Carbon dioxide (CO2) is used by plants when they photosynthesize – they use CO2 and water to create food for growth using the energy they absorb from light. Fresh air has on average 350-500 PPM (parts per million) of CO2 but marijuana plants can potentially utilize a much higher level than this.
Some growers choose to supplement their air with additional CO2 as in the right circumstances this can dramatically increase growth rates and eventual yield. For growers operating a closed system this is vital: in a closed system air is circulated within the grow area and not drawn from outside so without supplementing CO2 it would become completely depleted from the air. In other situations CO2 should probably only be considered after all other factors have been optimized or it will be of little or no benefit, for example:
- If your plants are already not getting enough light then they will have no opportunity to use any additional CO2 in photosynthesis
- If temperatures are too high then the stomata on the undersides of leaves will close to minimize water loss through transpiration and thus prevent absorption of the CO2
- As CO2 is heavier than air, If your air circulation within the grow area is not good enough then the CO2 will simply settle at the bottom of the grow room and be unable to absorbed through the leaves.
Homemade and DIY CO2 solutions are often ineffective and sometimes outright dangerous so if you do decide to use CO2 by far the most effective way is to buy a CO2 bottle and a kit from a hydroponics store. Although CO2 is not in itself poisonous or deadly, in an extreme case increasing its level within a room can deplete the room’s relative oxygen content leading to possible asphyxiation (of you, not your plants!). If you are using CO2 you should always use a reliable monitor and regulator, and care must always be taken when handling or working around compressed gas bottles or canisters of any kind. Take particular care to ensure that any bottles or canisters are firmly secured.
Generally speaking with a bottle or canister setup the CO2 is run from the bottle, through a regulator and into the grow room through tubing pierced with holes. The tubing should be setup at a relatively high level above your plants as CO2 is denser than air.
If your system has a timer then link it to your light times as it is only necessary to run your CO2 system at the same time as your lights – plants only use CO2 during photosynthesis which occurs in the light, during the dark period they actually absorb oxygen and release CO2 themselves.
In addition to the more standard regulators, there are also hi-tech controllers available, including those made by EcotechnicsTM which beyond simply timing CO2 release can become a veritable climate control panel: extractor fans can be linked through both a thermostat and the controller to set flow based on temperature or humidity, while CO2 is added according to a timer, and linked to a monitor to provide predictive dose-settings.
Other approaches include using a propane or natural gas burning CO2 generator. It can be more discrete than using bottled CO2 as even if you are using bottled gas for the generator it is a lot less unusual to be receiving a delivery of propane than a delivery of CO2. The downside of a gas-burning generator is that it adds both heat and humidity to the grow area, another reason you should only use CO2 if you are in control of these factors already.
A big obstacle to using CO2 effectively in an open system is your ventilation. If your exhaust fan is changing the air in the grow area every few minutes then either your CO2 system will not have much chance to enrich the air or a lot of that enriched air will effectively get thrown away. If it is possible to do so then while running CO2 you should only use the minimum extraction needed to cope with temperature levels.
Your circulation fans SHOULD be running during this time though: as stated above CO2 is denser than air and absorbed through the underside of the leaf. Adequate air circulation is vital to ensuring the CO2 is accessible to the plants. It is easy to overlook circulation below the leaf canopy, but when using CO2 it is even more important than usual to ensure adequate air circulation at all levels in the grow area. A close eye should be kept on your plants’ water and nutrient requirements when starting to use CO2 as the greater potential growth that CO2 gives may lead to an increase in your nutrient and water consumption.
Provided all of these conditions are met then any increase in CO2 will be beneficial to plant growth, but the goal of enrichment would be to reach around 1200-1500ppm. At these levels it may be possible to add 20-30% to eventual yield, and shorten the overall growing period by accelerating vegetative growth. Some growers also report shorter flowering times as a result of using CO2. Any concentration higher than 1200-1500ppm may actually be counter-productive or pointless as plants also close their stomata (through which they absorb gases)when CO2 levels are too high.
Reports vary as to other benefits of CO2 use: some growers find their plants better able to handle high temperatures, some find increased level of THC production while others have claimed the increased yield has reduced potency. It’s also not unheard of for growers to use CO2 as a form of pest control – ramping up the CO2 levels with all extraction off to choke an insurgent insect infestation. With so few side-by-side trials to really assess it’s hard to be sure of every claim about CO2 but what is not in doubt is the fact it will increase yield and speed-up growth when other conditions are good.