The US Department of Energy estimates that trees and shrubs that are well selected, well placed and well planted can reduce the energy required to heat and cool a home by up to 25%. This may result in a $100 - $250 saving in heating and cooling costs for the average household and investment in a well designed landscape usually pays for itself within 8 years. These energy savings result from evaporative cooling by leaves, reduction of solar gain by summer shading and from reduction in winter heat loss due to lower wind velocity. Although the principles involved in landscaping for energy efficiency are simple, selection and placement of trees and shrubs can make a big difference in the effectiveness of the planting.
While each situation is unique, the best approach for shading a home is to plant large, spreading shade trees to the west and southwest of the building. Trees should be tall enough to cast a shadow across the roof, siding and windows on the west and south of the house in the afternoon. Since the sun June through September is 15 to 40 degrees south of directly overhead, trees planted off the southwest corner will shade the south side of the house in the afternoon (sun angles, shadow length). However, it is best to avoid planting large trees closer than about 30 feet from the house. Also, be certain not to plant trees taller than 20 feet within 35 feet of a power line. Always plant deciduous shade trees, especially on the south side of the house, since they will drop their leaves, allowing for solar gain by window on the south and west sides.
In situations where there is limited space on the west side of the house, it may be possible to select an upright or “columnar” tree. Very upright cultivars of maple, European hornbeam (hornbeam) and hybrid oak can be planted in a row or group to provide shade. In some cases, a trellis covered with vines can do an excellent job of shading where space is tight. Again, make certain that the plants chosen will lose their leaves in winter. Some trees, like pin oak, are notorious for hanging on to their brown leaves all winter.
According to the laws of physics, shading the outdoor air conditioning compressor unit should increase its efficiency. In fact, tests have shown 3-10% gains in efficiency are possible, but only if plants do not interfere with air flow. Foliage that is too close to the sides of the unit will make the fan work harder. Blocking airflow out of the top of the unit will direct hot air back toward the intake, reducing efficiency.
Trees and shrubs can be very effective at reducing the wind velocity near the home, leading to significant savings in heating costs. When a curious rural Colorado resident tracked natural gas usage for 20 years after installing a windbreak, he found that the windbreak reduced his gas usage by about 40% when fully developed. However, just as with shade plantings, tree and shrub selection and placement can have a big influence on windbreak effectiveness. The objective of a planting a windbreak is not to “block” the wind, but to re-direct it away from the building. The most effective windbreak is one using several rows of evergreen and deciduous trees and shrubs of different heights (wb design). Using this approach, wind velocity is reduced to a distance from the trees equivalent to 15 times the height of the tallest trees. Wind velocity 200 feet down wind from 25 foot trees will be about 25% of the velocity on the windward side. This can significantly reduce heat loss of from a building from air leaks. Windbreaks planted to the north and west of the home are most effective, since this is the direction from which the prevailing wind comes in winter. Where space is limited, one or two (staggered) rows of evergreen tree will reduce wind velocity considerably. When using this approach, do not place the windbreak closer than 75 feet from the house. Otherwise, the evergreens may reduce solar gain to the house.
Of course, trees will have no effect on energy usage if they don’t grow. A pin oak planted in a soil with a pH above 7.0 will never shade your roof. A group of white pines planted in a poorly drained low spot will never develop into an effective windbreak. Look around your neighborhood and see what kinds of trees are growing the best. In sites with poor soil, species such as eastern redcedar, hackberry and black locust that are often considered low-class trees, may be the most likely to develop into a functional windbreak.
REVISED: September 29, 2015