Insulation helps the interior of a building remain at a relatively stable temperature by keeping air trapped in tiny pockets. These air pockets keep heat trapped on one side of the insulation. In the summer that side is the outside, and in the winter that side is the inside. Proper placement of plants in your landscaping can help your insulation keep heat trapped outside in summer.
How Plants Can Help
Plants can assist your insulation during the summer in one of three ways: First is through creating shade. The plants will shade windows and roofs and keep the sun's heat out of your house. Second is through a mechanism very much like the way insulation works. By creating an area of relatively still air, the plants will keep heat trapped on one side. Third is through transpiration, which is the way that water evaporates from plant leaves. Transpiration causes the air around a plant to become cooler.
Using Shade Trees
Plant shade trees on the east, west and south sides of your home. You will want as large a crown on your shade trees as possible in order to keep shade on your house as long as possible. Plant the trees about ten feet away from your house, which is far enough that your foundation will not be damaged by the roots but still close enough to cast shade on your roof.
Using Vines and Shrubs
Vines and shrubs create air pockets that act like another layer of insulation. The vines and shrubs should ideally be about one foot from the house. This is especially true in moist climates, where growing a vine against the house could lead to moisture collecting and potentially damaging the house.
The "Green Roof"
The "green roof" is an old idea that is coming around again, particularly in Europe. There are two varieties of green roof. One is a park-like atmosphere, including trees, shrubs and paths for people to enjoy and walk around in. This kind of green roof is primarily used by businesses.
The other is more appropriate for homeowners. This type of green roof is more of a rooftop meadow of low-care wildflowers or groundcovers intended primarily to provide environmental benefits rather than for walking in. The green roof makes air pockets within the dirt to help the work of the insulation. Additional benefit can be gained if the plants used have a larger leaf surface.
Using Trees in the Winter
You can also use trees in winter to keep your house warmer. Choose deciduous trees for your shade trees, because after the leaves fall, the winter sunlight will shine through the bare branches. Also, you can use evergreens as a windbreak to keep the wind chill factor from affecting your home and making your furnace work harder. The United States Department of Energy recommends planting windbreak trees "at a distance from your home of two to five times the mature height of the trees."