Section 1: Introduction

The greenhouse atmosphere, or greenhouse air quality, can impact many aspects of plant growth and crop quality. The degree of control that can be exercised over the greenhouse atmosphere will at least partially depend upon the type of greenhouse structure being used and the technology available. There are three basic areas that relate to the atmosphere of the greenhouse that should be considered: carbon dioxide, humidity and various types of air pollutants.

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.

Because of the positive responses to increased CO₂ concentrations, some greenhouse operations have found it beneficial 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.                            

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.

Section 3: Humidity

Because of evaporation of water from floors, root substrates and other surfaces as well as transpiration (loss of water from plant leaves), the relative humidity in a greenhouse is often high, especially when vents are closed. High humidity promotes the development of certain diseases (i.e. black spot, powdery mildew) as well as various physiological abnormalities (i.e. leaf edge burn in poinsettia and blossom end rot of tomatoes caused by a calcium deficiency induced by high relative humidity) in some greenhouse crops. Additionally, high humidity can increase condensation on the inside of the glazing, thus reducing light levels and causing water to drip onto plants.

During the summer, vents are usually open and the ambient relative humidity out-of-doors is the humidity at which the greenhouse will be maintained (although the relative humidity in the greenhouse may still be somewhat higher than that outside due to evapotranspiration). However, during cool months when vents are closed, very high relative humidities can occur inside of the greenhouse. To control the relative humidity, greenhouse managers may vent the warm saturated air out of the greenhouse. Additionally, horizontal airflow fans may be used to circulate air within the greenhouse which helps to reduce the effective relative humidity experienced by the plant by reducing the boundary layer around the plant surfaces.

In some cases, such as propagation houses and seed germination chambers, it may be desirable to increase the relative humidity level. For propagation, mist, irrigation booms or fog systems (see the "Irrigation and Water Quality" learning unit for more information) are most often used to increase the relative humidity and reduce water loss from cuttings by transpiration. If seed are being germinated, fog or a fine mist may be used. Large water droplets should be avoided as they result in splashing and may displace seed. In germination chambers, fog systems are usually used to maintain a 100% relative humidity level without actually applying additional water to the root substrate in the plug trays. This allows for a more even moisture level to be maintained and reduces the amount of free water that accumulates of the floor.

Section 4: Air Pollutants

Various air pollutants or potentially harmful compounds can occur in the greenhouse atmosphere. Carbon monoxide (CO) is dangerous to people. It may be generated by malfunctioning heaters, vehicles, arc welders other combustion engine machinery. Unit heaters without internal heat exchangers should be avoided as they may emit CO into the greenhouse if not functioning properly. Additionally, poorly or improperly maintained and vented unit heaters may result in CO entering the greenhouse. A CO concentration of 50 ppm is generally considered to be the upper limit for human safety (the OSHA PEL limit).

In addition to generating CO, ethylene may be generated by such devices as malfunctioning heaters, vehicles, arc welders other combustion engine machinery. Ethylene (C₂H₄) is a gas that can be highly injurious to plants. Very low concentrations (as low as 0.05 ppm) of ethylene can cause plant damage. Typical plant symptoms of exposure to ethylene include epinasty (malformed leaves that curl or corkscrew often in a downward direction), "sleepy" flowers (flowers appear wilted and curled), and abscission or abortion of flowers and fruits. Sources of ethylene should be avoid and excluded from the greenhouse environment, and unit heaters should be checked periodically to be sure that they are functioning correctly (follow the manufacturer’s recommendations on proper maintenance and operation) and the exhaust fumes properly vented to outside of the greenhouse. Ripening fruits and vegetables also produce ethylene so these products should not be stored in coolers, growth chambers or other enclosed spaces with plant materials.

If CO₂ burners are used in greenhouses, low sulfur fuels should be used to avoid producing injurious levels of SO₂.

Herbicides can be damaging to plants even at very low concentrations. Caution should be taken to insure that herbicides applied out-of-doors do not drift (and are not pulled in by fans) into the greenhouse. Some herbicides are labeled for greenhouse use, and herbicide labels should always be followed. However, even when used in greenhouse according to label directions, problems can occur. For example, if herbicides are sprayed onto active heating pipes, the high temperature can cause volatilization of phytotoxic components even though this would not occur under normal application conditions.

Numerous chemicals including paints and cleaning materials may release potentially damaging volatile chemicals and should be used in or around greenhouses with caution.

© M.R. Evans, 2008, 2009, 2011, 2014