More than 100 species of native and introduced evergreen conifers are found in the forests and cities of North America. These conifer species have evergreen needles and bear seed cones; however, they vary considerably in where they are found, how big they grow, how fast they grow, and how suitable they are for home landscapes. Evergreen conifers provide both winter and summer energy conservation benefits. If appropriately planted and cared for, they can reduce chilling winds in the winter and provide shade in the summer.
Because evergreen conifers keep their foliage year-round, they are well-suited for use in windbreaks to abate harsh winter winds. A windbreak is a dense grouping of trees, often situated in several parallel rows, that is planted adjacent to a house, building, or outdoor space to intercept and slow prevailing winds. By slowing wind speed, windbreaks help conserve energy used to heat buildings. If situated correctly, these same trees can also provide shade in the summer.
Much like the human body, a building is kept warm by an energy source that elevates the core temperature, then circulates the warmth around the extremities. The drywall, fiberglass, brick, and vinyl that cover the walls are like a coat, helping to insulate the building from heat loss. When cold air passes across the exterior walls, heat is transferred from the wall surfaces to the cold air. This is analogous to windchill for people. A windbreak of trees slows down the wind speed so that a stable layer of slightly warmer air forms around the building exterior, minimizing the temperature difference between the building and the outside air. Less heat is is then "pulled" from the building to warm the surrounding air and there is less demand on the building's heating system.
To maximize the heat conservation benefit of a conifer windbreak, the proper species must be selected and placed in the proper orientation and distance from the building. For best effect, the trees chosen for a windbreak must attain a height similar to the building. Most conifers are capable of growing to that stature. In fact, many conifers may grow too large for typical residential landscapes, which can lead to conflicts with driveways, sidewalks, rooftops, and power lines. Moreover, conifers may differ in their requirements for soil fertility and drainage. Most pines are adapted to dry, infertile soil, but some species only thrive on cool, rich soils. Measure the space in which your trees can grow and have a soil sample tested prior to selecting the conifer species for your windbreak.
For best effect, a conifer windbreak must be situated to block the prevailing winter wind. For much of North America, these winds prevail from the northwest, although local differences may exist (Diagram 1). Information on prevailing wind patterns in your vicinity may be obtained from the National Weather Service.
Windbreaks are most effective when the trees are planted relatively close to the building being sheltered. A good rule of thumb is a distance no more than the mature height of the trees being planted. At greater distances, the windbreak becomes less effective. However, do not plant the windbreak too close to the building, because this can lead to nuisances from errant branches and roots.
|Diagram 1. Landscape planting for energy conservation. http://extension.missouri.edu/explore/images/g06910art01.jpg|
Evergreen conifers can also help conserve energy in summertime by casting shade upon houses, garages, and other buildings. By casting shade, the trees are intercepting solar radiation that would otherwise strike the building surfaces and warm them. By keeping building surfaces cool, less heat is transferred to the inside air. Therefore, the air conditioning system has to be run less frequently to keep the building at a comfortable indoor temperature.
To maximize the shading benefit, trees should be planted on the west and south aspect of the building to intercept the hot afternoon summer sun. In northerly latitudes, it is undesirable to plant large evergreen conifers on the south aspect of a building. The dense shade that they cast in winter keeps the sun from heating the building. Without this free natural heat source, more energy must be consumed to warm the building. Deciduous shade trees are suitable for the south aspect because they shed their leaves in winter.
In summary, homeowners should balance their aesthetic preferences, energy conservation needs, and anticipated tree growth when choosing evergreen conifers for their home landscapes. Diligent research regarding the best location to plant on site should also be undertaken. For maximum energy conservation benefits, conifers should be planted to block prevailing winter winds from the north and west, yet kept away from the south aspect to take full advantage of passive solar heating in winter.
By: Eric Wiseman, Associate Professor, Urban Forestry and Horticulture, Virginia Tech University