University of Vermont Extension
OH 47

Landscape to Conserve Energy

By Leonard P. Perry, Extension Professor

Plants properly placed in the landscape planting offer many functions ranging from serving as buffers of noise from highways, to cleaning the air from dust and dirt. However, a function often overlooked is the role plants may serve in decreasing energy required to keep a home more comfortable in both summer and winter.

Effects of shade:

Ornamental plants can provide cooling shade for walls, windows, and the roof of a house. The proper type of plant and its correct placement will also allow the rays of the sun to warm a house during winter.

A tree or trees located on the west side of a house or building shields it from the hot afternoon sun in summer while trees to the east cast shade in the morning, although the latter is usually less effective for energy conservation. Deciduous trees, such as maples, oaks and lindens, provide shade in summer and light penetration for greater warmth in winter. Plant medium to large trees 15 to 20 feet from the side of a structure for shading a roof or wall. Maximum shade is obtained if branches extend slightly over the roof; however, leaf problems in the eaves and gutters must be considered.

Shrubs, espaliered plants, and vines grown on a bare wall may act as heat control devices. These plants may insulate walls by absorbing and reflecting the sun's rays before they strike the wall. Roses and certain vines, such as clematis, can be grown on a sturdy trellis to shade windows in summer while permitting sunlight to enter in winter.

Dead air space:

A row of evergreens placed next to a wall creates an area of "dead" air between the plants and the wall. This still or dead air has much less cooling power than moving air. The temperature difference between the inside of a home and the outside still air space is reduced and held relatively constant, which greatly decreases the loss of heat through the walls. In the summer, this still air space also insulates a home from hot air and reduces an air conditioner's load.

Effects of wind:

In landscaping, it is important to consider the direction from which winds blow in various seasons of the year, the effect these winds have on human comfort, and how winds can be controlled by placement of landscape materials. Typical northwesterly winter winds can accelerate the rate of air exchange between a house's exterior and interior environments, resulting in greater fuel consumption. Research has shown a fuel savings of 23 percent between a house landscaped to minimize air infiltration versus a home without such landscaping. It is desirable, where space permits, to create windbreaks that will intercept and redirect winter winds before they reach the house.

Summer winds, while often from the south and southwest, affect human comfort as well. A 9 MPH wind can make an actual air temperature of 85 degrees F feel more like the high 70s, bringing air temperature back into the range of human comfort (68 to 80 degrees F and 30 to 70 percent Relative Humidity). Channeling summer breezes into and through living areas promotes human comfort and reduces the need for air conditioning.

Plant placement for wind control:

As wind strikes a windbreak, it moves over and around the obstruction. This wind direction alteration creates a small area on the windward side of the windbreak and a larger area on its leeward side that is protected from the full force of the wind. To be most effective, windbreaks should be vertical rather than sloping. Space four or five rows of deciduous plants or two to three rows of evergreens for best results. The zone of protection extends leeward for a distance equal to 30 times the windbreak's height. Maximum protection is provided at a distance five to seven times the windbreak's height. Windbreaks are most efficient when their length is 11.5 times greater than their mature height.

Wind channels are designed to promote and guide wind circulation and must be oriented so that this deflection is funneled into a desired area without a substantial decrease of initial velocity. Align plants to the northeast, or plant in a line that curves gradually to the northeast, so that southerly breezes can be captured and directed into an otherwise windless area on the north side of a house. As wind strikes deciduous shade trees, part of the mainstream will be forced under the canopy and through the gap between the lower edge of the canopy and the ground. Such a planting on the southern side of a house would help promote internal air circulation within a house.

The design of a successful wind control device requires an understanding of the physical principles of wind control and how different materials can be used to take advantage of these principles, but it is equally important to understand how the materials will look in relation to the surrounding landscape and how well they will hold up under continued exposure to wind. Landscaping for energy conservation should be considered as one segment of a complete landscape design.

(Adapted from Elton M. Smith, The Ohio State University)

Edited in December 1997, based on material from 1991.

Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. Lawrence K. Forcier, Director, University of Vermont Extension, Burlington, Vermont. University of Vermont Extension and U.S. Department of Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.

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Last reviewed April 24, 1998.