Carbon
Dioxide
CO2 can be added to the greenhouse in several ways.
Natural gas or propane burners hooked up to sensors can be used to generate
CO2 or flue gases from a hot water boiler burning
natural gas can be captured and recirculated. Venting exhaust gases into
the house is dangerous unless systems, such as carbon monoxide (CO) monitors,
are in place to monitor pollutants. The most expensive, but safest option
in terms of possible air pollution is compressed CO2,
which is unlikely to contain combustion gases as contaminants. See the
air pollution section for a further discussion of combustion gas problems.
Another advantage of compressed CO2 in warm-winter
areas is that it does not add heat or water vapor to the greenhouse.
In northern latitudes, a minimum carbon dioxide concentration of 1000 mmol mol-1 is recommended during the day as long as the greenhouse is not venting. Supplemental carbon dioxide at a concentration up to 400 mmol mol-1 is sometimes applied at low vent rates. In southern latitudes, greenhouses are vented so frequently for most of the year that CO2 enrichment is not common, especially in smaller ranges. In North Carolina, USA, CO2 enriching tomatoes less than 8 hours daily did not significantly increase yield [30].