Never before has the demand for energy been as high -- and never before have homeowners become so increasingly aware of the energy savings possible with landscaping. Although it is not possible to control temperature, wind, and other natural elements, certain landscape practices can help modify the climate in and around the home.
By placing trees, shrubs, vines and landscape structures properly, homeowners can reduce the energy required to keep homes comfortable during winter and summer. Along with the reduction of energy bills, a well-planned landscape adds beauty, interest and increased property values.
Although homeowners have intuitively used landscaping to save energy for many years, we are only beginning to realize the magnitude of the savings possible. According to one government study, winter heating bills may be reduced by as much as 15 percent, while summer cooling energy needs may be cut by as much as 50 percent.
Houses gain or lose heat in 3 basic ways:
- air infiltration - passage of air through cracks and around doors or through open windows and doors. The average home loses 20-30% of heat in winter by air infiltration;
- heat conduction - conduction of heat through materials of which the house is built. Controlling the temperature difference and air movement between inner and outer surfaces of walls, floors and ceilings is the best opportunity for reducing heat conduction. Heat conduction represents up to 50% or more of the total heat exchange between a home and the outside environment;
- solar radiation - heat is transmitted into homes by penetration of the sun's rays. Up to 90% will be transmitted into the living area if rays are received perpendicular to a single pane. Sunlight will be increasingly reflected by the glass as the sunlight departs from the perpendicular.
The role of landscape vegetation in conserving energy varies with the different microclimates across North Carolina. In the cooler, northwest areas of North Carolina, where enormous amounts of energy are consumed in winter heating, control of air infiltration is paramount. Hotter southeastern areas place more emphasis on use of shade to control heat conduction and reduce the need for summer air-conditioning. Three basic landscape applications which have proven to save energy are: (1) the use of shade trees, (2) windbreaks, and (3) the use of foundation plants.
Trees can reduce summer temperatures significantly. Shading the roof of a house from the afternoon sun by large trees can reduce temperatures inside the home by as much as 8 to 10°F.
Deciduous trees provide summer shade, then drop their leaves in the fall. This allows the warmth of the sun to filter through their bare branches in winter and helps warm the home. If a home can be situated to take advantage of shade from existing trees on southeast and west exposures, energy expended to cool the house can be reduced.
If there are no existing trees, the owner can select and place trees that ultimately will provide shade. The temptation is to plant the fastest growing species available. However, this is usually a poor choice for several reasons. Trees that grow at more moderate rates usually live longer, are less likely to break in wind and ice storms, and are often more resistant to insects and diseases.
A carefully selected and planted tree with a moderate growth rate often will respond to good care by increasing its rate of growth. Recommended shade trees for North Carolina would include: Red Maple, Sugar Maple, Pecan, Birch, White Ash, Ginkgo, Honeylocust, Sweetgum, Tulip Poplar, Blackgum, Sycamore, White Oak, Red Oak, Willow Oak, Water Oak, Bald Cypress, Linden, Zelkova, River Birch, and Hickory.
Smaller trees such as Crape Myrtles and Dogwoods can be planted closer to the house and used for shading walls and window areas. Since they are deciduous, they will provide shade during the summer and allow light and sun to penetrate during the winter season (see HIL-621, The Use of Small and Intermediate Size Trees in the Landscape).
Always remember proportions. Ask the nurseryman or county agent how fast and how large a certain tree will grow.
Another way to reduce energy consumption with trees and shrubs is to provide shade for the outside protection of a split system air conditioner. A study by the American Refrigeration Institute shows that shading of this type can reduce the temperature inside the home as much as 3°F. However, shrubs planted near the compressor should not obstruct the air flow or access for service. In addition to reducing energy consumption, screening outdoor air conditioning equipment with plantings enhances the esthetic value of the home.
In addition to shading roof areas, plants can protect walls from heat and cold. Vines, shrubs and certain trees can be used as espaliers (plants trained to grow flat against walls). The foliage cover insulates the wall against summer heat and cold winter winds. Trees, shrubs and vines can be highly effective in reducing noise and dust pollution.
Arbors and slatted wooden overhead structures can be effective either attached or adjacent to the home or farther out in the landscape. If adjacent to the home, they provide the bonus of shading walls and windows, thus reducing heat and glare and providing cool, restful sitting and viewing areas. Carolina Jasmine, ivy, wisteria or grape vines are popular vines which are well adapted to most of the state.
Although hedges have been utilized for many years, their value has increased with the advent of higher fuel costs. Winter winds in North Carolina usually blow from the northwest and accelerate the rate of air exchange between a house and the outdoor environment. Savings of up to 23 percent have been recorded in comparing completely exposed homes and a house landscaped to minimize air infiltration. Summer winds normally blow from the south or southwest with generally positive effects on human comfort. Tall trees on the south and west can reduce temperature while allowing breeze to pass beneath and through the foliage canopy.
Windbreaks obstruct and redirect the flow of wind. As wind strikes an obstruction, it can move over, around or through it. The extent of protection on the leeward side is related to the height and length of the windbreak. Impenetrable windbreaks create a strong vacuum on the protected or leeward side, which reduces the protection. Windbreaks composed of living plants allow some of the wind to penetrate, which makes them more effective.
Several evergreen trees which grow into large windbreaks and also screen objectionable views are: Hemlock, Cedar, Southern Magnolia, White Pine, Loquat, and Deodar Cedar.
Most homeowners need to consider the size requirements of the living hedge. Six to twelve feet evergreen shrubs for good windbreaks are: Camellia, Sasanqua, Cleyera, Elaeagnus, Holly varieties, Ligustrum, Waxmyrtle, Oleander, Osmanthus, Photinia, Pittosporum, and Viburnum. If space is limited for a hedge, consider some type of construction fence or wall.
In addition to traditional windbreaks, shrubs can also be used closer to the home for winter protection. This is more practical for small areas and subdivision lots where space does not allow the use of conventional windbreaks. For this type of protection, a combination of dense evergreen plants and groundcovers are most appropriate. They should be planted close enough to eventually form a solid wall and far enough away from the house (about 4 to 5 feet, minimum) to create a dead air space. This relatively still or dead air has much less cooling power than moving air which can decrease the loss of that through the walls.
Evergreen shrubs which should be considered as foundation plantings would include many dwarf or slow-growing types. This would include Dwarf Hollies, Boxwoods or Junipers.
Good landscaping practices offer one of the most practical methods of reducing energy consumption in homes. When the homeowner considers the added benefits of the increased real estate value and more attractive homes and communities, the investment becomes an even greater bargain.
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Publication date: Sept. 30, 1993