http://www.organicgardening.com/feature/0,7518,s1-4-76-1295,00.html?trafficsource=OGNews_2007_11_06snip...
answered by Willi Evans Galloway
In a nutshell: Shade trees, such as maples and oaks, act like natural air conditioners if you plant them on the south and southwestern sides of your house.
The whole story: If your home feels more like a greenhouse in summer, it's because roofs absorb solar energy, paved surfaces reflect it, and windows let in hot sunlight. Trees come to the rescue by shading your home and reducing air temperatures with evapotranspiration—a process in which the tree releases water vapor that lowers ambient temperatures.
The U.S. Department of Energy reports that carefully sited trees can cut the average household's energy consumption by 25 percent. To get a tree's maximum cooling benefit, you must plant it centrally and to the south of the house, or in the southwest corner. In this location, a deciduous tree will shade out hot afternoon sun in the summer, while still allowing light to warm the house in winter. You can plant a group of small trees—those that grow up to 25 feet tall—such as crape myrtles or flowering dogwoods 6 feet from your home, but the size and fine texture of the trees lets a lot of sunlight through.
