One of the most important ways humans interact with the environment is through the way we landscape our homes. Planting the wrong plant in the wrong place can have a very negative environmental impact. For example, selecting plants that require pesticide spraying can lead to the pollution of the Biscayne Aquifer, the primary source of Dade County's drinking water. Planting invasive exotic trees such as schinus (Schinus terebinthefolius) or malaleuca has resulted in huge losses of natural habitats in the Everglades as well as other natural areas. In fact, the most important characteristic of environmental landscaping in south Florida is to ensure that the tree species used are not invasive.
	Landscaping can have a positive environmental impact when native plants that are appropriate to a residential site are used. Even better is the planting of groups of native plants that are found together in a natural habitat such as those found in a typical hardwood hammock. Appropriate native trees and shrubs are generally more disease and drought resistant and are important for providing food and habitat for our indigenous animals. If a house is located near a natural habitat, it is even more important to plant native plants to provide a buffer zone and possibly contribute to a wildlife corridor between natural areas. Non-invasive exotic species of flowering and fruit trees and palms can be used selectively to increase the diversity and aesthetics of South Florida landscapes.
As the population of South Florida continues to increase, there will be increasing demands on our limited water supply.	As the population of South Florida continues to increase, there will be increasing demands on our limited water supply. This will lead to future water shortages, particularly during the dry season in the spring. Thus it is important to plant predominantly drought-tolerant trees. Using heavy mulch and grouping those plants that require more water into one area can reduce water demands during this dry season.

	Planting Trees The "environmental" way to plant a tree is usually just the best way to plant a tree to ensure health and rapid growth. Although it is often mentioned and nearly always ignored in practice, the planting hole should be large. The latest research in tree growth rates for Florida indicates that it is more important for the planting hole to be wide rather than deep. In South Florida limestone, the hole should be about five times the width of a small root ball or about three times the width of a large root ball. After a tree is planted level with the ground, a heavy layer of mulch (6 to 10 inches) should be applied to an area at least as wide as the original planting hole (it should be kept a few inches away from the trunk.) Environmentally, it is important to use mulch from locally-chipped yardwaste (Metromulch in Dade County) or mulch made from malaleuca trees.
	Trees and Global Affixing Although the scientific debate continues, some recent scientific studies corroborate the possibility of a significant increase in global temperatures due to the accumulation of greenhouse gases, particularly carbon dioxide, in the atmosphere. Planting trees to mitigate against global warming has been proposed by a number of scientists and environmental organizations (particularly American Forests through their Global Releaf Program). In one case, an electrical utility was required to plant trees in Central America to offset the projected emissions of CO2 from a new power plant. This is based on the most direct impact of trees, the conversion of carbon dioxide into oxygen. In South Florida, the average tree will remove about 60 lbs of CO2 from the atmosphere each year. When using trees to reduce global warming, it is important to purchase trees that are initially small in size. When trees are grown to a large size in an energy-intensive nursery, they not only cost considerably more but also have significantly greater embodied energy. These fossil-fuel inputs come from the extensive use of irrigation, fertilization, and pesticide spraying at the nursery as well as the transportation requirements of large specimens. Another factor is that a smaller specimen planted in a wide hole has a greater growth rate and thus converts CO2 at a faster rate. In South Florida, the optimum size trees for survival and fast growth in residential landscaping while minimizing embodied energy are those with heights of 6-10 feet in 3-15 gallon containers. Trees planted along a street should be 10 feet in height to improve survival rates.
In South Florida, the most important use of trees to reduce the greenhouse effect is to strategically plant trees to reduce air conditioning requirements of a building.	Energy Conservation Landscaping In South Florida, the most important use of trees to reduce the greenhouse effect is to strategically plant trees to reduce air conditioning requirements of a building. A tree planted to shade an air-conditioned building is 10-15 times as effective in reducing CO2 emissions as through photosynthesis. Thus a single tree can reduce CO2 emissions by about 600-900 pounds per year. Trees can reduce air conditioning by (a) blocking solar radiation from the building, the adjacent ground, and the foundation (b) creating cool microclimates near the residence by evapotranspiration, and (c) either channeling or blocking air flows through and around the residence. The following are guidelines for optimizing the energy savings from reducing air-conditioning requirements through landscaping.

