CO2 enrichment means adding carbon dioxide to the growing air above ambient, which is now about 420 ppm outdoors. CO2 is the raw material for photosynthesis, and at high light levels it becomes the limiting factor. Enrichment raises the light saturation point and the optimum temperature, so plants can turn more light into growth. Chandra and colleagues (2011) found cannabis photosynthesis increased substantially at elevated CO2.
In practice
Enrichment pays only when light is already high, roughly above 800–1,000 µmol/m²/s, and the room is sealed or exhausts rarely; venting enriched air wastes it. Common targets are 800–1,200 ppm in veg and early flower, sometimes to 1,500 ppm at high PPFD, with a slightly higher temperature, about 26–30 °C (79–86 °F). Most growers stop enrichment in the last week or two of flower.
Safety first. CO2 is odourless and displaces oxygen. NIOSH sets an exposure limit of 5,000 ppm averaged over a working day, with 40,000 ppm immediately dangerous to life. Use a monitor with an alarm, never enter a heavily enriched room for long, and keep burners and tanks away from living areas.
Units and formula
CO2 in ppm (parts per million by volume). 1,000 ppm = 0.1%.
Don't confuse with
- ppm (nutrients): TDS readings for solutions, an unrelated scale.
- Ventilation: brings CO2 back to ambient but cannot exceed it.
