Carbon Dioxide Supplementation

Carbon dioxide (CO2) is essential for photosynthesis. The amount of CO2 available to plants affects their net photosynthetic rate. Providing additional CO2 in indoor cultivation serves two purposes: (1) It can prevent a reduction in plant growth, which can happen in tightly sealed indoor environments if CO2 drops below ambient outdoor levels. (2) It can spur the rate of photosynthesis by providing elevated levels of CO2, resulting in bigger yields.

In this blog post, we’ll discuss the role of CO2 in photosynthesis, the ideal levels of CO2 for plants and best practices for supplementing CO2 (also called CO2 enrichment or CO2 fertilization) in your grow room.

CO2-plant relationship

Plants take in CO2 through tiny pores on the undersides of their leaves called stomata. They do this by diffusion: The stomatal cells open and close, allowing gas exchange to occur. Leaf surface area, light levels, ambient air temperatures, relative humidity, water stress and CO2 and oxygen (O2) concentrations in the air and leaves all influence this process.[1]

In photosynthesis, plants use light energy to turn CO2 and water into glucose and other simple carbohydrates, releasing O2 back into the atmosphere. Chemically, the process is as follows: 6CO2 + 6H2O + light energy ® C6H12O6 + 6O2.

Plants also respire, taking in O2 and releasing CO2. However, the amount of CO2 they release is much less than the amount they absorb. Plants can absorb so much CO2 that it lowers the level of CO2 in their immediate surroundings, particularly in enclosed spaces like greenhouses. In this way, plants can progressively deprive themselves of a source of CO2. This imbalance causes the environment to be CO2 deficient, creating the need for external CO2 supplementation in enclosed spaces to support optimal growth.

Figure 1. Some water sources used for irrigation and their related contaminants. Image source: Malakar, Arindam, Daniel D. Snow, and Chittaranan Ray. 2019. “Irrigation Water Quality—A Contemporary Perspective.” Water 11 (7): 1482. https://doi.org.10.3390/w11071482.

The concentration of CO2 in the environment strongly influences the rate of CO2 absorption: The higher the environmental CO2, the more CO2 is available for plants to absorb.[2] This is especially true of plants with a C3 photosynthetic pathway, including cannabis.

C3 plants use the RuBisCO enzyme to combine CO2 and other molecules into a 3-carbon compound, which is their photosynthetic product. By contrast, C4 plants produce a 4-carbon compound that does not require as much CO2.[3] For this reason, C3 plants yield 40–100% more when CO2 levels are 800–1,000 parts per million (ppm), while C4 plants yield only 10–25% more.[4]

CO2 levels and photosynthetic rates

Outdoors, ambient CO2 levels are usually around 340 ppm[5] by volume. When CO2 levels drop below this level, plant growth slows significantly, and in tightly sealed indoor environments, CO2 levels can drop as low as 200 ppm. That is why CO2 enrichment is often necessary indoors.

Better still, even when indoor CO2 levels equal outdoor ambient levels, increasing them above 340 ppm usually boosts photosynthesis. For most crops, increasing CO2 levels to 1,000 ppm increases photosynthesis by about 50% over ambient CO2 level photosynthetic rates.[6]

One study that examined the effect of different CO2 concentrations on Cannabis sativa L. found that when CO2 was above 700 ppm, it significantly heightened not only net photosynthesis but also transpiration, stomatal conductance and water-use efficiency.[7]

Figure 2. Effect of different levels of CO2 on net photosynthesis (PN), transpiration (E), stomatal conductance (gs), internal CO2 concentration (Ci), ratio of internal to external CO2 concentration (Ci/Ca) and water-use efficiency (WUE) on the leaves of Cannabis sativa. Source: See footnote 6.

