Carbon dioxide (CO2) enrichment is the practice of increasing the concentration of this gas within a greenhouse to enhance plant growth and yield. The Earth’s atmosphere typically contains about 400 parts per million (ppm) of CO2. In a sealed greenhouse, actively photosynthesizing plants can rapidly consume available CO2 during peak daylight hours, causing levels to drop dramatically, sometimes as low as 150 to 200 ppm. This depletion limits plant development, even when conditions like light, water, and nutrients are optimal. Supplemental CO2 is introduced to overcome this deficiency and increase the rate at which plants convert light energy into biomass.
The Photosynthesis Link
Photosynthesis is the fundamental process where plants use light energy to convert water and CO2 into sugars for growth, releasing oxygen as a byproduct. Plants absorb CO2 through tiny pores on their leaves called stomata. Raising the CO2 concentration above ambient atmospheric levels accelerates this conversion process.
Elevated CO2 levels increase the efficiency of the primary photosynthetic enzyme, RuBisCO, allowing the plant to produce carbohydrates more quickly. This leads to faster growth rates, larger leaves, stronger stems, and higher yields.
The positive effect of CO2 enrichment continues only up to the CO2 saturation point, which is typically between 1,000 and 1,300 ppm for most greenhouse crops. Concentrations exceeding this range offer little additional benefit and may sometimes hinder plant growth. Maximum benefit is achieved when enrichment is paired with high light levels, as supplemental CO2 increases the light intensity a plant can utilize.
Practical Methods for CO2 Generation
Three primary methods deliver supplemental CO2 into a greenhouse environment, suited for different scales of operation.
Combustion Sources
The most common method for large commercial operations involves combustion sources, such as CO2 generators that burn natural gas or propane. These specialized burners produce CO2 and water vapor as byproducts of complete combustion. Combustion systems require a continuous fuel supply and proper venting to ensure complete burning, which prevents the production of harmful carbon monoxide. The heat generated by these burners can also provide the secondary benefit of warming the greenhouse.
Compressed Gas Injection
Compressed gas injection uses liquid CO2 stored in pressurized tanks or cylinders. The liquid is vaporized and released as a pure gas into the greenhouse through regulators and tubing. This is considered the cleanest and most precise method because it introduces only CO2 without heat or combustion byproducts. While highly controllable and suitable for all greenhouse sizes, it requires a higher initial investment for the storage and distribution infrastructure.
Organic and Fermentation Methods
For small-scale or hobby growers, organic and fermentation methods offer a less expensive alternative. These methods include using dry ice, composting organic matter, or fermenting a sugar and yeast solution. However, these methods are generally less reliable and provide poor control over the CO2 concentration released. Dry ice, which is solid CO2, sublimates directly into gas but is best suited for very small spaces or short-term use. These low-cost options are not scalable for large commercial production.
Determining Optimal Concentration and Timing
The efficiency of CO2 enrichment depends on maintaining the correct concentration and timing the application to coincide with plant activity. The goal is to raise the CO2 concentration above the ambient 400 ppm to a target range, typically between 1,000 and 1,500 ppm. Many crops show optimal performance around 1,200 ppm, though the ideal level varies by plant species and growth stage.
Timing and Temperature
CO2 application timing is entirely dependent on light, as plants only use CO2 during photosynthesis. Enrichment should begin shortly after sunrise, once plants are actively photosynthesizing, and cease before sunset. Applying CO2 at night is wasteful, as plants switch to respiration. Furthermore, higher temperatures are required for plants to fully utilize the additional CO2. Supplemental CO2 increases the optimum temperature for maximum photosynthesis, allowing growers to operate warmer, which benefits tropical and heat-loving crops.
Managing Ventilation
A significant operational challenge is managing ventilation, which is necessary for temperature control but causes CO2 to escape. Enrichment systems must be automatically shut off when vents or cooling fans are activated to prevent the immediate loss of the supplemental gas. Sophisticated climate control systems integrate CO2 dosing with light, temperature, and ventilation status to maximize efficiency and minimize waste.
Safety Protocols and Monitoring
While CO2 benefits plants, elevated concentrations pose a health risk to humans, acting as an asphyxiant by displacing oxygen. Levels above 5,000 ppm can cause dizziness and lack of coordination, making continuous monitoring necessary. Greenhouses utilizing enrichment must install reliable CO2 sensors and alarm systems that provide real-time data and alert personnel to unsafe conditions.
These monitoring devices should link to the environmental control system to automatically shut off the CO2 source if levels exceed a safe human threshold. Combustion systems carry the additional risk of incomplete burning, which produces the toxic, odorless gas carbon monoxide. Therefore, CO2 generators must be paired with both CO2 and carbon monoxide monitors to ensure safe operation.
Safety training is paramount, ensuring all personnel understand the potential dangers, recognize alarm signals, and know proper ventilation procedures. Adequate ventilation must be available to rapidly reduce CO2 levels if an alarm is triggered or before staff enter for extended periods. Following these protocols protects workers and prevents excessive CO2 levels, which can also harm crop health.