Carbon dioxide (CO2) supplementation, also known as CO2 enrichment, is a standard practice in controlled environment agriculture (CEA) to optimize crop production. By increasing the concentration of this atmospheric gas, cultivators can push past the natural limitations of the outdoor environment. This technique is employed during the flowering phase to maximize the plant’s ability to generate biomass and complex structures that contribute to final yield.
The Biological Mechanism of CO2 Enrichment
Plants rely on carbon dioxide to perform photosynthesis, the process that converts light energy and water into glucose, the sugar that fuels cellular growth. Ambient atmospheric CO2 levels are typically around 400 parts per million (ppm), but plants can utilize significantly higher concentrations. In a sealed grow environment, CO2 is often raised to a range of 1000 to 1500 ppm to achieve a higher rate of carbon assimilation.
Elevating the CO2 concentration allows the plant’s stomata, the microscopic pores on the leaves, to partially close while still absorbing enough carbon. This partial closure reduces water loss through transpiration, improving the plant’s overall water use efficiency. The higher carbon availability also enables the plant to utilize more intense light, raising the light saturation point and accelerating the growth cycle. This increased metabolic activity leads to faster development and a larger final harvest.
Why CO2 Efficiency Declines Near Harvest
As the flowering cycle nears its conclusion, the plant undergoes a programmed biological aging process known as senescence. During this final phase, the plant’s metabolic priorities shift away from building new vegetative mass. The overall rate of photosynthesis slows, meaning the plant’s capacity to utilize high levels of supplemental CO2 diminishes.
The plant redirects its energy and stored nutrients, particularly nitrogen, from the leaves to the developing flowers and reproductive structures. This resource mobilization supports the final stages of ripening and the production of secondary metabolites like terpenes and cannabinoids. Since the plant is no longer focused on structural expansion, providing extra CO2 does not translate into proportional gains, making continued supplementation wasteful. Furthermore, maintaining high CO2 levels can inhibit the production of ethylene, a naturally occurring plant hormone that aids in the final ripening process.
Identifying the Optimal CO2 Cutoff Point
The ideal time to stop CO2 supplementation is when the plant’s physiology signals that it is fully dedicated to ripening and no longer prioritizing structural growth. This practical cutoff point is typically in the final seven to fourteen days before the planned harvest date. Stopping enrichment should align with the start of the final nutrient reduction phase, often called flushing.
Visual indicators are the most reliable method for determining this window, such as observing the plant’s trichomes, the tiny resin glands on the flowers. When the trichomes begin to transition from fully clear to a cloudy or milky appearance, the plant is signaling its peak production phase and the start of the final ripening. A further indicator is the browning or recession of the flower’s white pistil hairs, which indicates that pollination receptivity has ended and the flower is maturing.
At this point, CO2 levels should be reduced back to ambient atmospheric concentrations, generally between 400 and 500 ppm. Ceasing supplementation saves money on CO2 costs and electricity, as the equipment is no longer needed to maintain high concentrations. Additionally, reducing the CO2 level is a safety measure, as the gas is heavier than air and can accumulate in the lower parts of the grow room, posing a risk to workers during final plant handling.