There are several variations on how plants like cannabis fix carbon. The most common of these is termed C3 photosynthesis. Cannabis, and all other crops that benefit from CO2 fertilization, use this pathway. Structurally, think of the inside of a cannabis plant's leaves as composed of tiny reaction sites stacked on top of one another, with empty space and plumbing in between. These sites harness light and turn raw materials into energy-rich building blocks to fuel plant growth. The building blocks generated by these reaction sites are simple sugars, and CO2 is a key ingredient.

CO2 molecules present within the leaf need to be channeled to provide a constant supply of fuel for photosynthesis.

The answer to this need is the enzyme RuBisCO, which binds to CO2 molecules and transfers them to the photosynthetic machinery. Under ambient CO2 concentrations (about 400 ppm) and otherwise favorable conditions, the activity of RuBisCO is the limiting factor on photosynthetic productivity. This means that when temperature or light intensity rises above the cannabis plant's tolerance level, RuBisCO is unable to keep up with the CO2 demands of the reaction sites and the excess energy becomes stressful. By adding additional CO2 to the equation, we boost the activity of RuBisCO. It encounters CO2 molecules more often and can transfer them more efficiently, allowing the plant to extend productivity beyond normal limits.

Air Circulation and CO2 Uptake in a Cannabis Grow Room

As RuBisCO uses up CO2 inside the leaf, more is drawn in through diffusion—the natural movement of molecules from higher to lower concentrations. To enter the leaf, additional CO2 must pass through tiny pores called stomata. Since this is a passive process, only CO2 contained in the air that immediately surrounds the leaf, (known as the boundary layer), is available. Poor air circulation leads to stagnant boundary layers that are rapidly depleted of CO2. This concept is critical to maximizing CO2 enrichment. Without fans actively mixing and replenishing the air in contact with your plants, they will run low on CO2, no matter how much is available in the surrounding room.

In addition to facilitating the passage of CO2, stomata also regulate water loss through transpiration. Leaves close stomata to reduce water loss, but doing so reduces CO2 uptake. It's a dry world out there, and C3 plants constantly regulate stomatal openings to balance CO2 uptake against water loss. Due to the large moisture gradient between leaves and the surrounding air, taking in CO2 is costly in terms of water.

Dr. Suman Chandra, lead author in several federally sanctioned studies on cannabis physiology, found that when CO2 concentrations are raised well above ambient, cannabis responds by partially closing its stomata. Without the need for CO2 driving them to open, the stomata naturally close to conserve water. This is important for several reasons. It means that cannabis water use, per unit area, may decrease with CO2 fertilization. It also makes air mixing even more important, since partially closed stomata will slow CO2 uptake. Finally, this can lead to higher leaf temperatures by restricting transpiration.

Temperature and Light Intensity

CO2 fertilization allows cannabis to thrive at higher temperatures and utilize higher light intensities, but these two factors need to be considered together. Light comes with more heat, especially in HID illuminated environments. Both parameters shift the photosynthetic machinery into higher gear and CO2 enrichment allows it to run faster and cleaner. However, even with CO2, pushing too hard with light and/or temperature can send your plants into stressful conditions.

The general recommendation for maximizing CO2 fertilization in greenhouse crops is to raise the growth temperature by five to 10 degrees Fahrenheit above the ideal temperature in the absence of CO2 enrichment. For cannabis, this means that the ideal bloom temperature is shifted into the mid to high 80s. It is important to note that ambient grow temperature does not usually represent the temperature that the plant canopy is experiencing.

A room temperature in the low 80s will translate to canopy temperatures closer to the ideal for growth, with CO2 enhancement. Some strains may enjoy an even higher temperature, but I don't recommend running your space above 83°F, unless you know your strains will respond favorably and you have tight control of other environmental parameters. Be cautious when pushing the temperature envelope, the difference between ideal and harmful can be a few degrees.

Ideal CO2 Levels for Growing Marijuana

One of the most hotly debated aspects of CO2 fertilization in cannabis cultivation is the proper concentration of CO2. The only cannabis-specific research done is this area is presented in Dr. Chandra's publications, where he found that raising CO2 concentration to 700 ppm resulted in an instantaneous increase in photosynthetic productivity of 38-48 per cent, depending on strain. Unfortunately, his work doesn't discuss the effects of CO2 fertilization at concentrations higher than 750 ppm.

As CO2 concentrations are increased well above ambient, the law of diminishing returns applies to the benefits. This means that the degree to which additional CO2 increases productivity drops as ppms increase, ultimately reaching the point at which plant stress occurs. As with most things, too much CO2 can have negative effects, leading to lower yields and leaf death at extremely high levels.

The concentration at which CO2 becomes detrimental to plant health varies widely between species. Tomatoes, for example, have an upper threshold of about 2,000 ppm, while chrysanthemums experience stress at concentrations greater than 1,200 ppm. In the absence of research to clarify the issue, my view is that our favorite plant likely falls on the higher side of the continuum, as cannabis is a highly productive annual capable of explosive growth.

Another consideration is that, over time, many C3 plants fail to maintain the productivity gains that they initially experience with CO2 fertilization. Understanding this goes back to RuBisCO activity.

Scientists studying the phenomena have found RuBisCO levels in many plant species lowers over time in response to elevated CO2. This occurs because the environmental cues that drive RuBisCO production are suppressed under enhanced CO2 conditions. The degree to which acclimation to enhanced CO2 occurs is species-specific, and few studies have evaluated this response at CO2 levels higher than 700-800 ppm. In cannabis it may be more effective to gradually increase CO2 concentrations over the crop cycle, as opposed to raising them to the highest level immediately.

As cannabis legalization continues to progress and the markets in legal states mature, the physiology of cannabis will hopefully be studied to the same degree as other crops. With these efforts will come a better understanding of how to best use CO2 fertilization in cannabis cultivation. Until that time, my advice is to enhance bloom in the 1,200-1,600 ppm range, with 1,400 ppm as a good rule of thumb.

If you are running CO2 in the vegetative phase, I don't recommend exceeding 800 ppm. This level provides your vegetative plants with a good boost and ensures that they see a significant benefit as they move into higher CO2 in flower. If able, I also recommend experimenting with different levels of CO2 fertilization and with gradually increasing CO2 concentrations across the bloom cycle.