Plants Can Reduce Energy Costs
by Dr. Robert Black, Consumer Horticultural Specialist

How you arrange the plant in your landscape can influence the costs associated with maintaining comfort inside your home. Plants can be used to either divert or channel air movement away from or toward your home. Air movement around the home may raise energy consumption by increasing conductive heat loss (in winter) and heat gain (in summer) through walls and windows, and infiltration of outside air around the edges of windows and doors.

In Florida, winds prevail from the north in winter. Planting situated on the north, northwest and, to a lesser extent, northeast exposures of the home can provide significant energy savings. The height and foliage density of trees used will directly influence their effectiveness as wind barriers. Evergreen trees with dense canopies provide the most complete protection, but extremely dense or solid windbreaks tend to concentrate their effects over a much shorter distance than those of moderate texture. A multi-layered canopy of shrubs and trees of moderate density planted in two to five rows is the most effective windbreak design, but even a single row of trees will provide some windbreak action. Windbreaks significantly reduce wind velocity for a distance equal to 10 times the height of the trees, less significantly to 20 times the height. The greatest amount of protection within a distance of five times the height of the windbreak.

In Florida, summer breezes prevail from the south and southeast. In north Florida, breezes during the "dog days" of July and August originate form the south or southwest; in south Florida they largely remain southeasterly. How best to use plants to interact with summer air movement is largely determined by the means with which the home will be cooled.

For a home in which air-conditioning will be used only minimally, trees and shrubs should be strategically situated to channel cooling breezes toward the windows. Low-branching trees should be avoided on the southeastern and/or southwestern exposure, or the low branches removed. Plants used to shade windows from the sun should be far enough away to not restrict air movement. Shrubs near the windows can be positioned to further funnel moving air into the house. Winter wind barriers in the north and northwest sides of the home will also deflect cooling breezes from the south back toward the house in the summer.

During Florida's 5-7 months of uncomfortably warm temperatures, some residents find it impossible to stay cool without air-conditioning, regardless the cost. Wind movement around the home during the season will substantially raise the energy costs for air conditioning by increasing the infiltration of hot, humid outside air around windows, doors, and through cracks.

Shrubs and trees should be positioned around the air-conditioned home to divert the prevailing southern breezes away from the house. A multi-layered summer windbreak should be designed along the southern exposures and away from the home. The tallest components of the windbreak should be the closest to the home. Along and close to the walls that face the direction of summer winds, a foundation planting of shrubs should be used to create a dead air space that will reduce or eliminate warm air infiltration. Deciduous shrubs, or at least more open-branched species than recommended for north-facing foundation plantings, should be used on the south side to allow the sun to heat those exposures in winter.

During the mild transitional months of fall and early spring, natural ventilation is desirable, even in homes that will be air-conditioned during the peak of the hot season. The south-facing foundation shrubs can be pruned in September to permit air movement, and then allowed to fill out again the following spring, if such pruning will not disrupt the aesthetic integrity of the landscape. Shade trees positioned between windows and prevailing summer winds should be low-branching to provide maximum protection against air movement. Additional tall shrubs can be placed nearby but on the windward side of east and west windows.