Quick answer: Plants take in CO₂ through open stomata, the same pores that release water vapour. Outdoor air holds about 420 ppm of CO₂; in a closed, sunny greenhouse, a crop can draw that down quickly, so many growers enrich to around 800–1,000 ppm during the day. Enrichment only pays off if the CO₂ stays in the house and reaches the leaves: that needs closed or nearly closed vents and good air movement. The conflict is humidity: venting to dry the air throws CO₂ away. Dehumidifying with vents closed lets growers hold both CO₂ and humidity.
Quick answer
- Outdoor CO₂: about 420 ppm. Common enrichment: about 800–1,000 ppm.
- Stomata trade water for CO₂: very dry air closes them and cuts uptake.
- Venting for humidity loses CO₂; dehumidification keeps vents closed.
How CO₂ reaches the leaf
- Air in the house carries CO₂ to the canopy.
- Leaf boundary layer: a thin layer of still air around each leaf slows CO₂ arriving; air movement thins it.
- Stomata: CO₂ enters through open pores, while water vapour leaves.
- Inside the leaf: CO₂ diffuses to the chloroplasts and is fixed in photosynthesis.
A sensor reading in the aisle is not the CO₂ at the leaves. In a dense canopy with poor air movement, CO₂ can be lower inside the crop than in the aisle. Air distribution through the canopy is how CO₂ is delivered; Wageningen research covers air distribution and climate uniformity in greenhouses (WUR).
Stomata, humidity and CO₂
Stomata open in light to take in CO₂, and lose water while open. When the air is very dry (high VPD), plants close stomata partly to save water, which also cuts CO₂ uptake. When the air is very humid, transpiration slows and nutrient uptake suffers. A moderate VPD (often about 0.8–1.2 kPa during the day) keeps stomata open; see the VPD chart. A 2024 review discusses how CO₂ and climate interact in plant growth (PMC).
The CO₂ cost of venting for humidity
Worked example: a 1 ha greenhouse with an average height of 3 m (30,000 m³) is enriched to 800 ppm. Venting at one air change per hour to lower humidity replaces 30,000 m³/h of enriched air with outside air at about 420 ppm:
30,000 m³/h × (800 − 420) × 10⁻⁶ × 1.8 kg/m³ ≈ 20 kg of CO₂ per hour lost.
Over 8 hours of daytime venting, that is about 160 kg of CO₂ a day that must be replaced to hold 800 ppm, or the enrichment level falls. Dehumidifying with vents closed removes the water and keeps the CO₂ in. See greenhouse humidity control methods and semi-closed greenhouses.
Temperature, light and CO₂
- Higher CO₂ lets plants use more light and often tolerate slightly higher temperatures productively.
- Enrichment is most useful when light is strong enough for photosynthesis and vents are closed (winter and spring days in temperate climates).
- At night, plants respire and do not need enrichment.
Follow crop-specific guidance for target levels.
Practical checklist
- Measure CO₂ at canopy level in several zones.
- Run horizontal airflow fans during enrichment.
- Stop or reduce dosing when vents open wide.
- Use dehumidification to control humidity during enrichment periods.
- For indoor farms and grow rooms, keep rooms sealed and dehumidify; see grow room dehumidifier sizing.
Our greenhouse dehumidifiers and ceiling-mounted and ducted units let growers remove moisture without venting.
FAQ
Frequently asked questions
What CO₂ level is best for greenhouse plants?
Many growers enrich to about 800–1,000 ppm during the day; follow crop-specific guidance.
Does humidity affect CO₂ uptake?
Yes. Very dry air makes plants close stomata and take in less CO₂; very humid air slows transpiration and nutrient uptake.
Why does venting reduce CO₂ in a greenhouse?
Enriched inside air is replaced by outside air at about 420 ppm.
How can I control humidity without losing CO₂?
Dehumidify with vents closed, and keep air moving through the canopy.
Sources
- Wageningen University & Research, greenhouse air distribution research (edepot 202121).
- 2024 review on CO₂ and plant growth (PubMed Central PMC11190854).
