Carbon dioxide (CO2) enrichment is a technique used in controlled environment agriculture to boost plant productivity. Supplementing the naturally occurring CO2 levels in a grow room accelerates photosynthesis, the process plants use to convert light energy, water, and CO2 into growth sugars. While atmospheric CO2 is typically around 400 parts per million (ppm), increasing this level provides the plant with an abundance of a primary building block. This supplementation results in a significant increase in growth rate, faster maturity, and greater yields.
Essential Environmental Prerequisites
Adding CO2 is only effective when several environmental factors are optimized, allowing the plant to fully utilize the gas. The most significant factor is the intensity of the light provided to the plants. Plants require high levels of light energy to process the increased carbon dioxide, meaning CO2 enrichment should only be considered when using high-output lighting systems.
Light intensity is measured by Photosynthetic Photon Flux Density (PPFD), which quantifies the amount of photosynthetically active radiation reaching the plant canopy. For CO2 to be beneficial, the PPFD should be above 700 micromoles per square meter per second (\(\mu\text{mol}/m^2/s\)). A useful guideline for maximizing efficiency is to match the PPFD value to the CO2 concentration in parts per million. For instance, a CO2 level of 1200 ppm requires light intensity of at least 1200 \(\mu\text{mol}/m^2/s\) to ensure the plant can process the available gas.
Temperature management must be adjusted when CO2 is supplemented because the elevated gas allows the plant to function efficiently at warmer temperatures. In a standard environment, optimal temperatures usually range between 68–78°F (20–26°C), but with CO2 enrichment, the ideal range shifts higher to approximately 75–88°F (24–31°C). This higher temperature is necessary to increase the plant’s metabolic rate, which drives the faster gas exchange needed for the boosted photosynthesis.
Grow rooms utilizing CO2 enrichment typically operate as sealed environments to prevent the gas from escaping. This sealed nature necessitates careful management of humidity and air circulation. While the environment must be sealed to maintain elevated CO2 levels, occasional ventilation is needed to prevent excessive humidity buildup and disease. Proper air movement within the room is also necessary to ensure the CO2 is evenly distributed across the entire plant canopy for uniform absorption.
Timing Based on Plant Growth Cycle
The timing of CO2 supplementation must be precisely aligned with the plant’s biological stage and daily light schedule to realize maximum benefit. You should not begin supplementing CO2 when plants are in the seedling stage or immediately after transplanting. Young plants are too small and delicate to utilize the added gas, and the ambient CO2 level of 400 ppm is sufficient for their needs. Initiation should occur once the plant is established and has entered the active, mid-vegetative growth phase, typically around the second or third week.
The vegetative phase represents the primary period where CO2 enrichment provides the greatest advantage. During this stage, the plant is focused on building significant structural mass, including stems and leaves, making it highly responsive to the boosted photosynthetic rate. Maintaining CO2 levels between 800 and 1200 ppm throughout the vegetative cycle will support rapid foliage and root development. This accelerated growth allows the cultivator to move the plant into the reproductive phase sooner, shortening the overall cycle.
Supplementation should continue into the early and mid-flowering stages, as bud production is an energy-intensive process that benefits from enhanced photosynthesis. During the peak flowering period, plants can effectively utilize CO2 levels between 1000 and 1500 ppm, supporting the development of larger, denser flowers. However, the benefits of enrichment begin to diminish significantly in the final weeks before harvest.
It is recommended to stop or reduce CO2 supplementation during the final two to three weeks of the plant’s cycle. Lowering the CO2 concentration back toward ambient levels (400–500 ppm) encourages the plant to focus energy on secondary metabolite production, contributing to improved quality. This cessation also prevents providing CO2 when the plant’s demand has peaked and is beginning to decline.
It is necessary to turn off the CO2 supply when the grow room lights are off, as enriching the room during the dark cycle is inefficient and wasteful. Plants only perform photosynthesis when light is available; at night, they switch to respiration, absorbing oxygen and releasing CO2. A functioning CO2 controller should be synchronized with the light timer to automatically stop the gas flow when the lights shut down.
Safe Concentration Levels and Monitoring
The goal of CO2 enrichment is to maintain a concentration that maximizes plant growth without reaching levels that are unsafe for the grower. The standard target range for optimal plant benefit is between 1000 and 1500 ppm, which is 2.5 to 3.75 times higher than the natural atmospheric level. Increasing the concentration above 1500 ppm provides diminishing returns and is considered a wasteful application of resources.
Accurate measurement is required to maintain this narrow range, which involves using specialized monitoring tools. Cultivators rely on CO2 controllers equipped with Non-Dispersive Infrared (NDIR) sensors to constantly track the gas concentration. These systems automatically introduce CO2 when the level drops below the set point and stop the flow when the target is reached, ensuring a steady, consistent environment.
Safety is a consideration, as high CO2 concentrations can pose a risk to human health. While plants thrive in the 1000 to 1500 ppm range, the maximum concentration considered safe for prolonged human exposure is 5000 ppm. Cultivators should install an alarm system and ventilate the grow room thoroughly before spending any significant amount of time working inside the enriched environment.