Shading the Air Conditioner Area The single most effective way to reduce your cooling needs is to shade the building's air conditioner and the immediate area around it. Air conditioners become less efficient as temperatures get hotter; preliminary measurements show that direct shade, plus evapotranspirative cooling, can reduce the temperature of the air around an air conditioner by six or seven degrees Fahrenheit. This can increase the efficiency of the air conditioner by about 10% during peak periods. To cool your air conditioner, plant several trees so that after a five-year growth, their canopies will completely shade the air conditioner and the adjacent area during mornings and afternoons for the entire cooling season. Shading the Building and the Adjacent Ground Position trees and shrubs to keep sunshine off the building and nearby ground. During summer months, you need to shade the east and west sides of a house and all windows except those facing north. In South Florida where air conditioning is used through November, south walls also need significant shade. To shade walls, plant trees so that, near maturity, the limbs extend almost to the roof. The canopy should reach within five feet of west or east walls and overhangs and three feet of south walls or overhangs. These trees will provide optimal shading patterns and create cool microclimates directly adjacent to the house through evapotranspiration. Beware of planting trees too close to the house. Roots can damage the foundation, and large limbs can cause severe damage if they fall. (To further reduce heat gain through the roof, paint it a light color and use attic insulation and radiation barriers.) Similarly, place tall shrubs within four feet of west, east, and south walls so that the inside edge of the hedge will reach within one foot of the walls within four years. To further optimize cooling, plant an "understory" of shrubs and groundcovers, especially if the trees are surrounded by concrete or asphalt. Peak Load Shading Air conditioning is the primary component of electrical peak demand. Because heat transfer through walls (particularly concrete/brick walls) causes a delayed impact on air conditioners, plant trees so they will shade the east- and south-facing walls to reduce peak afternoon consumption. Plant other trees along the west wall to reduce air-conditioning during the late evening, after the electrical peak load period. Influencing Wind Movement You also can plant trees to influence wind movement around and through a residence. If you use minimal air-conditioning in your home, position trees and shrubs so they funnel breezes into the windows, thereby maximizing natural cooling. Prune back the low branches of surrounding trees to allow prevailing summer breezes to pass through the house. If you use air-conditioning most of the year, facilitating winds with trees can actually increase air-conditioning use because warm breezes increase warm air infiltration and heat the interior. You can avoid this by placing shrubs and trees so winds are channeled into the dwelling when the windows are open, but away from it when the windows are closed. In South Florida, the prevailing summer winds are from the southeast. Consequently, air infiltration through the windows can be reduced by locating tall shrubs close to and on the north sides of east-facing windows and the west sides of south-facing windows. When the windows are opened during mild periods, these same shrubs will facilitate natural ventilation through the windows. Energy Saving If these recommendations for energy conservation landscaping are followed, one can expect significant reductions in air-conditioning consumption. Experiments conducted in South Florida during the warmest months of the year demonstrated energy savings of 58% during the daytime. These results, along with studies of the impact of vegetation on heat flow through residential walls, suggest that air-conditioning can be reduced by up to 40% in older homes and 20-30% in homes built since 1980. Trees also can be used to counteract the "urban heat island effect." This is the heating up of urban areas due to the replacement of vegetation with heat-absorbing surfaces like asphalt and dark roofs. Recent computer and analyses indicate that planting large numbers of trees in our cities can reduce this heat build-up. These studies, along with the research on the effectiveness of energy-conserving landscaping, has led to the establishment of American Forests' Cool Communities Program. This is a program sponsored by the U.S. Department of Energy and the Environmental Protection Agency to encourage the use of energy conservation landscaping and light surfaces in six urban communities, one of those being Dade County. If energy conservation landscaping is utilized in urban areas around the world, their potential in reducing global warming is significant. Homeowners can help make this happen by using the concepts outlined in this paper to plant environmentally sound trees in their own residential landscape.

Jack Parker, landscape architect, is a Professor of Chemistry and Environmental Science and Chairperson of Environmental Studies at Florida International University.