Carbon dioxide (CO2) enrichment is a technique used in controlled environment agriculture, such as indoor grow rooms, to boost plant growth. Natural air contains about 400 parts per million (PPM) of CO2. For plants grown under high-intensity indoor lighting, this ambient level often limits how fast they convert light energy into biomass. Increasing the concentration of this fundamental gas removes this bottleneck, allowing plants to achieve significantly higher rates of growth and yield. This process requires precise management of other environmental factors like light and temperature to be effective.
The Role of CO2 in Plant Metabolism
Photosynthesis, the process plants use to create food, relies on three main inputs: light, water, and carbon dioxide. The enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) fixes atmospheric CO2 into sugar molecules. When CO2 concentration inside the leaf is low, RuBisCO may mistakenly bind with oxygen instead, initiating a wasteful process called photorespiration.
Photorespiration is a metabolic inefficiency that consumes energy and fixed carbon without producing new growth. Increasing the CO2 concentration provides RuBisCO with a greater abundance of its preferred substrate. This effectively suppresses photorespiration, allowing the plant to commit energy toward productive growth. Elevating the CO2 level accelerates the rate at which the plant accumulates dry mass.
Determining Optimal CO2 Concentration Levels
For most C3 plants, the recommended target range for CO2 enrichment in a sealed grow room is between 1,000 and 1,500 PPM. Maintaining this level leads to a substantial increase in biomass and overall crop yield. This elevated concentration allows the plant to maximize its photosynthetic capacity, especially when provided with intense lighting.
The concept of a “CO2 saturation point” means adding more gas above a certain level provides diminishing returns. Pushing the concentration past 1,500 PPM is not advised because the photosynthetic rate plateaus, wasting resources. Excessively high levels, particularly above 2,000 PPM, can also damage the plant’s leaf tissues over time.
Adjusting Temperature and Light for CO2 Efficiency
CO2 enrichment requires corresponding adjustments to other environmental parameters, primarily light and temperature. When carbon dioxide is abundant, plant enzyme systems operate more effectively at higher temperatures. The optimal leaf temperature for photosynthesis increases because high CO2 concentration suppresses photorespiration.
In a CO2-enriched environment, the ambient temperature must be raised by approximately 5 to 10 degrees Fahrenheit (2 to 5 degrees Celsius) above non-enriched conditions. This means operating in a range of 75°F to 86°F (24°C to 30°C) for peak metabolic activity. Elevated CO2 levels cause the plant’s stomata to partially close, reducing water loss through transpiration. This allows the plant to tolerate the necessary temperature increase without significant heat stress.
The effectiveness of CO2 enrichment is directly proportional to the intensity of light provided. Plants utilize extra carbon dioxide only if they have enough light energy to drive the photosynthetic reaction. Growers must use high-intensity lighting systems, such as powerful LEDs or high-pressure sodium (HPS) fixtures. High CO2 levels become a growth factor only when Photosynthetically Active Radiation (PAR) reaching the plants is very high, often exceeding 1,000 µmol/m²/s.
Practical CO2 Delivery Systems and Safety Protocols
Growers typically use two primary methods to introduce supplemental CO2 into a sealed grow room environment.
Compressed Gas Systems
The first method involves compressed gas systems, which use cylinders of liquid CO2 connected to a regulator and a solenoid valve. This method is clean, precise, and easily controlled by an external environmental controller. It is a popular choice for operations of all sizes.
CO2 Generators
The second common method is a CO2 generator, which burns fuel like natural gas or propane to produce carbon dioxide as a byproduct of combustion. Generators are economical for very large grow spaces and produce heat, which benefits cooler climates. However, they also produce water vapor and, if combustion is incomplete, dangerous byproducts like carbon monoxide, requiring careful monitoring.
Regardless of the source, CO2 delivery must be managed by an environmental controller and monitor to ensure stable levels during the lights-on cycle. CO2 is only consumed by plants during the day, so the system shuts off when the lights are out. Safety monitors and alarms are necessary, as CO2 is an odorless, colorless gas that is heavier than air and can accumulate at dangerous levels. Concentrations above 5,000 PPM are hazardous to human health, acting as an asphyxiant.