Enviroscaping to Conserve Energy:            a Guide to Microclimate Modification

Introduction

In California, temperatures fall below freezing on an average of 10 to 20 days every year (Black, 1999). Even in Los Angeles County, winter fronts with associated winds can cause uncomfortably cool temperatures for short periods of time. In addition, California's long, hot summers create a high demand for air conditioning during 4 to 8 months of the year. Approximately 50 percent of the energy consumed by a home in California is used to maintain interior comfort. Most of the energy (about ¾) used to heat and cool our homes comes from the non-renewable fossil fuel energy resources, natural gas, coal, petroleum, and nuclear.

California's Major Sources of Energy

Page last updated: 7/1/2008

Electricity (2007) Source    Natural Gas 45.2%    Nuclear 14.8%    Large Hydro 11.7%    Coal* 16.6%    Renewable 11.8%    Natural Gas (2006) Source    In State 13.5%    Canada 23.4%    Rockies 27.7%    Southwest 40.3% Crude Oil (2006) Source    In State 38.8%    Alaska 16.1%    Foreign 45.0%

California homes use approximately 1819 trillian Btu (a heat unit called British thermal unit) of energy every year. As much as 180 trillion Btu of this energy could be saved by effective management of the microclimate that surrounds our homes. One Hundred & Eighty trillion Btu is enough energy to power approximately 1,541,000 homes in California.
This energy savings would reduce the strain on individual pocketbooks and the state's bank account. California currently imports from other states approximately 69 percent of the petroleum and 90 percent of the coal burned at the power plants to make our electricity. This energy savings would reduce the strain on our environment, too, by reducing the quantities of greenhouse gases and pollutants produced when fossil fuels are burned.
A microclimate is any small, local area within which the effects of weather are both relatively uniform and easily modified. Microclimate modification involves the best use of structural and landscape design elements to maximize or limit sunlight, shade and air movement. Structural modifications involve the design of the house and associated construction (walkways, fences, patios). Landscape modifications (enviroscaping) involve the use of plants to further increase or decrease the impact of sun and wind upon the local environment.
The "passive" methods of microclimate modification discussed in this publication are simple, low-cost improvements that can decrease the energy costs associated with maintaining interior comfort.

Sructural Elements of Microclimate Modification

When choosing or designing a new home, several decisions strongly influence the degree to which interior comfort requires energy inputs for heating or air conditioning. The homeowner should incorporate effective insulation in ceilings and walls, and weather stripping around windows and doors, even if local ordinances do not require such practices. Total energy savings of 50 percent to 60 percent can be realized if conservation practices are followed in new home construction (Buffington, 1988).

Orientation

In California's hot climate, a house is more energy efficient if it is oriented with the long axis running east-west ( Figure 1 ). With this orientation, the short walls of the house receive most of the direct morning and afternoon sun, thereby reducing the total heat load on the structure. An east-west orientation can save as much as $80 to $120 per year in cooling costs alone for the average home. In the winter, when the sun is lower in the sky, the south & west facing long wall receives the heating benefits of solar radiation. Divergence of up to 12°F, in either direction from this orientation, is allowable to compensate for the prevailing wind direction or other specific site requirements.

Figure 1. A house is more energy efficient if it is oriented with the long axis running east-west.

Roof and wall colors

Light-colored materials reflect sunlight; dark materials absorb the radiation. A house with dark walls and roof is less expensive to heat in winter, but more costly to cool in summer. Light-colored walls and roofs lower cooling costs but increase the need for winter heating. In California, the use of light-colored materials is more cost effective and energy efficient, since the cooling season is considerably longer than the heating season.

Fencing

Fencing is primarily used around homes to ensure privacy or mark boundaries. Fencing also directly influences air-circulation patterns. Air movement can affect the energy efficiency of the home, depending on the season of the year, direction of prevailing winds, and degree of dependence on air conditioning for home cooling. Air movement around the home may raise home energy consumption by increasing conductive heat loss (in winter) and heat gain (in summer) through walls and windows, and the infiltration of outside air around the edges of windows and doors.

Figure 2. Solid fence design deflects breezes.

In California, winter's prevailing winds are from the north or northwest. Thus, a solid fence on the north side of a house can provide a barrier against cold winter winds ( Figure 2 ). Winds from the south, southeast and southwest predominate during the summer months when effective air circulation is generally desired. Open fencing, especially with bottom clearance, maximizes air flow and reduces reliance on air conditioning for cooling ( Figure 3 ).

Figure 3. Open fence design allows for air circulation.

