Grow science · 11
CO₂ for plants
CO₂ is the raw material of every leaf. More only helps when light, climate and roots can keep up.
The baseline
Outdoor air holds around 420 ppm CO₂. In a closed grow tent with strong light, plants can pull that down noticeably within hours if the air isn’t exchanged. That’s one reason why exhaust and fresh intake air matter even without enrichment.
What enrichment does
With more CO₂ in the air, the leaf can take in more carbon through a smaller stomatal opening. Photosynthesis gets faster and the plant loses less water per gram of growth (stomata 101).
| CO₂ | What it means |
|---|---|
| ~420 ppm | Outdoor air, the normal baseline |
| 800–1000 ppm | Moderate enrichment, used for seedlings and many greenhouse crops |
| 1000–1300 ppm | Where most crops get close to their maximum benefit |
| above ~1500 ppm | Little extra benefit; can start to hold growth back |
Typical greenhouse guidance. The best level depends on crop, light, temperature and stage.
Light comes first
CO₂ only pays off when light is the thing the plant has plenty of. Below a strong PPFD (roughly 800+ µmol/m²/s in flower) the plant can’t use the extra carbon, and the money goes out the exhaust. With enrichment, plants can handle more light and a slightly warmer room, but only if water, nutrients and VPD keep up.
When it doesn’t work
- Open exhaust: CO₂ gets blown straight out. Enrichment needs a sealed or mostly sealed room.
- Lights off: no photosynthesis, no use. Only enrich during lights-on.
- Weak light or bad climate: fix those first. They limit growth long before CO₂ does.
Stay safe
CO₂ is invisible and odourless. Indoor air above about 1000 ppm already feels stuffy, and the occupational limit in Austria and Germany is 5000 ppm over a workday. If you enrich, use a controller with a sensor, keep it out of living spaces and ventilate before you spend time in the room.
Questions growers ask
What CO₂ level is best for plants?
For most crops, enrichment to about 1000–1300 ppm gets close to the maximum benefit when light is strong. Outdoor air has around 420 ppm. Above about 1500 ppm there is little extra benefit.
Does CO₂ help without strong light?
Barely. CO₂ enrichment pays off when light is plentiful, roughly from 800 µmol/m²/s PPFD in flower. With weak light, the plant can’t use the extra carbon.
Should I add CO₂ at night?
No. Without light there is no photosynthesis, so plants can’t use extra CO₂. Only enrich during lights-on.
Is CO₂ dangerous for people?
In high concentrations, yes. Air above about 1000 ppm feels stuffy, and the occupational limit in Austria and Germany is 5000 ppm over a workday. Use a controller with a sensor and ventilate before spending time in an enriched room.