Does Lighting Affect Growroom CO2 Levels?

By Lynette Morgan | Last updated: November 16, 2022

lettuce plants under growroom lighting

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.

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Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants

Profile Picture of Lynette Morgan

Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.

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