Ventilation at night

I have squirrel cage fans exhausting stale air and bringing in fresh. Should I leave them off at night so the plants can absorb the C02 that they produce when it is dark? I am concerned that if I leave the vent fans off for the 12 hour dark cycle of this flowering room, the plants will not have sufficient fresh air during the night.
Harry K.,

Plants have several versions of photosynthesis. Cannabis is considered a “C-3” plant. These plants do not absorb or store CO2 during the dark period. “C-4” plants, mostly grasses, and “CAM” plants, mostly cactus and succulents, save CO2 during the dark. This is why CO2 supplementation is so important in enclosed cannabis gardens. There are about 380 parts per million (ppm) CO2 in the atmosphere. In a garden with no ventilation this is used up quickly. Once the amount of CO2 goes down to 200 ppm, photosynthesis stops until the CO2 is replenished.

Low CO2 is a limiting factor, too. In a brightly lit indoor garden, plants get all the water, nutrients and light they need. When CO2 levels are increased to 2000 ppm, cannabis photosynthesizes at a faster rate. Cell metabolism stays normal so there is a large surplus of sugar produced. Sugar is the basic building block of plant growth.

Venting the room serves several purposes. It replenishes CO2 in spaces where enrichment is not used and removes excess humidity and heat from the space.

During the dark period, cannabis does not use CO2. There is little heat generated but humidity does build up. For this reason you should use the vent fans in the dark period connected to a humidistat.

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