CO₂ Enrichment in High-Density Greenhouses: Control Parameters That Matter

Holding CO₂ at 800–1000 ppm sounds like a fixed target. In practice, the concentration that actually helps a crop changes hour to hour, and dosing without accounting for that is a reliable way to enrich the air above the canopy without enriching the plant.

Uptake is conditional, not constant

Photosynthesis only uses additional CO₂ when light and temperature are also in range. On an overcast morning or during a ventilation-driven temperature swing, stomata partially close and CO₂ uptake drops — dosing at the same rate as a bright, warm afternoon during that window mostly raises ambient concentration without a matching increase in fixation. The inverse matters too: on a high-light day, under-dosing leaves photosynthesis light-saturated but carbon-limited, which is lost yield that a fixed schedule will never catch.

The three parameters that actually need to talk to each other

A dosing strategy that works ties CO₂ setpoint to two other readings, not just a clock:

• PAR/light intensity — dosing should scale up as usable light increases, and step down as it falls, rather than following a static daily curve
• Vent and screen position — CO₂ dosed while roof vents are open is dosed into the outside air; enrichment needs to gate against ventilation state, not run independently of it
• Canopy temperature, not just air temperature — stomatal response tracks leaf temperature more closely than the greenhouse air sensor several metres away

What this looks like in a control system

The practical implementation is a dosing curve driven by live PAR and vent-position inputs rather than a single ppm setpoint, with dosing suppressed automatically whenever vents open past a threshold. Getting this right does not require more CO₂ input overall — in most high-density retrofits, total consumption drops because dosing stops running during ventilation events that used to waste it. The gain is in when the gas is delivered, not how much.