Residential energy use comprises about 26% of all the energy used in the state of Florida (Florida Energy Office 1992). In south Florida, about 35% of this annual residential energy expenditure is for cooling the home during Florida's 5- to 7-month-long summer, and about 10% is used for heating in winter (Cook 1993). As utility rates escalate, so does the cost of staying comfortable in the home.
Glass windows and doors can account for between 30 and 60% of a building's total heat gain in the summer (Cook 1986). As much as 270 Btu (heat unit called British thermal unit) of direct and diffused solar radiation can enter a home or building through each square foot of glass on the east and west sides. For example, if sunlight strikes only 50 square feet of a clear glass window (sliding-glass door) on a west wall, the cooling effect of more than one ton of air conditioning is required to remove the heat gained from this source alone. This is more than eight times the heat gain caused by conduction and infiltration. Even windows facing north or south can have twice as much indirect radiant heat gain than that from conduction and infiltration combined.
Before central heating and air conditioning, homes were designed and built to take advantage of natural heating and cooling. For example, a tree with high branches offers shade in the summer and insulation from cold winter winds. Today, passive methods of climate control are once again of interest because we are now aware of fossil fuel supply limitations and the environmental effects of fossil fuel use. New information has substantially improved many passive, energy-saving landscaping concepts (known as enviroscaping) from the past.
Landscape plants can improve the appearance of our surroundings and modify the extremes of local climate (microclimate modification). Plants provide shade, insulate the home from heat loss or gain, and cool the air that surrounds their leaves through transpiration (release of water from plant pores).
Trees are the main types of landscape plants used around the home for passive energy conservation. They provide shade, influence air movement around the house and, once established, require little maintenance. The energy-conserving impact of a particular tree species depends on 1) whether it keeps its leaves during the winter and 2) the shape of a tree and density of its foliage.
Planting Site Selection
House walls are the most practical to shade because new tree plantings take many years to cast an effective shadow on the roof. Heat transmitted through the roof is best reduced by using attic insulation, radiant barriers and ventilation. This is because tree limbs over the roof can present both a nuisance (litter clogging rain gutters) and a risk of damage or injury should heavy limbs fall off in a storm. Existing vegetation that can provide roof shade without undue risk, however, should be incorporated into the home site design.
The correct placement of trees shading the home involves consideration of the angle of the sun's rays in summer and winter, mature tree height and structure height. In general, the target areas for shading during Florida's warm months are the walls on western, eastern and southern exposures, in that order. Though an exposure facing due south receives little direct sun on June 21, by August the sun is low enough in the sky to increase heat loads considerably on south walls. Windows provide the most direct entry for heat into the home. Consequently, special attention may need to be given to walls containing the most windows.
The benefits of new shade trees should be felt within 5 years. To accomplish this goal, a distance of 7 to 20 feet from tree to wall is recommended. Lot size and the mature tree height directly influence this distance. The closer a tree is to the house, the longer its shading effects last during the day. The shadow of a tree planted 10 feet from the home moves across the shaded surface four times slower than a tree planted 20 feet away.
If winter windbreak effects are desired, trees should be planted on the north and northwestern exposures of the home. This is the prevailing direction of blustery, winter winds in most of Florida (see EES-5 Florida Climate Data ). The effects of summer breezes, which usually prevail from the southeast and southwest in Florida, are often desirable during mild, transitional times of the year. Where air conditioning exclusively cools the home for most of the hot season, summer winds can reduce cooling efficiency by increasing hot, humid air infiltration around window and door fittings or cracks in siding and masonry. In this case, a tree windbreak located on the southeast exposure of the house deflects the energy-robbing winds from the home.
The chart at the end of this publication provides information that will help you choose one or more species best suited to your house and landscape. The trees are alphabetized by scientific name. The size category specifies the mature height of the tree (small, small-medium, medium, medium-large, and large).
