MadSci Network: Botany

Re: Why does a greenhouse make plants grow quicker

Date: Fri Nov 9 20:27:59 2001
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Botany
ID: 1005268723.Bt

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 

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. 


The Greenhouse Effect

Greenhouse Effect Project

Greenhouse Effect

Mastalerz, J.W. 1977. The Greenhouse Environment. New York: Wiley.

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