and efficient than beginner ones, but they also require more work and attention.

You can use any way of dispensing CO2, connected to a CO2 controller that will shut off the flow of CO2 once it reaches a certain level, and open it again once it gets too low. If all you have is a normal CO2 meter, you can still control the CO2 levels by opening and closing a solenoid valve using a timer. (Solenoid valves are valves that are opened and closed with an electromagnetic charge). Whichever kind of system you use, you must know the exact PPM (parts per million) of CO2 in your grow room.

CO2 needs to be introduced into your room through a silicone tube, with one outlet per plant near the bottom of the trunk. You can also use a 2m tube to go around the grow area with holes facing the center, towards the plants.

Once everything’s installed and ready to go, you’ll need to know exactly how to use CO2. Well, it’s used in the flowering period from the 21 st day onwards, once the buds start to take shape and are slowly popping up at the tips of all of the branches. You’ll need to change your air filtration so that the extractor only works for around 15 minutes an hour because if it’s left on it will get rid of all of the CO2 and all of the effort will have been for nothing. You can use another timer to program the CO2 controller so that it doesn’t turn on when the extractor is on. CO2 should only be administered when the lights are on, as the extraction should be on constantly when the lights are off.

CO2 increases your plants cell walls and multiplies them rapidly, but make sure that you fertilize them also as they’ll end up light and pretty down looking if they get a lot of CO2 but not any nutrition. They’ll also need a slightly higher heat than usual, around 28-32ºC so that the water in the leaves can evaporate slightly faster and the plants can absorb the nutrients straight away. Basically, we want the plants to absorb the nutrients but get rid of the water fast. You’ll need a dehumidifier to lower the ambient humidity to normal levels, because once the temp is raised and your plants begin evaporating water, humidity levels will raise a lot.

Here’s a guide on what you should do and the strength of the CO2 in your grow room from the 21 st day of flowering onwards. EC levels apply if you’re growing in hydro or aeroponics. If you want to measure them in soil you’ll need to measure the water that comes out from the bottom of the flowerpot once you’ve watered; if more is needed you can add it in the next watering, and if it’s too high then the next watering should just be water on its own.

  • Day 21 of flowering: Begin with 800 PPM, and keep it at that when the extractor isn’t on. When watering, you’ll need to raise the EC every time to raise the CO2 levels. For this first week you’ll need about 1.7 EC using normal irrigation water.
  • Day 24 of flowering: Raise the CO2 to 850 PPM, and the EC to 1.8.
  • Day 27 of flowering: CO2 to 900 PPM and EC to 1.9
  • Day 29 of flowering: From this day onwards you’ll need to increase both CO2 and EC every two days. 950 PPM and 2.0 EC.
  • Day 31 of flowering: 1000 PPM and 2.1 EC.
  • Day 33 of flowering: 1050 PPM and 2.2 EC
  • Day 35 of flowering: 1100 PPM and 2.3 EC
  • Day 37 of flowering: 1150 PPM and 2.4 EC
  • Day 39 of flowering: 1200 PPM and 2.5 EC. From this day onwards, increase levels every day.
  • Day 40 of flowering: 1250 PPM and 2.6 EC
  • Day 41 of flowering: 1300 PPM and 2.7 EC
  • Day 42 of flowering: 1350 PPM and 2.8 EC
  • Day 43 of flowering: 1400 PPM and 2.9 EC
  • Day 44 of flowering: 1450 PPM and 3.0 EC (this is the max EC level)
  • Day 45 of flowering: 1500 PPM and 3.0 EC
  • Day 46 of flowering: 1550 PPM and 3.0 EC
  • Day 47 of flowering: 1600 PPM and 3.0 EC
  • Day 48 of flowering: 1650 PPM and 3.0 EC
  • Day 49 of flowering: 1700 PPM and 3.0 EC
  • Day 50 of flowering: 1750 PPM and 3.0 EC
  • Day 51 of flowering: 1800 PPM and 3.0 EC – This is the max CO2 level you can have in your grow room. Continue the rest of the flowering period without raising anything, and make sure to do that root wash 10 days before harvesting.

If you notice your plants get weak or yellowish at any moment, or worse, then stop using CO2 immediately and try and find out what’s going wrong. Either too much CO2 is accumulating or we’re giving them too little and it’s too warm. Make sure you follow the parameters exactly or using it can actually do more harm than good. If done properly, your harvest will be ready a few days earlier and you’ll get a higher yield.

If you have any questions feel free to leave a comment.

Author: Javier Chinesta
Translation: Ciara Murphy

How to Use CO2 in Cannabis Grows; here's a step by step guide on how to correctly use CO2 to get the most out of your plants.


Carbon dioxide (CO2) is necessary for the correct performance of the plants, together with water, light or nutrients, because plants need to consume CO2 – in atmospheric air – to provide photosynthesis, and, consequently, their basic vital processes – otherwise, the plant won’t be able to produce energy, unable to develop properly, so, although totally unknown for beginner growers, CO2’s fundamental for professional growing.

  • 1 Why many people don’t know anything about CO2?
  • 2 How does CO2 work for Marijuana?
  • 3 Ideal CO2 levels for growing Weed
  • 4 How to maintain optimum CO2 parameters
    • 4.1 How to dispense CO2 to our growth
    • 4.2 How to calculate necessary CO2 level for growing
  • 5 Is CO2 dangerous?
  • 6 CO2 and light intensity?
  • 7 Homemade and cheap CO2 Dispenser for Cannabis grows

Why many people don’t know anything about CO2?

