There are many questions surrounding carbon dioxide’s effectiveness on improving plant health and increasing yields. I want to take a look at some of those questions and try to clarify a few things.

First and foremost, it is important to start a CO2 discussion with the most important biological function of a plant—photosynthesis. Without this chemical process, plants are unable to provide food for themselves. Many gardeners believe nutrients provide all the food a plant needs to develop. The truth is that nutrients are not consumed as food. Nutrients contain minerals that the plant uses in conjunction with light and CO2 during photosynthesis to produce the complex sugars it needs for energy and growth.

It is the leaves that provide plants with all their food because they turn sunlight into food energy. Chlorophyll makes this energy transformation possible. Leaves also make the oxygen in the air that we breathe. Chlorophyll is a pigment found in the cells of leaves, is formed only in the presence of light and is what makes plants green.

It is contained in the chloroplasts and has the ability to capture light energy. Sunlight shines through the top of the leaf and reaches the next layer of cells. The light energy is trapped by the chlorophyll in the chloroplasts. In the chloroplasts, a process that uses water changes the light energy into a kind of chemical energy. This chemical energy is stored in the chloroplasts. The chloroplasts use the chemical energy to make food.

Air enters the leaf through the stomata and moves into tiny spaces around the food-making cells in the leaf. CO2 from the air passes through the walls and membranes of the cells. CO2 enters the chloroplasts where the previously stored chemical energy converts the CO2 into sugar. Tubes in the plant carry the sugar from the leaf cells to other parts of the plant, such as roots, stems and flowers. Cells in these parts of the plant store some of that sugar.

Does CO2 help with clones and rooting?

An often overlooked and under studied aspect of plant response to CO2 is the below-ground processes. When exposed to increased CO2, roots have been observed to become more numerous, longer, thicker and faster growing in many plant species.

When cloning plants, root growth appears three to five days sooner with CO2 enrichment versus without. Although some things are known about root response to enrichment, much remains to be learned. Nevertheless, it is clear that plant roots, like other parts of the plant, typically do better in enriched air versus ambient air.

What does light have to do with CO2?

Photosynthesis has two parts. The light-dependent reactions and the light-independent ones. The light-dependent part is the use of light to steal electrons from water. This process produces oxygen. The light-independent part is carbon fixation. Plants produce CO2 during respiration when they break down sugars, just as humans do.

They do this day and night, but during photosynthesis they tend to take more out of the air than they put in. They reduce CO2 output during the carbon fixation steps of the light-independent reactions of photosynthesis. Both processes go on all of time, except carbon fixation tends to be more active during the day. Plants only release oxygen during the day since they require light to do it.

Does CO2 really improve yields, or just make for a healthier plant?

The answer is both. The goal of CO2 enrichment is to reduce the time from seedling to harvest and to speed up growth and increase yields. Plants grown with elevated CO2 levels are better able to resist insects and diseases, which makes for healthier plants. For example, in a recent study lettuce was grown in a greenhouse with ambient air versus lettuce grown in a greenhouse with enrichment.

The test showed the lettuce grown in the greenhouse with ambient air was ready to harvest in 59 days. In the greenhouse with CO2 enrichment, the lettuce was harvested in 48 days. By weight, the CO2-enriched greenhouse lettuce weighed 30% more. In studies here at our farm, we have found that by supplying tomato plants with elevated CO2, those plants produced 20% more tomatoes than plants that did not receive the elevated levels. By harvestable weight, the plants receiving more CO2 out-produced those without by 25%.

Tests performed on strawberries showed that strawberries grown with elevated levels of CO2 contained more sugars and physical mass to support a greater number flowering sites. The fact that there were more flowering sites led to a greater number of strawberries being formed, which led to more overall production.

Can you have too much CO2?

Too much CO2 can be detrimental for plants. When levels get too high, the plant’s ability to perform transpiration during photosynthesis is reduced. With lower transpiration rates, fewer nutrients are drawn through the plant, thus less food enters the plant and growth slows down. High CO2 levels can cause necrosis spots to appear on leaves. These dead tissue spots are an invitation for bacteria and mold to appear. The bacteria and mold feed on the dead tissue and can cause plant damage, low yields and in some cases crop failure. It has been shown that CO2 levels around 1,200 to 1,500 ppm provide optimal growth. With levels above this, you are only wasting CO2 and potentially asking for trouble.

Final Thoughts

CO2 is an often overlooked aspect of indoor gardening. While plants can survive without it, you are not maximizing your garden’s potential. Even slight elevations can benefit your garden. With ambient air, CO2 levels hover around 400 ppm. Raising this level to 1,000 to 1,200 ppm will give you greater rewards.

When beginning CO2 enhancement in a garden, the first thing you will notice is your garden is greener. This is proof your garden is benefiting from the CO2 you are providing. Greener plants mean more chlorophyll is present in the leaves and that more photosynthesis is taking place.

There are many options available for CO2 enrichment. Do your research and find the option that works best for you. You and your garden will be better off in doing so.