Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
12/27/99
Contacts:
Jeff Iles, Department of Horticulture, (515) 294-0029,
iles@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the week beginning Dec. 31, 1999
Landscape Plants Can Reduce Energy Consumption
By Jeff Iles
Extension horticulturist
Iowa State University Extension
At last! The Canadians have finally sent a little winter our way. Yes, near zero temperatures, measurable snow and falling on one's keister after mistaking slick parking lot ice for asphalt are the joys of a winter season too long in coming. But with frigid temperatures come heating bills. Very high heating bills if your house is a bit on the leaky side. What to do? Plant trees and shrubs of course! You were expecting me to say something else?
Trees and shrubs planted in key positions around homes can reduce energy consumption and save money. For example, properly placed trees can slash air-conditioning demand in summer by as much as 50 percent. Thoughtfully placed trees also can reduce winter heating costs by 4 to 22 percent. But to achieve maximum energy savings and environmental improvement, appropriate tree and shrub species must be planted in strategic locations and in the correct relationship to buildings they are to benefit. Simply stated, the goal is to get maximum shade in summer but minimum shade in winter.
But before we plant the first tree, let's remember that a lot of the unwanted heat in a house in summer comes from the sun shining through windows. And because the summer sun is so high in the sky, almost twice as much solar energy enters through east and west windows as the south windows. Therefore, highest priority should be given to planting shade trees due west of west-facing windows followed by planting trees east of eastern windows.
Trees located in these positions offer the most advantageous combination of solar control and energy savings by blocking early morning and late afternoon sun in the summer, but offering no obstruction to winter sunlight. Select trees that can be planted within 20 feet of windows and will grow at least 10 feet taller than windows. If space permits, use tree combinations to create a continuous planting opposite all major west- and east-facing windows.
But don't waste your time planting trees on the south side of your home. Remember, in summer the midday sun is almost directly overhead. A large tree oriented to the south will cast little, if any shade on buildings to the north because its shadow will fall directly under the tree. In winter, when the sun is lower in the sky, the same tree will cast an undesirable shadow on structures to the north for most of the day.
Deciduous shade trees (those that drop their leaves in autumn) are the preferred natural heating and cooling regulators in temperate climates like ours. To obtain maximum benefit, the "ideal" shade tree should have a broad crown and dense foliage in summer when shade is most desirable. Then when temperatures begin to cool in fall, the ideal tree would lose its leaves, permitting the suns energy to penetrate a sparsely-branched canopy. Trees meeting these criteria are classified as "solar-friendly". Kentucky coffeetree, green and white ash, river birch, red and sugar maple, and American linden are good examples of solar friendly trees because they provide dense summer shade and sparse winter branching.
In general, large-growing trees are best because they provide the maximum environmental benefit per tree. But, because solar friendly trees are most effective when planted close to east and west sides of buildings, only sturdy trees with good branching habits that resist injury from storms should be planted. Do not plant large, fast-growing, weak-wooded species like silver maple and cottonwood next to homes. And if energy efficiency is your primary goal, avoid trees that drop their leaves late in the fall (Norway maple), trees that retain their leaves throughout winter (oaks), trees with exceptionally sparse branching (ginkgo) or those that are densely branched (littleleaf linden).
If a site isn't large enough to accommodate large-growing tree species, you might consider using woody plants to establish a zone of insulating dead air space along the walls of buildings. Plants like arborvitae and juniper installed close to buildings will create a layer of still or slow-moving air that can slow heat loss in winter and heat gain in summer. This technique is most effective when plants are installed in a continuous line that extends along the walls to be protected and around the corners.
Other, large-growing conifers like pine, spruce and fir, can be used to shield homes from piercing north and west winds during winter. Finally, landscape interest can be created by using a variety of plants, but take care to group like kinds together.
Energy-conserving landscapes don't happen by accident. Rather, careful planning and preparation is required to realize environmental and financial benefits from strategically placed woody plants. So, before you plant that next tree or shrub, make certain it is positioned for aesthetic beauty and energy conservation.
ml: isugarden
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