Energy Saving Landscapes
Trees help people create a nicer environment in which to live. In addition to helping make landscapes visually pleasing, they make our environment cleaner and more comfortable. Trees provide shade in the summer and shelter from harsh winds in winter. Correctly placed shade trees, windbreaks, and foundation plantings can reduce heating and cooling costs by an estimated 25-30%, with some estimates as high as 50%. Tree canopies keep air temperatures at ground level cooler, and when summer temperatures are cooler, fewer air pollutants form.
Home Energy Use
In Minnesota and other northern states, people usually spend about ten times more for heating than for cooling, even if homes have air conditioning, but energy savings in all seasons should be considered.
Winter Heating Considerations
Both the sun and wind affect the temperature of homes in winter. A significant amount of solar energy can be gained from the sun shining through south windows in the winter when the sun is low in the sky. East and west windows can also provide solar energy gain in winter. The solar energy may make up as much as 5-20% of the total energy needed to heat the home. Escaping warm air, along with cold wind entering a home, increase heating costs and account for 25-40% of the heating requirements. The stronger the wind, and the colder the temperatures, the greater the effect.
Summer Cooling Considerations
Very little solar energy enters our homes through the roof and walls because of insulation. Approximately half of unwanted heat in a house in the summer comes from the sun shining through the windows. Because the summer sun is so high in the sky, almost twice as much solar energy enters through the east and west windows as the south windows, especially if there is a roof overhang on the south side of the house.
How Trees Can Help
When planting trees for energy conservation, try to:
- Create windbreaks to block harsh winter winds, but allow cool summer breezes to flow through;
- Increase the tree canopy to cool the surface area;
- Shade east and west windows in the summer, where most solar energy enters the house; and
- Avoid shading south windows in winter, where most solar enters the house.
An area of relatively calm air is formed downwind from the windbreak, a distance of approximately 10 times the height of the windbreak. There can be some wind reduction as far as 25 times the height of the windbreak downwind. To be effective, the windbreak should consist of trees and shrubs that are tall enough, dense enough, and in a long enough line to protect the house. The most efficient windbreaks will consist of at least one row of dense evergreen trees whose branches extend to ground level. Usually the windbreak is planted in rows perpendicular to the wind direction. In most cases, the windbreak will run to the north and west of the building. A windbreak that permits 50-60 percent of the wind to penetrate (such as evergreen trees) is better than a solid barrier (such as a solid fence) because it creates a larger area of protection on the downwind side.
Small residential yards will not have space for large evergreen trees, but the canopy of tall deciduous trees also provides a great deal of shelter. For the greatest effect, mature trees should cover at least half the canopy space. This will provide some shelter from winter winds, and a significant amount of shading from hot summer sun.
Shade trees should be planted due west and east of windows. Shade trees to the west and east will shade the late morning and afternoon sun, which adds the most solar heat to homes in summer. Trees should be planted within 20 feet of windows and should grow to a mature size of at least 10 feet higher than the window.
Trees planted to the south of the home will have an adverse effect on energy savings. In summer the midday sun is high, almost directly overhead. The shadow of a tree will fall directly under the tree, and miss the house, providing no shading. However, in winter, when the sun is at a lower angle, the branches will provide some shading to the house, rather than letting the full solar heating benefits get through.
In addition to shading the house, trees should be planted to provide shade to air conditioners, parking places and paved areas in summer. This will allow the air conditioner to run more efficiently and will prevent heat buildup in cars and on pavement.
The ideal shade trees for energy conservation are those with dense summer foliage and fine twigs in winter. Trees that lose their foliage by the time the heating season begins are best. Mature deciduous trees in summer block 60 to 90% of the sun. In winter, a mature tree's branches and twigs will block approximately 30 to 50% of the sun.
Typically, the bigger the tree, the more benefits it will provide from an energy saving and air cleaning view. For those reasons, you can select trees that will grow as big as the space permits. Keep in mind, however, that trees need space for both roots and branches, and many city locations can be harsh environments for trees. Be sure trees are planted where adequate water is available. Choose trees that are adapted to the particular site, considering soil conditions, light conditions, and moisture conditions. Trees planted close to the house should be strong and resistant to damage from disease, insects, and storms.
How Shrubs and Other Plants Can Help
Foundation plantings of shrubs and small trees can also significantly reduce energy costs. In addition to reducing the amount of wind that actually hits a home, shrubs planted next to the house can provide insulation because they create a dead airspace next to the foundation. Plant shrubs so that when mature there will be approximately 1 foot of space between the plants and the building wall.
Vines growing on a house wall, or on a trellis attached to a wall, will absorb the sun's heat and shade the wall's surface, allowing less conduction of heat from the sun to enter the house. In winter, the vine will reduce the amount of wind hitting the house walls, and will provide a small insulating layer of air.
Using Plants to Control Snow
If drifting snow is a problem, windbreaks of trees and shrubs can act as living snow fences to control the location of snow drifts. Lower shrubs planted on the windward side of the windbreak will trap snow before it blows next to the home or buildings. Winds will funnel around the ends of a snow fence. If possible, the row of plants should extend at least 100 feet beyond the snow drive problem area. Because of the decrease of wind velocity, snow will settle immediately downwind from a windbreak or snow fence. The windward row of a living snow fence should be placed at least 100 feet from the building or area that needs protection. A minimum of two rows of evergreens and one row of shrubs is most effective for snow control.
Consumer Energy Information: EREC Fact Sheets, U.S. Department of Energy, on internet at http://www.eren.doe.gov/erec/factsheets/landscape.html.
Energy Saving Landscapes: The Minnesota Homeowner's Guide, Department of Public Service, Energy Information Center, State of Minnesota.
Homeowner's Guide to Landscaping that Saves Energy Dollars, Ruth Foster, David McKay Company, Inc., 1978.
Landscaping for Energy Conservation, Colorado State University Cooperative Extension, L. Walker, on internet at: http://www.ext.colostate.edu/pubs/garden/07225.html.
Snow Fences and Proper Setbacks from the Road, Dan Gullickson, Urban and Community Forester, MN/DOT Office of Environmental Services, 1998.