Greenhouse carbon dioxide (CO2) enrichment is a growing technique used to increase plant productivity. More than 90% of the dry matter of a plant is created from the photosynthetic conversion of atmospheric CO2. By manipulating greenhouse CO2 levels, plant growth can be accelerated. Why is that? Well, it is simple math! The primary energy source for a plant is photosynthesis, which follows the following chemistry equation:
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So by giving plants more carbon dioxide, they can produce more plant matter. Doubling CO2 concentration has been experimentally shown to increase total crop yield up to 30% in greenhouse and growth chamber conditions. There is a catch though! Photosynthetic uptake of CO2 and production of plant biomass are temperature and light-dependent, with ~25°C being ideal. So in addition to the plant's potential capacity for CO2, the greenhouse environment can create limiting conditions.

Throughout the day plants use up the CO2 available inside the greenhouse. The air directly around the leaves becomes depleted and needs to be replaced with fresh air. Greenhouse CO2 enrichment systems are sometimes used to increase plant productivity. With good horizontal airflow, less CO2 enrichment is needed (1000 to 1200 ppm). CO2 injectors, CO2 generator, or combustion heaters can be used to increase CO2 levels inside the greenhouse.

CO2 is consumed by plants throughout the day as they conduct photosynthesis. The air directly around the leaves becomes depleted of CO2 and needs to be replaced with fresh air (or have CO2 levels added with an enrichment system). Proper greenhouse ventilation ensures that the depleted air around the plant will be replenished. If CO2 is not replenished, photosynthesis rates in the plant decrease and result in overall decreased plant productivity.

In photosynthesis, the carbon from CO2 is used to create sugars and starches that the plant can store. The oxygen from CO2 is a waste product that must be released at night when pores open in the plant’s leaves. Greenhouse exhaust fans and shutters help release the spent air from the building and replenish it with fresh air.