by Kim Dieter

Created on: April 09, 2010

A greenhouse is an ideal location to start seeds before the normal growing season, protect plants during cold weather or grow plants that cannot be normally grown outside.  Greenhouses are designed to keep plants at the optimal growing temperature.  But they can become too cold or too hot to produce healthy plants. Day length and humidity are also critical for the production of some greenhouse plants.  It is important to consider how to control the greenhouse climate to maintain optimal greenhouse temperatures, light and humidity.

For maximum growth, many plants prefer a daytime temperature 10 to 15 degrees warmer than the nighttime temperature.  Peppers, for example, prefer a 70 to 80 degree daytime temperature and a 60 to 70 degree nighttime temperature.  When the temperature gets too hot or too cold, normal plant growth is reduced or stops.  For peppers, normal growth is reduced when the temperature drops below about 50 degrees or rises above 90 to 95 degrees.

 Greenhouses are enclosed spaces often constructed of wood, aluminum or galvanized steel and covered with glass, fiberglass panels or double-wall plastic.  They can get very hot in warm weather and cold in cool weather.

The site where the greenhouse is constructed is important.  The greenhouse should be located on the south or southeast side of buildings or shade trees.  Avoid locating the greenhouse on the north side. The greenhouse should have full sun throughout the morning at a minimum. Greenhouses often require a heating system and ventilation or cooling system.

 It is important to consider the time of year a greenhouse will be used.  If the greenhouse will be used to start plants in the late winter and early spring, a heating system will be required. There are a variety of electric, natural gas, propane and other greenhouse heaters available.  However, an exhaust fan with shutters may be the only requirement for cooling and ventilation because the outside temperature is low.

If the greenhouse will be used during the hot summer months, an evaporative cooling system is a must.  Shade cloth to cover the greenhouse may also be required to keep the greenhouse temperature below 90 degrees.

The heating and cooling systems should be connected to waterproof thermostats. The temperature will be automatically maintained at optimal levels.

 Day-length is also a consideration for greenhouse plants.  Some plants such as poinsettia and chrysanthemums are short-day plants. They flower when they receive less than 12 hours of light each day.  The opposite is true for long-day plants such as pansies and dahlias.  They flower when the day length is greater than 12 hours.  Artificial light should be provided if necessary to maintain the optimum light requirements for long-day plants. Or, the greenhouse can be covered for part of the day to accommodate short-day plants.

 The humidity inside the greenhouse must match the requirements of the growing plants.  A psychromometer or similar device can easily measure the level of humidity in the greenhouse.  Desert-loving plants like cactus require much lower humidity than some tropical plants. Sometimes it is difficult to adjust the humidity in a smaller greenhouse.  Therefore, it is critical to select plants that grow well in the humidity present in the greenhouse.

With careful planning, the greenhouse climate can be successfully managed to produce healthy plants.

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