There seems to be a lot of misinformation about how much co2 (carbon dioxide) is available to indoor plants, and how to think about whether and how much co2 to add if you decide to enhance co2. I took my calibrated co2 meter on a hunt for the truth.
As an aside for newbies, don't confuse carbon dioxide (CO2) that your plant uses with carbon monoxide (CO) which is a poisonous by-product of burning fossil fuels.
First myth - you should add CO2 indoors because there is less co2 in a closed indoor environment than what an outdoor plant gets. This is just not true. A normal indoor situation of an inhabited structure with central heating/cooling has as much as twice the amount of co2 indoors compared to outdoors. You do not have to add co2 to equal outdoor conditions. For example, in my case the outdoor co2 is at 400 ppm (national average is 390 ppm, thank you climate change) and indoors it's 650 ppm.
Second myth - adding co2 will improve your grow. This is only true under special conditions. The reason experienced growers add co2 is that if all other environmental conditions are optimized (water, nutes, light, temperature, humidity, medium, container, ph, insects, etc.) then and only then will increased levels of co2 (typical target is 1,500 parts per million) be used by cannabis plants to grow faster. Adding co2 without dialing in your grow will not change anything. if you are just starting out then don't worry about co2 for your first few grows.
Third myth - a couple of plants in a closed closet will exhaust available co2, requiring action on the grower's part to compensate by (a) actively ventilating to bring in fresh air or (b) by adding co2. In the vast majority of cases this is not true either. Home construction of internal walls is just not that airtight. An easy experiment to test your grow room - close off any inputs you've created and turn on the exhaust - the fan will run and air will continue to be forced out of the room indefinitely. If you have a tight room you can lower the air pressure (ie create negative pressure) but you won't be able to pull a vacuum because air will leak into the room from thousands of tiny cracks. The rule of thumb for an un-vented room is a complete air exchange every two hours. That new air will have more co2 than outdoors. In addition, the higher partial pressure of co2 outside your grow room will cause co2 to migrate into your room faster than other components of air. I have checked repeatedly and the co2 concentrations in my grow room are always within 100 ppm of the rest of the house, and considerably higher than outdoors, even without noticeable negative pressure and with a closet full of plants. Unless you're growing in a refrigerator or tent or similar (or you are growing dozens of plants) you don't have to do anything to get enough co2 for your plants.
Fourth myth - carbon filters emit co2. no, they don't. Carbon filters remove smelly compounds from the air, they have nothing to do with carbon dioxide. Thanks to mydixiecrat for helping to point this out.
Fifth myth - you need expensive co2 generating and monitoring equipment to maintain the proper levels of co2 if you choose to enhance. Okay, I bought into this myth and I believed it enough to get a $400 co2 monitor and a $450 gas generator. So, I thought I'd get a little extra back from my investment. I have the gas generator running in my flowering room but nothing for my veg room. I'm going to use my monitor from the flowering room to test whether it's really possible to build a ghetto sugar/yeast co2 system that produces a steady and predictable concentration of co2. I'll keep updating this thread with results - maybe the next person can save the $850 plus propane costs with a couple of kitchen supplies. Results so far show that a sugar/yeast system is quite good at providing a relatively steady, predictable and meaningful amount of carbon dioxide.
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