Understanding CO2 in Gardens

Many indoor growers believe that carbon dioxide (CO2) is essential to plant development and thus is directly correlated to yields achieved in gardens. To a certain extent, this is true. In fact, increasing a garden’s CO2 from ambient levels of around 300 PPM to supercharged levels of around 1,800 PPM can almost double plant growth. However, there are limits to this reasoning and limitations to what the plant can physically handle.

CO2 & Your Growroom

The first question that needs to be answered is just how much CO2 does a plant need exactly? And immediately following that question, the next thought should be, how much CO2 is too much?

Generally speaking, indoor plants should not need a CO2 infusion, as ambient CO2 levels are sufficient enough at 300-400 PPM. However, there are reasons to add CO2 to indoor grow spaces. For instance, adding CO2 can help bulk up cannabis flowers, especially for strains that are notoriously low yielders. Furthermore, in gardens where lighting is intense, the more light that is provided to a plant the more CO2 it needs for photosynthesis — but this is only true to a certain point. Once a garden climbs past 2,000 PPM of CO2, plants begin to lose their capacity for CO2 uptake and any overage of this amount becomes wasted gas.

Now consider the following: The dry matter of a plant is comprised of 90 percent carbon, oxygen and hydrogen, with all of that carbon coming from the CO2 in the air that the plant leaves breathes in. These CO2 molecules are necessary during the light photo period only and (if you read this column last month you might remember this next part) are used in photosynthesis to form glucose, or plant food. The light photo period is key, as it requires approximately 10 photons (the particle energy that leaves harvest from light) to provide enough electrons to supply enough energy to split a single CO2 molecule into carbon (C) and oxygen (O) atoms for use in food production.

A lot of growers like to use lumens as a measure intensity to determine how much CO2 to supply their plants. And while ratios have been established using lumens to derive a basic formula for garden quantities of CO2 (more on that below), lumens are not the true barometer for measuring such levels. Considering the photon requirements mentioned above for splitting CO2 molecules, a much better measurement of light intensity is PAR, or photosynthetically active radiation, because light not hitting the leaf surface is wasted in terms of photon harvesting and CO2 splitting. So in the end, it is really the higher the PAR, the higher the CO2 levels should be.

In addition to lumens and PAR, there are a couple more considerations for CO2 usage. The proximity of your light sources to your garden canopy is also a factor as this distance also affects light intensity. The closer your lamps are to the garden, the more intense the light will be and a greater use for CO2 your plants will have. The final consideration will be your garden temperature — a delicate subject when dealing in CO2.

It is important to understand that higher temperatures will cause stomatal closing on the leaf. When stomates close the absorption of CO2 ceases and photosynthesis grinds to a halt. Carbon dioxide emitted into gardens whose temperature rises above 86 F (30 C), becomes more and more useless to your plants as temperatures climb. When temperatures climb in the growroom two things happen; First, water within the plant decreases, causing water stress and triggering the stomates to close; Second, plant respiration increases, causing an increase of CO2 concentration within the leaf and thus stomatal closing again.

Lastly, remember that when growroom temperatures climb out of the proper temperature range, most growroom cooling systems kick in, many of which rely on exhaust systems to remove the heat from the garden. This will subsequently (and hopefully) shut off the garden’s CO2 infusion system and then vent out the heat along with the CO2 enriched atmosphere of the garden – a negative for the plants and a big waste of CO2. Keeping your garden temperatures at optimal levels (68 – 72 F) is essential when using CO2.





1,000-watt HID

4 feet from canopy


1,000 lumens/ sq. ft.

3 feet from canopy

500 PPM

1,250 lumens/ sq. ft.

2 feet from canopy

1,000 PPM

2,500 lumens/ sq. ft.

1 foot from canopy

2,000 PPM

5,000 lumens/ sq. ft.


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