Maximizing the benefits of elevated CO2

By Daniel Banks

You bought the tank/burner and the regulator, maybe you even sprang for a pricey controller, and now it’s time to rock, right? Well, it depends on a few other factors. While the effects of CO2 enrichment on the growth and productivity of Cannabis are undeniable, many growers fail to grasp the details and maximize the potential benefits.

To increase the effectiveness of CO2 enrichment in the production of top quality Cannabis it is important to understand a few basic principles concerning how the plant consumes CO2.  Cannabis has an enzyme inside its leaves called RuBisCO that facilitates the conversion of CO2 into plant matter and ultimately those tasty buds we are all concerned about.  You can think of RuBisCO molecules as baseball mitts and CO2 molecules as baseballs.  RuBisCO catches the CO2 and takes it were it needs to go.  

In order for CO2 to enter the leaf and interact with RuBisCO it must first pass through tiny mouths on the underside of the leaf called stomata.  Diffusion, the process of molecules moving from higher to lower concentrations, is what brings CO2 into the stomata.  As RuBisCO works, the concentration of CO2 inside the leaf drops and more CO2 is drawn in to regain equilibrium. The CO2 that is drawn into the leaf comes in from the boundary layer that surrounds the leaf.  The term boundary layer describes the tiny layer of air that is in direct contact with the leaf surface and the stomata.  It is the concentration of CO2 in this minute layer that drives the diffusion of CO2 into the leaf. I know plant physiology can be painful, but stick with me.

If the boundary layer of the leaf is very still and stagnant, then its concentration of CO2 will deplete, which leads to a decreased rate of photosynthesis and growth even if the surrounding room has a high concentration of CO2.  In order to stop this from happening it is important to disrupt the boundary layer by moving fresh air across the leaf.  A good way to think of the boundary layer is as the lungs of the leaf.  By disrupting it by well placed fans and by avoiding over-crowded canopies we allow the plant to effectively breathe.  If you were to hold your breath while reading this you would eventually pass out whether or not the air around you contained oxygen.  In much the same way, you can raise ambient CO2 concentrations to 2,000 ppm but see little effect on plant growth if the air around the leaf is not mixed properly.  Remember, the quality of the air in your room only matters because it provides the source for the air in direct contact with the leaf!

Recent research published by a plant scientist at Mississippi State (he has access to the little known federal marijuana production facility) found that 700 ppm of CO2 increased photosynthesis by an average of 44% in the short term in four Cannabis varieties known for their high THC yield. Now I can’t tell you for certain what the ideal level of CO2 enrichment is but I can say that it is not necessary to ever go above 2000 ppm and that in some plant species high levels of CO2 can actually hurt plant growth.  One more detail to keep in mind is that when growing at elevated CO2 Cannabis may benefit from higher temperatures and more intense light, although the exact degree to which both should be increased will be strain specific.

Within the plant science community it is well known that plants like Cannabis can acclimate to elevated CO2 concentrations and lose some of the benefits.  To understand this phenomenon we have to go back to RuBisCO. When we increase the amount of CO2 reaching the leaf we are effectively working the available RuBisCO harder. But the plant is only able to work as hard as it needs to, not as hard as you might want it to.  For this reason a plant kept at a steady level of elevated CO2 stops making/activating as much RuBisCO, which could cause the rate of photosynthesis to drop.  A good comparison is fitness training at altitude. The human body adapts to less oxygen and when you come down to a lower altitude you get a boost from all the available oxygen because your body is used to having less. But the boost doesn’t last and your body goes back to its normal low altitude oxygen usage.

So if you are using CO2 enrichment and you want to top out at 2000 ppm I would advise stepping it up gradually.  Again, due to a lack of research in this area I can’t say with certainty what the best way to step up your CO2 concentration is.  If you asked me to take a shot at it, I would advise upping ambient to 700 ppm after plants are established and increasing the concentration by 300 ppm every two-three weeks for the life of the crop maxing out in the range of 1500-2000 ppm.  

Best of luck and happy growing!

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