Why Control CO2 Levels at Night in Plant Grow Rooms?

Author: Michael Forster, PhD, Edaphic Scientific 

This is an abbreviated version of a longer article on the same topic which can be found here.

Growers can artificially increase the amount of atmospheric carbon dioxide (CO2) available to plants to improve growth and yield. But at night, plants do not uptake CO2 for photosynthesis. Maintaining artificially high levels of CO2 at night may, therefore, be wasteful. Low cost technology is now available to automatically switch on and off the CO2 gas supply during the day and night.

In this article, we outline 5 reasons why you may consider turning off the CO2 supply at night via your CO2 monitor and controller.

1. Decrease CO2 Costs

Turning off CO2 supply at night will lead to a cost saving on your gas supply. Injecting CO2 gas into a grow room can be expensive. A calculation can be used to determine how much gas you need for a given room size and how long a gas cylinder will last. By turning the valve for the CO2 gas cylinder off at night, almost instantly your CO2 gas cylinder costs may decrease by up to one half.

2. Decrease Electricity Use

Turning off CO2 at night can also save on electricity costs. Electricity is required to run CO2 detectors, flow meters, valves and cylinders. Therefore, if energy can be saved in any way possible then this will add a cost advantage to the grower.

3. Increase Plant Growth

On reason why growers artificially increase CO2 in grow rooms is to increase plant growth rates and overall biomass. But some scientific studies have found that growth rates can increase even more if CO2 is turned off at night and only artificially enhanced during the day (see Griffin et al 1999). The increase growth rate is due to lower plant respiration rates at night when CO2 is switched off.

However, other science experiments have not found a consistent stimulus in growth across all species and cultivars. You should always experiment with your own crop and circumstances to see if you do find a growth stimulus, and even at what level of elevated CO2 this may occur.

4. Minimal Impact on Plant Yield

Generally, atmospheric CO2 enhancement experiments increase the yield in economically important crop species. As an increase in atmospheric CO2 leads to larger plants, perhaps it is not surprising that the total amount of fruit and seed on the plant also increases. A scientific experiment on soybean found that, when CO2 was switched off at night, seed output was only slightly lower than when CO2 remained on for 24 hours a day (See Bunce 2005).

5. CO2 Not Used in Dark Period

About 95% of the plants on earth are C3 plants (including most plants grown in greenhouses). These plants require oxygen at night for respiration. Since the CO2 is not used anyway, there is no reason to add it to the air!


Switching the CO2 supply off at night can save on costs while also improving or, at worse, having no effect on plant growth and yield. However, the results vary widely across species and even cultivars within species and there is no one single solution for all situations. For your particular circumstances and crop you may consider experimenting with both approaches to find our which one is more suitable for you.



Posted by CO2 Meter on March 29, 2016.
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