Plants build themselves from CO₂. Outdoor air carries about 420 ppm. A crop in a closed greenhouse consumes CO₂ far faster than leaks replace it — and on the brightest days, when the crop grows fastest, it runs out fastest.
How fast it falls
In bright sun a sealed house can fall from ambient to below 250 ppm within a couple of hours. Below roughly 300 ppm, CO₂ — not light, water or fertiliser — becomes what limits growth. It is counter-intuitive, because it happens on your best days and nothing on the plant shows it.
The fix is air exchange
Extraction and vents bring outside air — and its CO₂ — back in. The trick is to run them on CO₂ as well as temperature, so the house is ventilated for air on cool bright mornings even before it is too hot.
Other gases worth knowing
| Gas | Why it matters |
|---|---|
| Carbon monoxide | Only where a gas or paraffin burner is used — a worker-safety alarm first |
| Ethylene | Burner exhaust causes flower abortion and leaf droop at levels too low to smell |
| Ammonia | Where manure is heavy — scorches young foliage |
When goals conflict, a good controller resolves them in fixed order: worker safety, then temperature, then humidity, then CO₂.