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Section 2: Carbon Dioxide

Carbon dioxide (CO2) serves as the carbon source for photosynthesis. During photosynthesis, CO2 and water are combined (carbon is reduced) to form carbohydrates and oxygen. Carbohydrates are compounds such as sucrose, fructose and starch. Carbohydrates serve as both energy sources as well as the building blocks of the plant.

Although the outside atmospheric (ambient) CO2 concentration varies by location and season, it is approximately 0.034% or 345 parts-per-million (ppm) at sea level. In enclosed greenhouses, the carbon dioxide level may be significantly lower. This is because plants utilize, and thus deplete the atmosphere of CO2 during the day. In fact, CO2 levels in greenhouses have been reported to be as low as 200 ppm.  Typically, the more tightly closed the greenhouse and the less venting that occurs, the greater is the potential for CO2 levels to drop significantly below the outside ambient level. This is problematic because as CO2 concentration decreases, photosynthesis, and thus plant growth, slows or even ceases. As a result, venting even in the winter, particularly for tightly sealed greenhouses, may be necessary in order to replenish the CO2 level in the greenhouse.

In addition to the fact that plants may absorb CO2 faster than it is replenished from the outside, many plant species have been shown to respond positively (through increased photosynthetic rates) to CO2 levels up to 2000 ppm, and most greenhouse crops respond positively to CO2 levels up to 1500 ppm. This is obviously much higher than even outside ambient CO2 concentrations. Therefore, in many situations in greenhouse environments, CO2 may be the limiting factor for photosynthesis and increasing CO2 concentration to above the outside ambient concentration may result in increased plant growth and productivity. Some common responses of greenhouse crops to increased CO2 concentrations are listed in the table below.

Common Responses of Some Greenhouse Crops to Increased Carbon Dioxide Concentrations




Decreased bud blasting, increased stem length, increased flower weight, increased number of petals, reduced cropping time


Increased number of flowers, increased stem strength, increased flower weight


Thicker stems, increased stem length, reduced cropping time, increased stock production


Increased vegetative growth and increased total fruit weight


Reduced crop time and increased product weight

Because of the positive responses to increased CO2 concentrations, some greenhouse operations have found it beneficial (from plant growth and economic perspectives) to inject CO2 into the greenhouse atmosphere (also referred to as CO2 enrichment) to increase the atmospheric CO2 concentration.

Increased CO2 levels are effective only during the day. However, vents must often be opened during the day (especially during summer and in southern climates). If vents are opened more than 5% of capacity or if exhaust fans are turned on, it is difficult to maintain increased CO2 concentrations. Therefore, the period of time over which CO2 injection can be used effectively may be limited depending upon season, climate, and the production systems being used. Additionally, to gain the maximum effect of elevated CO2 levels, supplemental lighting, increased temperatures (5°F to 10°F higher depending upon crop) and increased fertility levels are often required. The plant cannot fix additional carbon dioxide if other factors such as light are limiting. Injecting CO2, as well as increasing lighting and temperature, increases production costs. The gain in production and quality must offset these increased costs. In other words, many crops may respond positively to CO2 injection, but the response or the level of response must pay for the increased inputs.

There are numerous methods of increasing greenhouse CO2 concentrations in a greenhouse. Carbon dioxide burners are located within the greenhouse and combust natural gas, propane, butane or kerosene to produce CO2. Complete combustion of these hydrocarbon fuels results in the production of CO2 and water. However, if these systems are not maintained and operated properly, they may produce harmful carbon monoxide (CO) or ethylene (C2H4) gases that can be injurious to plants and dangerous for people. Additionally, natural gas used in CO2 burners should contain no more than 0.02% sulfur (w/w) or sulfur dioxide may be produced. Sulfur dioxide combines with water on plant surfaces to form damaging sulfuric acid. Kerosene should contain no more than 0.06% sulfur. Burners used to produce CO2 also will result in the formation of nitrous oxides. High concentrations of nitrous oxides can be injurious to plants. However most researchers have found that if the CO2 level is maintained within acceptable ranges, nitrous oxide concentrations will generally be within acceptable levels or at least the negative impact of the nitrous oxides will be significantly less than the beneficial effects of increased CO2 concentrations.                            

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Some greenhouse operations burn natural gas to operate boilers used for heating. During the day, the boilers are operated and heat water that is stored in tanks for night-time heating. The flue gases are passed through a manifold to separate CO2 from flue gases produced from burning natural gas for heating. This CO2 is then pumped into the greenhouse during the day.

In addition to burning hydrocarbons to produce CO2, greenhouse operations may also use systems that inject carbon dioxide into the greenhouse using compressed liquefied carbon dioxide.

The optimal CO2 concentration will depend upon the crop and cultural conditions. However, typically, CO2 concentrations are increased to 1,000 to 1,500 ppm for most greenhouse crops. If the CO2 concentration is too high, plant damage may occur. For example, above 1500 ppm CO2, damage was reported on cucumber. On gerbera, CO2 concentrations of 1600 ppm caused foliar chlorosis and necrosis and CO2 concentrations of 2600 and 4500 ppm caused leaf dieback. Common CO2 toxicity systems included leaf roll and foliar chlorosis and necrosis.

When deciding on whether or not to inject CO2 into the greenhouse, it is very important to conduct research on the crop(s) of interest to determine the optimal CO2 concentration, the optimal light, temperature and fertility regime, as well what types of plant growth responses might be expected. It is also important to determine the cost of increasing the CO2 concentration including the costs of the required increases in light, temperature and fertility. The increased production, reduced cropping time and/or increased plant quality achieved will need to cover the cost of the increased inputs.

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