CO2 can be provided to your cannabis garden in a number of ways, the easiest ways are to grow outside, install an intake drawing air from outside and/ or human interaction with your garden. Other more complex and artificial ways of providing adequate CO2 for larger gardens include pressurized tanks that are mostly needed when the grow room is completely isolated from outside sources of CO2. Cannabis plants can grow extremely dense flowers with a sufficient supply of CO2, even in a situation with temps reaching 90 degrees!
Intakes and Exhaust: A passive intake hole or ducting from outside is an easy and efficient way to provide sufficient CO2 to your indoor garden. An simple carbon filter connected to an exhaust will pull the air from outside and then pass it through a carbon filter before it is released back into the environment. An intake system also reduces temperatures that may inhibit your plants production, however temperatures may be too cool when lights are off in temperate weather. It is important to install a carbon filter to reduce odors being released out of the exhaust. Installing a mesh screen over the intake will help reduce unwanted insects from entering your cannabis garden.
Tanks of CO2 cost very little to fill and are great because you can set them up with a release valve and regulator that only lets CO2 out when the PPM of your room drops bellow 1500-1200 PPM.
Burners are usually a simple propane gas burner mounted inside a metal box that has a emergency shut off switch in case of tipping. These units can also be equipped with a sniffer to make sure the unit only turns on when your room needs CO2. In order to keep the temperature down, some of these units also have the ability to connect to your inline duct system to pull the heat associated with the constant burning pilot out of your grow!
Human Interaction: This is a great method for those that have a small closet garden or have a closed environment that has no intake or outtake but instead uses AC cooling to cool bring down the temp. Not a preferred method.
PRO TIP: Supplemental CO2 will keep your plants growing when you have temperatures higher than 85 F.