|MadSci Network: Botany|
Plants grow well in greenhouses because they typically get much more care than field grown plants, including more frequent irrigation, fertilization, pest control, pruning, staking, and sterilized soil. Potted plants can also be respaced as they get crowded and shade each other to optimize light for photosynthesis. Greenhouse production is very labor intensive but is justified because the plants have a high value. Greenhouses also provide protection from damaging winds, rain, snow, and hail. The greenhouse temperature is often much closer to optimum for plant growth because greenhouses can be heated or cooled. The humidity in a greenhouse is often higher than outdoors, which can be beneficial to plants by reducing water stress. Electric lights are sometimes used in greenhouses to increase photosynthesis. If the outdoor environment for plants is optimum for plant growth, plants may grow faster outdoors than in a greenhouse because as much as half of the sunlight is blocked by greenhouse glazing and structural supports. Greenhouses get hot for two reasons. The main reason is because greenhouses are an enclosed space, and there is little air exchange unless they are vented. Light heats air inside the greenhouse, and the heated air is trapped in the greenhouse, causing the temperature to rise. The second reason is the so- called "Greenhouse Effect", which occurs because short wavelength visible light easily passes through the greenhouse glass but then loses some energy and heats the air or items inside the greenhouse and becomes longer wavelength infrared waves, or heat. The infra-red waves cannot as easily pass out again through the glass. The Greenhouse Effect is used most frequently to describe the heating phenomenon that occurs on a global scale, only with greenhouse gases instead of glass. Research (Mastalerz, 1977) indicates that only 22% of the greenhouse temperature increase is due to the Greenhouse Effect of the glass blocking infra-red waves from escaping. The rest, 78%, is due to the greenhouse being an enclosed space. Greenhouses covered with clear polyethylene plastic still get hot inside on sunny days but have no Greenhouse Effect because polyethylene allows infra-red waves to pass freely. References The Greenhouse Effect Greenhouse Effect Project Greenhouse Effect Mastalerz, J.W. 1977. The Greenhouse Environment. New York: Wiley.
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