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Landscaping for Energy Efficiency
Plant a tree and you get more than just summer greenery, autumn color and spreading branches in which to climb. Well-placed trees also can lower your heating and cooling bills. The same can be said for shrubs, vines and even ground-hugging plants (groundcovers).
Your house heats up when the sun beats down on it and when heat moves in and out by conduction through walls or infiltrates the building envelope. Properly located plants can affect how much your home heats up in summer and loses heat in winter.
You can expect landscaping to save about 25 percent in heating and cooling costs. Government studies have quantified some of these savings. In a Pennsylvania study, for example, shade trees reduced air-conditioning needs by 75 percent. Another study conducted in Colorado showed that landscaping provided a 25 percent reduction in winter heating bills and a 50 percent savings in summer cooling bills. The U.S. Department of Energy estimated that as few as three shade trees could save $100 to $250 per year in cooling costs.
Despite the variability of these findings, it is obvious the energy savings from well-placed trees, vines, shrubs and groundcovers can be significant and more so now with recent surges in energy costs.
Evergreens that grow branches right to ground level are the best choice for a winter windbreak.
Winter winds that chill your home by forcing cold air in and drawing heated air out can account for up to half the heat lost on a cold, windy winter day. You can limit this heat loss by making your home more airtight and by reducing the wind's force against your home.
A windbreak planted on the windward side of your property (the side toward the wind) can redirect and slow wind at a distance downwind of 15 to 20 times the height of the plants, and cut fuel consumption by up to 40 percent, a study in South Dakota showed. Wind speed is most dramatically reduced at a distance downwind of about three times the height of whatever you plant.
For most homes, the windward side is on the north or northwest, since winter winds generally come from that direction across much of the continental United States, but check local conditions to determine the best place to plant a windbreak on your property.
Don't leave any gaps in a windbreak "for a driveway, for instance" or the funneled wind will come howling through the gap with even greater force. In addition, a windbreak must be somewhat wider than the width of the area to be protected, to allow for eddy currents winging around the ends of the windbreak.
An added benefit of a windbreak is its ability to position drifting snow. Snow accumulates on the ground on the leeward side of a windbreak (the side away from the wind) beginning at a distance of one to three times the height of the plants, so position a windbreak accordingly. Another option, if you have sufficient space on your property, is to plant a row of shrubs 50 to 100 feet or so on the windward side of a windbreak; a good portion of snow will accumulate between this trip row and the main windbreak.
Because windbreaks are meant to mitigate winter wind, evergreens are the best plant choices, and the best evergreens are those that grow branches right to ground level. If possible, plant multiple rows, with trees staggered from one row to the next, and plant different species of different heights in each row.
You also can use a combination of plants and a wall or a fence to create an effective windbreak. Wind is sucked into the vacuum created on the lee side of an impermeable barrier, so any wind barrier should allow some air to pass through to prevent the creation of a vacuum.
Trees and shrubs can help reduce winter heating bills even on a small property. A few well-placed evergreens near entryways can decrease the amount of cold air that finds its way indoors every time a door is opened. And evergreens planted near the exterior walls of your home can create a zone of relatively still air between the plants and the walls. This dead air space will reduce heat transfer, functioning much like the dead air space in the insulation in your walls.
Foundation plantings, then, can serve some purpose besides beautifying our homes and hiding foundations, as long as the plants are evergreens. And the benefits are year-round, as they also will reduce heat transfer into your home in summer, saving in cooling costs.
The most dramatic effect a planting can have on energy costs is if it creates shade. Shade plantings work even in small yards, since the plants must be close to the house to block the high summer sun. These plants are most effective when keeping the sun's rays off the roof and from streaming in windows.
Deciduous trees (those that lose their leaves in winter) provide summer shade when planted on the south side of a house. However, in addition to creating summer shade, we also want to maximize the heat gain from the sun's rays in winter. A deciduous tree with a high crown and a clear trunk will allow the low winter sunshine to reach your home's south-facing walls and windows.
Even bare branches can block up to a third of the sun's rays, so it's best to use tree species that naturally make few fine twigs. Gingko biloba, honeylocust, and white and green ash are good choices, since they'll block the summer sun but allow winter sunlight into the home. For a one-story home, choose trees that mature at about 20 feet high, and plant them 15 to 20 feet from the south side of the home.
Summer days often stay sultry through late afternoon, with late-day sun beating down on the western side of your home. Use deciduous trees to block those late-afternoon rays. Because afternoon sun is lower in the western sky than midday sun, choose trees with low branches. Large deciduous shrubs closer to the house also can create shade. There's no need to worry about the low, bare branches of these trees or shrubs blocking desirable winter sunlight, because in winter the sun does not wrap far enough around the southern sky to be blocked by the plants.
Where space is too limited for trees to shade the west side of a house, a vine can be planted instead, held off the wall by wires, wooden latticework or an arbor. If the wall is masonry, a self-clinging vine such as Virginia creeper or climbing hydrangea will block the sun as well as dress up the wall.
Sun beating down on a deck or paving near your house heats up the surrounding air, and the hotter the air just outside your house, the more energy will be required to keep the inside air cool. So why not also use plants to cool the air near your home?
Summer air temperatures in the shade are up to nine degrees cooler than in the sun. Because colder air is heavier than warmer air, it settles near the ground, so temperature differences become much greater as much as 25 degrees when comparing temperatures above blacktop with temperatures in the shade above ground where plants are growing.
Deciduous trees or vine-clad arbors will not only shade decks and paving in the summer, keeping these surfaces cooler, but also will cool the air by evaporation, as they lose water through pores in their leaves. Even ground-hugging plants, then, can do their part in keeping things cooler outdoors.
If the heat exchanger for your whole-house air conditioner is located outside, it will run less and gobble up less energy if it is shaded. Just make sure not to place plants so close to the unit that airflow is impeded.
As far as costs are concerned, the trees, shrubs and vines you plant around your house to reduce heating and cooling bills typically pay for themselves in reduced energy costs in eight years or less, according to government studies. Of course, the nice thing about investing in trees, shrubs and vines is that they increase the value of your property while making your home a more comfortable place in which to live.
Lee Reich, Ph.D. ( http://www.leereich.com ) is a horticultural writer and consultant based in New Paltz, N.Y.