The greenhouse atmosphere, or greenhouse air quality, can impact many aspects of plant growth and crop quality. The degree of control over air quality will be at least partially dependent 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 pollutants.

Carbon Dioxide Injection

Carbon dioxide serves as the carbon source for photosynthesis. During photosynthesis carbon dioxide and water are combined (carbon is reduced) to form carbohydrates and oxygen. The outside atmosphere at sea level contains approximately 0.034% or 345 parts-per-million (ppm) carbon dioxide. In enclosed greenhouses, the carbon dioxide level may be significantly lower and may be below what is required to maintain the photosynthetic level at the compensation point. Some plant species have been shown to respond positively (through increased photosynthetic rates) to carbon dioxide levels up to 2000 ppm, and most greenhouse crops respond positively to carbon dioxide levels up to 1500 ppm. Therefore, in certain situations, carbon dioxide may be the limiting factor for photosynthesis.

Because of the positive responses to higher carbon dioxide levels, some greenhouse operations have found it beneficial to inject carbon dioxide into the greenhouse to increase the carbon dioxide concentration. Some of the responses of greenhouse crops to carbon dioxide injection are listed in the table below.

Increased carbon dioxide levels are effective only during the day. However, vents must often be opened during the day. If vents are opened more than 5% of capacity or if exhaust fans are on, it is difficult to maintain increased carbon dioxide levels. Therefore, the period of time over which carbon dioxide 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 carbon dioxide levels, supplemental lighting, increased temperatures (5^oF to 10^oF 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 carbon dioxide, as well as increasing lighting and temperature, increases production costs. The gain in production and quality must offset the increased costs. In other words, many crops will respond positively to carbon dioxide injection, but the response or the level of response must be something for which the consumer will pay.

Carbon dioxide injectors range from $450.00 to over $2,000.00. These systems burn natural gas or kerosene to produce carbon dioxide. If these systems are not maintained and operated properly, they may produce harmful carbon monoxide or ethylene that can be injurious to plants and dangerous for people. Additionally, natural gas used in carbon dioxide 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. Greenhouse operations may also use systems that inject carbon dioxide into the greenhouse using liquefied carbon dioxide. Typically, carbon dioxide levels are increased to 1,000 to 1,500 ppm.

Humidity Control

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, blossom end rot of tomatoes) in some greenhouse crops. Additionally, high humidity can increase condensation on the inside of the glazing, thus reducing light levels and water dripping 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 higher than that outside due to evapotranspiration). However, during cool months when vents are closed and heating is required, very high relative humidities can occur. To control this, growers will periodically increase greenhouse temperatures to saturate the air with water vapor and then vent the warm saturated air out of the greenhouse. Additionally, the use of horizontal airflow fans will help 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 relative humidity level. For propagation, mist or fog systems 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 used. this allows for a more even moisture level to be maintained and reduces the amount of free water that accumulates of the floor.

Pollutants and Toxic Substances

Carbon monoxide is dangerous to people. It is generated by malfunctioning heaters, vehicles, arc welders and other combustion engine machinery. Unit heaters without internal heat exchangers should be avoided as they emit exhaust and carbon monoxide directly into the structure they are designated to heat.

Ethylene is injurious to plants. It can cause epinasty, "sleepy" flowers, and abscission of flowers and fruits. Malfunctioning heaters, arc welders and combustion engines can generate ethylene. Plants are highly sensitive to ethylene, and very low concentrations (as low as 0.05 ppm) can illicit undesirable plant responses.

Herbicides can also be damaging to plants. Caution should be taken to insure that herbicides applied out-of-doors due 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 may 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.

Carbon Cycle Company International

© 2005, M.R. Evans