If shade requirements are immediate, give careful attention to the growth-rate designation in the chart. A fast-growing tree increases in height by 3 or more feet per year and provides shade benefits within 5 years. Most fast-growing trees, however, are both short lived and weak wooded, two undesirable characteristics. In such cases, it may be desirable to plant both a small, rapidly growing tree and a moderate or slow-growing species nearby. The fast- growing tree could then be removed once the other species provides shade benefits.
Site conditions directly influence the establishment and the life of a landscape tree. Coastal residents should heed the salt-tolerance ratings of the listed species. The "General Comments" column on the chart givesspecific tolerances (or intolerances) of a particular species. Tailor your choices to match the conditions in your site. For instance, a tree requiring well-drained soils does not prosper where standing water accumulates after a heavy rain. If this condition applies to your home site, choose trees for wet-soil tolerance as indicated (e.g., red maple, Acer rubrum; pond apple, Annona glabra; and bald cypress, Taxodium distichum).
Relative drought tolerance is also indicated for each species. These ratings refer to Florida conditions only and should be interpreted as follows: High—survives without supplemental irrigation after establishment; Moderate—requires supplemental irrigation during very dry periods to maintain satisfactory appearance and health; and Low—little or no drought tolerance. Drought tolerance also varies with soil type, water table, and other environmental conditions.
Whether a tree is evergreen or deciduous ("Leaf Persistence" on the chart) affects its performance. Deciduous trees, which drop their leaves in winter, are recommended for use on south, southeast and southwest exposures. In summer, they provide desired shade. In winter, their bare canopy allows the sun's rays to warm the home, creating additional energy savings. On the other hand, evergreen trees, which have leaves all year, on the north and northwest exposures provide the most effective barrier to cold, winter winds.
The shape of a tree influences how long shade lasts. Spreading, round and vase-shaped canopies provide the longest periods of shade during the day. With attention to both this category and the shade-density rating, home-shading methods can be fine tuned to meet individual needs and desires.
Interest in native plant materials has increased greatly in the state, so all native species are marked with an asterisk (*) on the chart. In some cases, native plants may be better adapted than exotic species to local soil and weather conditions.
A few common landscape trees in south Florida are not on this list. Australian pine (Casusarina equisetifolia), punk or cajeput tree (Melaleuca quinquenervia), earleaf acacia (Acacia auriculiformis), carrotwood (Cupaniopsis anacardioides), ear tree (Enterolobium cyclocarpum), bishopwood or toog (Bischofia javanica), laurel fig (Ficus microcarpa), and Brazilian pepper (Schinus terebinthifolius) are exotic species that are considered invasive, or have other serious environmental problems such as invasive root systems. These species are not recommended for south Florida landscapes.
Planting and Maintenance
All new tree plantings benefit from soil preparation, regular irrigation, and, in some cases, protection from insects, disease or weather extremes. Young trees require a period of regular aftercare to ensure proper establishment. Knowledgeable nursery employees and county extension agents are good sources for answers to individual problems.
Detailed information on proper tree placement, shading patterns, and microclimate modification are in the following publications available at your county extension office:
Cook, Gary. 1986. A guide to selecting window and glazing options for Florida buildings. IFAS/Florida Energy Extension Service EES-36. Gainesville, FL.
Cook, Gary. 1993. Personal communication. Gainesville, FL.
Florida Energy Office. 1992. Florida energy data report: 1970-1990. Dept. of Community Affairs, Florida Energy Office, Tallahassee, FL.
Key to South Florida Tree Chart
Leaf Persistence: D = Deciduous, E = Evergreen, S = Semi-Evergreen
Form: C = Columnar, O = Oval, P = Pyramidal, R = Round, S = Spreading, V = Vase-Shaped
Growth Rate: S = Slow (less than 6 inches per year), M = Moderate (6 inches to 3 feet per year), F = Fast (more than 3 feet per year)
Shade Density: L = Light, M = Medium, H = Heavy
Size: S = Small (up to 25 feet), M = Medium (25 - 40 feet), L = Large (more than 40 feet)
Drought Tolerance: L = Low, M = Moderate, H = High
Salt Tolerance: L = Low, M = Moderate, H = High
Tree selection for south Florida.