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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.