Part II of the three-part series “The Cost of Doing Nothing” demonstrates why delaying CO₂ enrichment is one of the costliest decisions an operator can make. Greenhouses that produce everything from lettuce to cannabis are, at their core, not simply crop producers, they are manufacturers of carbon. Every molecule of CO2 entering that system has the potential to be converted into biomass, flower, and ultimately, revenue. In other words, every molecule of carbon is a dollar waiting to be printed.
In Part 1 of this series, we explored the financial and operational impact of commercial lighting investments. Henry Ford once said, “If you need a machine and don’t buy it, then you will ultimately find that you have paid for it and don’t have it.” Ford also said, “The man who stops advertising to save money is like the man who stops a clock to save time.” Replace “stops advertising” with “doesn’t enrich CO₂,” and you have a statement that perfectly captures the silent killer of cannabis greenhouse margins. Delaying enrichment is not financial discipline, it is a symptom of financial indecision and an obvious lack of appropriate ROI analysis.
The Fatal Misconception: ‘It’s Just a Gas’
Many underestimate the impact of CO₂ enrichment because they cannot see it. Investing in something invisible, tied to a headache of building department and fire safety regulations, is a recipe for hesitation. Skeptics often argue that pumping in a "random gas" to increase yield by 25%–30% sounds like snake oil. They are wrong. CO₂ isn’t just a gas; it is the raw material that fuels reduced cost per pound and, ultimately, profit.
AER: The Hidden Lever in Every Greenhouse
Air Exchange Rate (AER), measured in Air Changes per Hour (ACH), defines how often the total air volume of a greenhouse is replaced with outside air. AER is critical for temperature and humidity control. The trick isn’t to eliminate air exchange, it’s to control it intelligently so that greenhouses can maintain optimal climate conditions while still retaining enough carbon to pay for every molecule injected.
Typical Air Exchange Ranges:
- Summer (Full Venting): 30–60 ACH; roof and side vents wide open, driven by heat, wind and exhaust fans.
- Shoulder Season: 2–8 ACH; controlled venting to reduce heat and relieve humidity while maintaining enrichment.
- Winter (CO₂ Retention Mode): 0.1–0.5 ACH; minimal exchange for maximum retention.
Here at Due Diligence Horticulture, AER is modeled dynamically across the growing year, resulting in an annualized average of approximately 7.5 ACH. This serves as the realistic baseline for our calculations.
RELATED: The Cost of Doing Nothing: How Delay and Indecision Are Sinking Cultivation Businesses
The Science of Photosynthesis
Photosynthesis is the process that forms the building blocks of life. Plants use three main photosynthetic pathways - C3, C4, and CAM - each adapted to different environments. Cannabis uses C3 photosynthesis, corn uses C4, and cacti use CAM. Unlike C3 plants, C4 and CAM plants have internal systems that concentrate CO2 inside their tissues. For this reason, CO2 enrichment is primarily valuable for C3 crops.
As a C3 plant, cannabis growth is directly limited by atmospheric CO₂. At ambient levels (~420 ppm), the enzyme responsible for fixing CO2 into sugar and ultimately biomass - Rubisco - operates under CO2-limited conditions. Rubisco can also react with oxygen through a process called photorespiration, which does not contribute to growth or yield. CO2 enrichment suppresses photorespiration by increasing the amount of CO2 available to Rubisco relative to oxygen.
When CO₂ levels rise to about 1,200 ppm:
- Rubisco operates at max capacity.
- Carbon fixation increases by approximately 25%.
- Every photon absorbed builds 25% more biomass.
The impact of CO2 enrichment on yield can be understood through the harvest index (HI) - the ratio of a plant’s economic yield (flower) to its total biomass. A 50% HI - typical for cannabis - means that 50% of the fixed carbon becomes marketable flower. CO₂ enrichment increases total biomass production and does not change harvest index in cannabis, meaning the plant maintains a similar proportion of flower to total growth. Thus, 25% more carbon fixation translates directly to 25% more bud.