Benefits of increasing photosynthesis

The benefits of increasing the photosynthetic rate with CO2 enrichment include:

  • Faster crop production: Plants have a light saturation point, above which greater light intensity no longer increases photosynthesis. However, additional CO2 increases the light saturation point. [8] By increasing light intensity and CO2, growers can stimulate growth and speed up the crop life cycle, especially if CO2 is enriched after transplanting.
  • Improved water-use efficiency (WUE): Elevated CO2 levels trigger the partial closure of stomatal cells, reducing transpiration, which involves water loss through vaporization from the stomata. Therefore, higher CO2 levels reduce water demand and increase WUE.[9]
  • Bigger yields: CO2 enhancement can boost yields. Research shows cannabis yields can increase by 10–25% when CO2 concentration is in the 800–1,000 ppm range.[10]

Figure 3. Benefits of CO2 supplementation. Image source: Wang, Anran, Jianrong Lv, Jiao Wang, and Kai Shi. 2022. “CO2 enrichment in greenhouse production: Towards a sustainable approach.” Frontiers in Plant Science 13: 1029901. https://doi.org/10.3389/fpls.2022.1029901.

How to supplement CO2

When lighting, nutrients, and other inputs and environmental factors are optimal, CO2 is usually the limiting factor for maximum plant growth.

Since plants only photosynthesize during the daylight hours, it is ideal to begin supplementing CO2 1–2 hours after sunrise and stop 2–3 hours before sunset.[11] When using HPS lighting at night, growers can continue supplementing CO2 to ensure adequate levels.

For improved economic efficiency, we recommend supplementing with 1,000 ppm of CO2 on sunny days when vents are closed and reducing to 400 ppm on cloudy days when light levels are below 40 watts per square meter. Environmental computers can be programmed to adjust CO2 levels according to light measurements. Once vents are opened beyond 10%, or when the second stage of exhaust fans is activated, strive to maintain a CO2 level of 400 ppm at the canopy.

Plants may also need more fertilizer. Since higher CO2 levels enhance root and shoot growth, plants can quickly exhaust available fertilizer in the growing medium. Some micronutrients may deplete faster; studies report low zinc and iron levels in crops where CO2 was enhanced. Lower transpiration and conductance[12] can also affect calcium and boron uptake, so growers often need to increase those micronutrients.[13]

Growers also need to be mindful of grow-room temperatures. Most biological processes, including photosynthesis, increase with an increase in ambient temperature. The higher the amount of available CO2, the higher the optimal temperature for crops.[14]

Finally, CO2 enrichment delivers diminishing returns as concentrations increase. Growth rates start to decline when CO2 levels go beyond 1,500 ppm—and plummet below normal growth rates at 2,000 ppm.[15]

Figure 4. Relation between CO2 concentration and rate of plant growth. Source: Roger H. Thayer, Eco Enterprises, hydrofarm.com.

When done properly, indoor CO2 enhancement can increase the rate of photosynthesis, driving faster plant growth and producing bigger yields. In our next blog post, we will explore the different sources of CO2 available to growers and their effects on cannabis.

Emerald Harvest Team

[1] 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.

[2] Ibid.

[3] Daley, Jason. 2018. “Why More Carbon Dioxide May Not Lead to More-Productive Crops.” Sierra, May 1. https://www.sierraclub.org/sierra/why-more-carbon-dioxide-may-not-lead-more-productive-crops.

[4] 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.

[5] Current atmospheric CO2 is higher than this number cited from the research. According to NOAA, the US’s National Oceanic and Atmospheric Administration, at the time of writing in September 2024, Monthly Average Mauna Loa CO2 was 422.03 ppm (vs 418.51 ppm one year prior): https://gml.noaa.gov/ccgg/trends/

[6] See footnote 1.

[7] Chandra, Suman, Hemant Lata Ikhlas A Khan, and Mahmoud A Elsohly. 2011. “Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2.” Physiology and Molecular Biology of Plants 17 (3): 291–5. https://doi.org/10.1007/s12298-011-0066-6.

[8] See footnote 4.

[9] See footnote 4.

[10] John W. Bartok Jr. 2017. “CO2 Enrichment for Cannabis.” Cannabis Business Times, August 30. https://www.cannabisbusinesstimes.com/article/co2-enrichment-for-cannabis/

[11] See footnote 4.

[12] Conductance is diffusion of gas such as CO2, water vapor and O2 through the stomata.

[13] See footnote 4.

[14] See footnote 1.

[15] See footnote 4.

1 comment

  1. October 10, 2024 at 4:49 pm
    Joey getz

    Useful information again.

Leave a comment

Your email address will not be published. Required fields are marked *

top