Does Lighting Affect Growroom CO2 Levels?

There is a connection to CO₂ levels in a growroom dropping when the lights come on. This is caused by the plants switching on photosynthesis as soon as there is sufficient light to power this process.

Why CO₂ Levels Fluctuate During the Light Cycle

As the plants sense the light, the stomata become fully open and are able to rapidly absorb the carbon dioxide from the surrounding air required for photosynthesis to occur. Plants use this CO₂ absorbed via the leaf surface, combined with water from within the plant and PAR (photosynthetically active radiation which your lights provide) to photosynthesize and produce assimilate (sugars) for growth and development. As a byproduct during the light phase, the plants release oxygen from the leaf surface. This only occurs in the light, so the plants don’t take up CO₂ in the dark when the lights are off and additional CO₂ does not need to be supplied until the lights come on.

The drop in CO₂ levels as the lights come on can occur surprisingly fast, particularly if the plants are large, grown at a high density, or in quite a limited room volume with minimal air exchange. This can also be seen in greenhouses when vents are closed to conserve heat in the early morning, sunlight powering photosynthesis can actually crop the ambient CO₂ to levels low enough to restrict photosynthesis and retard growth somewhat.

How to Prevent CO₂ Levels from Dropping

The best way to counter this drop in CO₂ as the lights come on is to either rapidly vent and draw fresh air in from outside (ambient CO₂ levels in the atmosphere these days are now over 400ppm) and ideally the growroom doesn’t want CO₂ to fall much below this while plants are actively photosynthesizing, so this may require a fairly rapid rate of air intake and venting out stale, CO₂ depleted air.

A more common approach in a growroom is to enrich with additional CO₂ so the plants are given an extra boost to increase the rate of photosynthesis rather than just making sure CO₂ depletion doesn’t occur. CO₂ can be enriched to 1,000-1,500ppm for many fast growing crops under warm growing conditions and good light. CO₂ enrichment has been well proven to increase growth rates and yields significantly and is widely used in controlled environment agriculture. Plants will make the most efficient use of CO₂ if they also have warm temperatures which are required to boost photosynthesis.

To prevent the CO₂ levels suddenly dropping as the lights come on, it’s a good idea to start CO₂ enrichment just before photosynthesis is triggered so that the environment is up to a good level as the lights fire up, this prevents any lag in photosynthesis which may otherwise occur if CO₂ drops rapidly during this time.

Monitoring CO₂ levels in your growroom is also a good was to not only ensure the CO₂ depletion does not occur, but to also monitor the rate of plant growth—healthy plants, growing rapidly under good conditions will consume large amounts of CO₂.