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.
"If you really want to cut your energy costs, you'll need to plant a larger tree farther out in the yard," says Mike Steede, George County director for the Mississippi State University Extension Service. Medium-size trees, such as Washington hawthorn (Crataegus phaenopyrum) and yellowwood (Cladrastis lutea), which top out at about 40 feet, can be planted 15 to 20 feet from the house and 35 feet apart. Planting at this distance ensures that the tree will shade the home without hanging over it and causing a hazard. Large trees, such as ginkgos, can overwhelm single-story homes, but if you have a two-story home or want to grow large trees (those that reach heights greater than 40 feet), plant them at least 35 feet from the house.
Steede suggests planting a larger, long-lived tree that grows at a moderate rate, such as a willow oak (Quercus phellos) or sugar maple (Acer saccharum), as opposed to a short-lived, fast-growing tree, such as a sycamore (Platanus occidentalis). It's best to choose a tree that's adapted to your area, resistant to local diseases, and long-lived. Check out the Mississippi State University Extension's publication Selecting Landscape Plants (available online at msucares.com) or the National Arbor Day Foundation's Tree Guide (online at arborday.org) for more trees that fit your yard and its conditions.