The Math of Enrichment
At the back-of-the-envelope level, CO₂ is incredibly cheap compared to the value of the plant material it produces. About a penny’s worth of CO₂ contains enough carbon to build roughly 30 grams of plant biomass - or about 15 grams of flower assuming a typical harvest index of 50%. At roughly $1 per gram (a conservative estimate), that penny of CO₂ could generate about $15 worth of flower, meaning growers could capture well under 1% of the CO₂ they release and still come out ahead.
Taking real greenhouse conditions into account, consider a 50,000-square-foot greenhouse with a 12-foot gutter height (600,000 cubic feet) with an average 7.5 ACH. That means 4.5 million cubic feet of air is exchanged every hour.
- Initial Load: It takes roughly 54 lbs. of CO₂ to initially raise the concentration to 1,200 ppm in this space.
- Replacement: At 7.5 ACH, the greenhouse replaces its entire volume 7.5 times per hour, meaning CO₂ must be restored at a rate of 405 lbs./hour (54 lbs. x 7.5).
- Uptake by Plants: At typical light intensities, roughly 50 lbs. of CO2 per hour is fixed through photosynthesis.
- Daily Use: For a flower-cycle photoperiod of 12 hours per day, daily consumption including air exchanges and photosynthesis is 5,460 lbs. ([405 lbs./hour + 50 lbs./hr] x 12 hours).
- Cycle Use: Over a 56-day flowering cycle, this equals 305,760 lbs. per cycle.
- Annual Use: Across 6.5 cycles per year, this results in approximately 1.99 million lbs. of CO₂ annually.
The Cost of Indecision
| Strategy | Annual CO₂ Cost | Annual Revenue | Net Gain vs. Ambient |
|---|---|---|---|
| No Enrichment | $0 | $14.4M | — |
| Liquid CO₂ | $597K | $18.1M | +$3.1M/year |
| NG Boiler | $157K | $18.1M | +$3.5M/year |
Calculations on flower yield only are as follows. Please keep in mind price per lb. sold is $575. You could play with the math as high as $3,500 depending on your business’s vertical capability and market. Also, yield baseline is low to begin with at 35 grams of flower per square foot harvest per week or per cycle. We see some greenhouses hitting as high as 70 grams of flower per square foot per week during peak season. This calculation is conservative to prove that even under harsh conditions (lower than average yields and lower than ideal pricing) CO₂ is still a game changer.
| Wkly Harvest ft² | g flower/ft²/cycle | Wkly flower lbs. harvested | Price/lb. sold | Annual Revenue | |
| Without CO2 | 6,250 | 35 | 482 | $575 | $14.4M |
| With CO2 | 6,250 | 44 | 605 | $575 | $18.1M |
Calculations for CO₂ are based on 1.99 million lbs. of CO₂ used in one year. Liquid CO₂ is estimated at $0.30/lb. vs. condensing boiler capture at $0.079/lb.
Delaying CO₂ enrichment results in $3.5 million/year in lost revenue, roughly $10,000 every single day in unharvested flower. Even with the reduced 12-hour photoperiod, the cost of the "invisible" gas is dwarfed by the massive gains in marketable biomass
Conclusion: Stop the Clock
Mature horticultural businesses succeed because they don't just "try" CO₂; they commit to it. By utilizing condensing boilers at 95% efficiency, scrubbing flue gas, and tying CO₂ injection to light integrals and proper air exchange strategies, they achieve massive yield efficiency at a fraction of the cost of liquid gas. Henry Ford was right: the real cost isn't the machine, it’s the harvest you’ll never see.
[Editor’s Note: Check back for Part III of “The Cost of Doing Nothing” series: “The Cost of Product Contamination,” coming soon. Subscribe to CBT’s enewsletter to make sure you don’t miss it!]