Mainly, atmospheric air contains the necessary CO2 for the correct performance of the plants – otherwise, they would die, so, when outdoor growing, no problems about CO2, and, when indoor growing with artificial light, a good ventilation system’s necessary to expel foul air and to introduce outdoor clean air.

This is very important because plants consume CO2 and expel oxygen, so, without proper ventilation, indoor air gets oxygen-jammed, CO2 levels get lowered and the plants are unable to develop; this isn’t a common problem, because, usually, current growers are concerned about ventilation.

How does CO2 work for Marijuana?

Cannabis uses CO2 and solar light to produce the necessary energy for its basic vital processes. One of these most remarkable processes is photosynthesis, a chemical reaction which happens when the plant receives solar light and extracts CO2 from the air by means of little holes at the bottom of the leaves (stomas); stomas function similarly to skin pores, but they have protective cells able to open and close – these pores regulate water, gas, oxygen (O2) and CO2 absorption in the plant, as well as water and O2 expulsion (kind of entrance/exit doors of the plant).

Then, CO2’s absorbed by the strain, heading it to the chloroplasts – organelles of the plant containing chlorophyll which absorbs light, providing photosynthesis; for more info on photosynthesis, check our posts on dark phase and light phase and discover all about this complex process of chemical reactions happening in our strains (basically, photosynthesis absorbs light, water and CO2, producing sugar to feed the plant, expelling oxygen).

Ideal CO2 levels for growing Weed

CO2 level in the air has a strong effect on photosynthesis and marijuana plant development – photosynthesis process gets boosted as air CO2 level increases (with proper solar/artificial light level).

Photosynthesis lowers down – and almost stops – if air CO2 concentration goes down to 200 ppm; this low CO2 level blocks the plant, because it can’t produce the necessary sugar level for its proper development. The main consequence of this situation’s a blocked growing, because the plant diminishes its metabolism to preserve energy and be able to survive – strains will be able to deal with their vital processes when having enough CO2. Besides, light, temperature and CO2 levels have to be increased for the plant to use the resources properly.

Under CO2 100 ppm, there’s no photosynthesis, and plants have a net sugar loss due to breathing; with CO2 100 ppm, breathing and photosynthesis are similar (no net loss/gain), and, with 400 ppm levels (atmospheric air), photosynthesis increases fast, similarly to the increase of CO2 levels.

The increase of photosynthesis diminishes when CO2 concentration goes up to 800 ppm and, if higher (800-1200 ppm), the increase is more remarkable.

How to maintain optimum CO2 parameters

As previously mentioned, good ventilation – providing fresh, clean, outdoor air to our indoor growth – is the best way to maintain the necessary CO2 levels for proper strain development – fans improve air flow too, as well as CO2 levels.

How to dispense CO2 to our growth

Usually, growers use CO2 tanks which expel gas gradually; generally, these tanks have a full kit – CO2 meters, press regulator, solenoid valve: Grow Shops provide a wide range of articles to generate CO2 for growths.

co2 tank and generator

On the other hand, there are CO2 generators – burning propane or natural gas, they produce the necessary CO2 for marijuana growing; other growers use dry ice (when evaporated, it expels CO2 too).

With little growing space, bicarbonate and vinegar can be a good solution.

How to calculate necessary CO2 level for growing

An example’s the best way to explain it; a 3 x 4 x 3 m room (long x wide x tall) contains 36 cubic meters, and it would require 0.036 cubic meters of CO2: with this simple formula, you can rapidly calculate the exact CO2 level your strains need for proper developing.

Is CO2 dangerous?

Average CO2 level’s not dangerous and, besides, it’s not flammable/toxic gas – with CO2 up to 50,000 ppm, it can be dangerous for our body; anyway, these levels are overrated, and this situation’s difficult to reach – 50,000 ppm’s a proportion 30 times bigger than the maximum percentage needed by a plant.

non-flammable and non-toxic co2

CO2 and light intensity?

CO2 levels are closely related to light intensity – main photosynthesis level increase happens when CO2 level goes 0-200.

If light’s dim (150 moles or 1,150 fc) (12,330 lux), photosynthesis level increases when CO2 goes up to 400 ppm; if CO2 concentration’s increased beyond those percentages without increasing light intensity, there won’t be any higher photosynthesis level – in fact, strains are unable to profit the highest CO2 levels till light intensity increases.

If light intensity’s 600 moles (4,600 fc) (49,310 lux), photosynthesis level increases as CO2 levels go up to 400 ppm; increase level diminishes slightly after that, but photosynthesis level keeps increasing as CO2 levels go up to 600 ppm.

If 600 ppm of CO2 are surpassed, photosynthesis level keeps increasing, but even slower, till increasing level stabilizes around 1,200 ppm – if the plants get 4,500-5,500 fc (48,240 lux) of light, the can use up to 1200-1300 ppm of CO2. This way, lumens are necessary to calculate the CO2 level to be provided to our indoor growth.

Homemade and cheap CO2 Dispenser for Cannabis grows

This has been an overview on all about CO2 and marijuana growing – if still in doubt, just leave your comment and we’ll answer as soon as possible.

Carbon dioxide (CO2) is necessary for the correct performance of the plants, together with water, light or nutrients, because plants need to consume CO2 …