You install a CO₂ cylinder, push levels to 1200 ppm, and wait for a 30% yield boost. Two weeks later — no visible difference. The reason is simple: CO₂ accelerates photosynthesis only when it is the limiting factor. If the plant is already limited by light or temperature — extra CO₂ simply goes unused. CO₂ enrichment amplifies a system that is already well-tuned. It does not fix a poorly tuned one.
Quick glossary: CO₂ (carbon dioxide) — the primary substrate for photosynthesis; atmospheric concentration ~420 ppm (2024); in enclosed spaces during active plant growth it can drop below 200 ppm, significantly inhibiting photosynthesis. ppm (parts per million) — unit of gas concentration in air; 420 ppm = 0.042% by volume. CO₂ compensation point — the minimum CO₂ concentration at which photosynthesis balances respiration; below ~50–100 ppm the plant does not grow even under adequate light.
Where CO₂ Actually Limits Growth
In a well-ventilated greenhouse or a room with frequent door openings, CO₂ stays near atmospheric 420 ppm. The plant has enough, and enrichment here delivers minimal benefit.
In a tightly sealed grow room or greenhouse at peak photosynthesis — with good lighting and temperature of 22–26°C — plants consume CO₂ faster than it is replenished. Concentration drops to 200–300 ppm within a few hours of lights-on. At 200 ppm photosynthesis is noticeably inhibited even under adequate lighting. This is where enrichment delivers real results.
Easy to check: place a CO₂ sensor at canopy level in the middle of the light period. If concentration stays above 380–400 ppm — enrichment will not produce a noticeable effect. If it drops below 300 ppm — there is something to fix.
Working Range: From the Lower Limit to the Maximum
300–400 ppm — the lower threshold for effective photosynthesis under good lighting. Below this — noticeable inhibition.
800–1200 ppm — the optimal range for most indoor crops with adequate lighting and temperature. Growth accelerates 15–30% compared to atmospheric levels.
1500 ppm — the upper limit of agronomic benefit for most crops. Some crops (tomato, cucumber) can effectively use up to 1500 ppm at very high DLI.
Above 2000 ppm — risk of toxicity for sensitive crops (lettuce, leafy greens); stomata close as a defense mechanism; further increases have no effect or a negative one. For people in the space — discomfort and reduced concentration.
5000 ppm and above — dangerous for humans; do not allow during work in the space without forced ventilation.
Three Conditions for Enrichment to Have an Effect
Condition 1 — adequate lighting. CO₂ is a substrate for photosynthesis, but photosynthesis only proceeds in the presence of light. With DLI below 15–20 mol/m²/day the plant simply cannot utilize additional CO₂ — it does not have enough photons for the reactions. CO₂ enrichment without increasing DLI is half a system.
Condition 2 — correct temperature. Photosynthetic enzymes are most active at 24–28°C. Below 18°C or above 32°C, elevated CO₂ is absorbed much less efficiently — reaction kinetics slow down or the enzyme denatures. Enrichment with poor temperature control is wasted gas.
Condition 3 — enclosed environment. With open ventilation, CO₂ escapes — and maintaining elevated concentration is impossible without constant supply. Enrichment is only worthwhile in a closed mode where concentration can be held. If ventilation is required for temperature or humidity — calculate whether enrichment makes sense at all.
CO₂ Sources: Cylinder, Generator, or Fermentation
Liquid CO₂ cylinder — the cleanest and most controllable source. Precise dosing, no waste heat or moisture. Downside — cylinder cost and logistics.
CO₂ gas generator (propane or natural gas combustion) — cheaper at scale but produces heat and water vapor alongside CO₂. In small greenhouses it can overheat and over-humidify — reducing or negating the CO₂ benefit.
Fermentation and organic substrates — release CO₂ as a by-product, but unpredictably and in small quantities. Not a reliable enrichment source for serious production.
Human respiration — we exhale CO₂, and in a small sealed grow room where people are constantly present, CO₂ concentration rises. But this is not a method of enrichment — it is an uncontrolled variable.
Three Mistakes That Cost the Most
Enriching CO₂ without measuring the baseline level. If the greenhouse is well-ventilated and CO₂ does not drop below 380 ppm — enrichment costs will yield no noticeable effect. Measure first — then decide whether enrichment is needed.
Increasing CO₂ without matching lighting and temperature. "1200 ppm and nothing changes" — because DLI is 8 mol/m²/day or temperature is 16°C. Enrichment amplifies a system where lighting and temperature are already optimal. Where they are not — fix those first.
Running enrichment with ventilation open during the day. Opening a vent to lower temperature while enriching CO₂ — the CO₂ escapes within minutes and all benefit is lost. Either a closed environment with CO₂, or ventilation — both goals cannot be achieved simultaneously without an air conditioner.
How to Know CO₂ Enrichment Is Set Up Correctly
A CO₂ sensor at canopy level in the middle of the light period reads 800–1200 ppm and holds steady. After ventilation is switched on, concentration does not drop below 400 ppm before the next enrichment cycle. Plant growth has visibly accelerated with the same lighting and nutrition parameters as before enrichment. If there is no acceleration — check DLI and temperature as the priority.
If you want to go deeper: Lighting: DLI, PPFD, and How to Calculate Daily Light Dose — explains why CO₂ and lighting are paired parameters and how to calibrate them together for real photosynthetic gains.