CO2 (Carbon Dioxide)
Definition: CO2, or carbon dioxide, is the gas plants combine with water and light energy to make sugars during photosynthesis. Outdoor air holds roughly 420 parts per million (ppm). Indoor growers sometimes enrich sealed rooms to 1,000 to 1,500 ppm so cannabis can make use of higher light intensity and grow faster.
How CO2 (Carbon Dioxide) works
Photosynthesis needs light, water, and CO2. When light is modest, the plant can’t use extra CO2, so enrichment does little. When light is strong, CO2 often becomes the bottleneck, and raising it lets the plant turn more of that light into growth.
When enrichment pays off
- Canopy PPFD above roughly 900 µmol/m²/s.
- A sealed or mostly sealed room, so enriched air isn’t immediately exhausted.
- Nutrients, water, and temperature already dialed in.
With elevated CO2, cannabis tolerates somewhat warmer canopy temperatures, around 80 to 85°F (27 to 29°C), which pairs well with high light.
The opposite problem
A small grow tent with weak exhaust can actually drop below outdoor levels during lights-on, because plants pull CO2 out of the air. Good air exchange alone keeps most home grows near ambient, which is enough for typical LED setups.
Ways to add CO2
- Tanks with a regulator and controller: precise, common in sealed rooms.
- Burners: produce CO2 and heat from propane or natural gas, suited to large spaces.
- Bags and fermentation: cheap but low and inconsistent output.
Safety
Above about 1,500 ppm, plants gain little. People need protection too: OSHA’s 8-hour workplace exposure limit is 5,000 ppm. Any enriched space needs a CO2 monitor, and gas burners also need carbon monoxide detection and proper ventilation. Turn enrichment off during lights-off, when plants don’t use it.