
- October 16, 2024
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Supplementing carbon dioxide (CO2) in the grow room is common in indoor cannabis cultivation. As we discussed in our blog post on CO2 supplementation, CO2 is essential for photosynthesis. By increasing the amount of ambient CO2, growers can boost their plants’ photosynthetic rates, producing bigger yields and shortening the crop life cycle for more frequent harvests. In this blog post, we will discuss the different methods of CO2 supplementation.
CO2 injection systems and tanks
CO2 injection systems and tanks rank among the most popular ways to supplement CO2 in grow rooms. This method uses CO2 compressed into liquid form, which is vaporized and distributed as a gas throughout the grow room.
Larger operations often use PVC pipes with holes in them to distribute the CO2, while smaller operations may just release it directly from the tank. While 20- to 50-pound tanks are available for smaller operations, larger operations require truck delivery and use of special storage tanks.
Growers also need a pressure regulator, solenoid valves, flow meters, CO2 sensors and timers for precise CO2 injection into the grow room. The pressure regulator lowers the compressed gas pressure to a manageable level for the flow meter to regulate, which dispenses a specific amount of CO2, measured in cubic feet per minute, to the plants while the solenoid valve remains open. A timer controls both the timing and the duration of the solenoid valve to inject the CO2 at regular intervals.[1]
The benefit of compressed CO2 is that it offers pure CO2 that can be introduced into the plant canopy at any time without risk of causing crop damage or producing heat and moisture. It also facilitates precise control over CO2 levels. However, depending on the size of the operation, it can be expensive. A 20-pound cylinder can cost $150‒200, with refills costing $20‒50 and lasting about two weeks for a 200-square-foot-sized grow room maintaining a CO2 concentration of 1,200‒1,500 parts per million (ppm).[2]
Figure 1. Solenoid valve

CO2 burners and generators
CO2 burners and generators are another common method. These combust hydrocarbons in fossil fuels like butane, propane and natural gas to maximize CO2 production and minimize heat generation. The rate of CO2 production is determined by the fuel combustion rate,[3] but several factors in the grow room or greenhouse play a role in maintaining the desired CO2 concentration, including:
- Temperature affects how efficiently plants absorb CO2.
- Plant density influences how much CO2 is needed for photosynthesis.
- Light impacts how much CO2 plants can effectively use—the more light, the more CO2.
The generator size and the amount of horizontal airflow in the grow room determine the number and placement of these units. Kept just above the plants, each unit covers around 4,800 square feet. Their hourly burner capacities range from 20,000 to 60,000 British thermal units, or Btu. Each unit costs around $1,000‒2,500 plus an additional $1,000 or so for gas and electrical installation.[3]
Figure 2. Power plant flue gas condenser

While an added benefit of these units is that they can complement heating systems, they entail higher risks than other methods. If fuel combustion is incomplete, CO2 burners and generators can produce carbon monoxide,[5] which is harmful to plants and at high enough levels is deadly to humans. Heat generation can also promote diseases like powdery mildew and botrytis.[6] Yet another risk is fuel contamination. While most fuel sources have low levels of impurities, it is important to check that sulfur does not exceed 0.02% by weight; otherwise, plant damage can occur.[7]
If growers own natural gas boilers, they can also use the flue gas to supplement CO2. This requires a flue gas condenser specifically designed to direct the gas into the grow room.
CO2 fermenter
CO2 is a byproduct of fermentation. Growers can capture CO2 by combining yeast, sugar and water in a tightly sealed plastic container. Fermenting a pound of sugar produces a half pound of CO2, but it also produces a half pound of ethanol, which can be burned to create even more CO2. While it is a faster process than decomposition, fermenters emit foul odors, require a larger space and make it difficult to maintain desired CO2 concentrations.[8]
Container methods
CO2 can also be supplied through containers, using dry ice or decomposed organic matter:[9]
- Dry ice emits CO2 as it sublimates. It is an inexpensive, efficient method: 1 pound of dry ice can maintain about 1,300 ppm CO2 in a 100-square-foot area and only costs $1–3. However, it has a short shelf-life and can be hard to store in normal conditions. While some grow operations use special cylinders equipped with gas flowmeters, it’s more common either to place small bits of dry ice around the grow room, replacing them frequently, or place dry ice inside insulators with small holes for CO2
- Decomposing organic matter naturally releases CO2. Organic waste placed in plastic containers is an inexpensive way to supplement CO2 and can be repurposed as compost. However, it is difficult to control CO2 levels, requires more space and substrate, and emits bad odors.
Figure 3. Dry ice

Chemical reaction
Finally, dripping acetic acid onto baking soda releases CO2. It’s not a popular method, as it requires large quantities of both compounds to significantly raise CO2 levels, making it cost prohibitive. To produce 1 pound of CO2, a grower needs 2 pounds of baking soda and 10‒12 liters of 5% acetic acid.[10]
Distributing CO2
Whichever source of CO2 is used, it’s important to have an adequate distribution system because CO2 does not travel far after diffusion.
One option is horizontal airflow fans and fan-jet systems, assuming the top vents are kept closed and exhaust fans aren’t in use. These move large volumes of air and uniformly distribute CO2.
Another option is a central header with small individual tubes that emit CO2 through evenly spaced holes placed either low in the canopy or under the benches of bench crops.
Most growers use sensors linked to a central computer to monitor and control the growing environment. A CO2 controller, typically an infrared gas analyzer (IRGA), can be used to monitor and maintain CO2 levels. Usually, a single IRGA is used per operation, but growers can use scanners or multiplexers to obtain readings from different areas.
Safely using CO2
Despite the many benefits of increasing the amount of CO2 in the grow room, this practice involves some risks that growers should be aware of.

- CO2 levels can get too high. This must be prevented. Not only can extremely high CO2 levels adversely impact plant growth and health, but they are also dangerous to human beings. In the US, OSHA[1] does not allow CO2 levels of more than 5,000 ppm. Increasing the ambient CO2 level from 340 ppm[2] to as high as 1,000 ppm is sufficient to achieve a significant improvement in plant growth and yield while remaining safe for plants and people.
- Other harmful gases may be emitted. Burners with a high flame temperature may form nitrous oxides, which can diminish plant growth or even cause necrosis. To prevent this when using flue gas as a CO2 source, use boilers with low-output nitrous oxide burners.
With the proper precautions and monitoring tools, growers can safely use CO2 to boost plant growth, speed up crop production and get bigger yields.
Emerald Harvest Team
[1] Poudel, Megha and Bruce Dunn. 2023. “Greenhouse Carbon Dioxide Supplementation.” OSU Extension. Published September 2023. https://extension.okstate.edu/fact-sheets/greenhouse-carbon-dioxide-supplementation.html.
[2] Ibid.
[3] Burning natural gas can produce 8.2 pounds of CO2 per hour. Source: See footnote 1.
[4] See footnote 1.
[5] The National Institute for Occupational Safety and Health (NIOSH). n.d. “Carbon Monoxide.” Last reviewed July 9, 2018. https://www.cdc.gov/niosh/topics/co-comp/default.html.
[6] Ontario Ministry of Agriculture, Food and Agribusiness and Ministry of Rural Affairs. 2022. “Supplemental carbon dioxide in greenhouses.” Updated July 8, 2022. https://www.ontario.ca/page/supplemental-carbon-dioxide-greenhouses.
[7] Ibid.
[8] See footnote 1.
[9] See footnote 1.
[10] See footnote 1.
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