Leafy green energy-savers that cost little, never wear out

Want an effective energy-saving strategy for your home that requires minimal initial investment and brings increasing savings over time? One that is long-lasting and easy to maintain? And that adds to the beauty of your home? Try landscaping.

''There is hard data to show that in most areas of the Northeast, landscaping can save at least 15 to 20 percent of energy costs for space heating and cooling ,'' says Anne Moffat, co-author, with Marc Schiller, of ''Landscape Design That Saves Energy'' (William Morrow & Co.).

Ms. Moffat is quick to point out that using landscaping for energy conservation is not a new idea, but rather an age-old strategy that was neglected when energy became cheap. Now it is coming back, not because of economics alone, but because there are advantages to choosing landscaping over mechanical energy-saving devices.

In contrast to mechanical devices, energy-efficient landscaping becomes both more effective and reliable over time, while being comparatively maintenance-free. Not only do many trees and shrubs not wear out, as machines do , but foliage is naturally self-regulating, requiring no thermostatic device telling it when to grow leaves for shade in the summer and then to drop them in the fall when sunlight becomes precious.

General climatic factors and specific local conditions determine which of the many landscaping options are best. In the Northeast, which Ms. Moffat describes as cool to temperate, appropriate planting can save on energy costs by affecting three factors: shade, wind velocity, and heat absorption.

''Just look at older rural residences, and you'll find they usually have trees on the upwind and south sides,'' she says. These trees serve two functions. First, trees on the south side shade the house during the hot summer months.

''A stand of trees with a light leaf canopy, such as honey locust or larch, screens sunlight more efficiently than closed Venetian blinds,'' she adds. Trees with a dense leaf canopy, such as oaks and maples, throw an even heavier shade.

In winter, trees on the upwind side serve as a windbreak. Anything that reduces wind velocity saves heat loss from the external surface of buildings, she continues.

The spacing of the trees with respect to one another and the house, along with the density and height of each tree's foliage, determines how much they affect the wind speed. Some windbreaks can reduce wind velocity by 30 percent. Evergreens planted in combination with shrubs are the traditional choice for windbreaks in the Northeast.

Not only trees can offer shade and prevent heat loss. Quick-growing vines, planted in the spring and trellised up the sides and over the top of south-facing windows, can produce as much shade by July as a mature tree. Some species, such as sweet pea and common morning glory, must be replanted every year; others, such as wisteria, are perennials. And the investment in vines and trellis is quite minimal.

''Look again at older houses,'' Ms. Moffat counsels, ''and you'll usually see shrubs planted densely around the foundation.''

The early settlers knew that significant heat loss occurs at the place where house and foundation join. By planting shrubs there, you create a pocket of dead air space between them and the foundation, considerably reducing heat loss at this vulnerable spot.

Taking advantage of the capacity of plants to use up heat is the third way landscaping can save on energy costs.

''Both bare earth and pavings, such as blacktop and concrete, absorb and retain heat, reradiating it back into the air,'' she notes. ''The air temperature above ground covers, however, is 10 or more degrees cooler than that above bare ground, and the difference is considerably more so above pavings.''

The most common ground cover, grass, is the cheapest to install but the most expensive to maintain. Others, such as creeping juniper, myrtle, and ivy, which require only an occasional weeding, feeding, and trimming each year, are much more economical choices than grass, which must be watered and mowed throughout the summer months.

Like the ground, the external surface of buildings also absorbs and reradiates heat. Trellising vines up the west side of a house can greatly reduce heat buildup during the late afternoons of summer, when the sun is low in the western sky.

Thoughtful planning is the key to maximizing the energy savings available through landscaping. In choosing species, consider the amount and patterns of future growth, especially how the plants will change in proportion to the house and existing plantings. Ease of maintenance, both now and in the future, is also an important factor.

''It may mean, too, that plants must be removed, even mature trees, for poorly placed plantings can actually increase energy consumption,'' Ms. Moffat says.

In the Northeast, the overall goal for energy-efficient landscaping, she adds , is to enhance the environment aesthetically while maximizing the ability of the sun and minimizing the ability of the wind to get to the house. All planting decisions should keep this in mind.