When air conditioning rather than natural cooling is used to cool the home in the hotter, summer months, prevailing winds should be blocked or diverted away from the house to reduce warm, air infiltration. In general, vegetation provides greater flexibility in directing air circulation and is a better choice than fencing expressly for this purpose.

Other Structural Features

In summer, large roof overhangs can help shade windows and walls, as well as walkways adjacent to the house. Arbors or trellises over outdoor living areas increase comfort and shade nearby walls. Decks should be built with bottom clearance to allow air to circulate below the structure. If possible, driveways should be located on the east or north side of the house to reduce heat buildup during warm afternoons.

Solid surfaces such as concrete and asphalt, which reflect a great deal of heat, should be kept to a minimum. Brick driveways build up less heat than either asphalt or concrete and produce less glare than concrete. Ground cover plants and organic mulches are the best option for covering large areas. Ground covers offer a cooling effect and are not energy intensive. Organic mulches reduce runoff, are inexpensive and an attractive alternative to pavements.

Landscape Elements of Microclimate Modification

Plants provide the most economical means of modifying microclimate around a home and represent an investment in future energy savings. Research in California has shown that energy costs for air conditioning at certain times of the day can be reduced more than 50 percent by proper placement of the right plant materials around a residence.

Plants in the landscape interact directly with the two primary comfort factors of California weather: sun and wind. Summertime heat gain in a home can be reduced by using plants to:

1. shade the residence from direct solar radiation,
2. either divert or channel air movement away from or towards the house,
3. create cooler temperatures near the home by evaporation of water from their leaves (transpiration).

Heating costs in winter can be decreased by selecting and properly locating plants so that:

1. the amount of direct solar radiation received by the home is maximized and
2. the effects of cold winter winds are minimized.

In many areas of California, the wasteful practice of completely clearing building sites of existing vegetation is being abandoned. Existing trees should be incorporated into new-home design whenever possible. Mature stands of native vegetation can often provide the desired energy-saving shade and wind control that would otherwise require years to develop from new plantings of nursery stock. Native plants are well adapted to local climate and soils, and established natives do not ordinarily require supplemental irrigation or fertilization, making them very energy efficient. Leaving too many trees around buildings, however, contributes to mildew, mold and other moisture problems in and outside the structures.

Shade

Trees

Trees are the most valuable landscaping tools for passively increasing the interior comfort of a home. Once established, most landscape trees require little maintenance and represent an appreciating investment in the home's value. Trees provide good shade and modify air movement effectively. How a particular tree species performs these functions depends on how tall it grows, whether or not the leaves stay on the tree all year, and the shape and density of the canopy.

**Trees on the west and northwest sides of homes provide the greatest energy benefit;          trees on the east side of homes provide the next greatest benefit,

Plant only deciduous trees on the south side of homes to allow winter sunlight & heat,  Plant evergreen trees as windbreaks,                                                                                   Shade trees can make paved driveways and patios cooler & more comfortable spaces,  Shading your air conditioner can reduce its energy use,                                                          but do not plant vegetation so close that it will obstruct air flow around the unit,               Keep trees away from overhead power lines and do not plant directly above underground water and sewer lines.

Trees that grow 60 or more feet tall are capable of casting shadows over the roof of a typical single family house. Unfortunately, new plantings of most large growing trees require 20 or more years to reach full size. The homeowner is probably better off investing in quality ceiling insulation, attic ventilation, and construction of radiant barriers (a layer of aluminum foil situated in the air space between the roof and the attic insulation) unless a preexisting tree canopy effectively shades the roof during the summer months.
Large trees overhanging the roof of a house do present the risk of damage from falling limbs, and the nuisance of clogged rain gutters full of leaf and twig litter. The planting sites of new trees must be carefully considered to optimize future shade benefits while minimizing these potential problems. If a solar water heating or electric system is on the roof, careful attention must be paid to the positioning of shade trees so that the efficiency of the system is not reduced.
Eastern and western wall exposures accumulate the most heat during the long days of summer. Tree shading should thus be maximized on these sides of the house. South walls also benefit from tree shade. Southern exposures relatively free of direct radiation in June can permit significant heat-load increases by August as the earth's position relative to the sun changes.
Windows are the most direct route for sunlight to enter the home. Trees (or shading devices such as awnings), therefore, should be positioned to shade them throughout the day.
Trees can provide valuable shading of sidewalls, particularly in older homes where walls have little insulation and retrofitting is prohibitively expensive. Small (up to 25 ft.) or medium-sized (25-40 ft.) trees perform this function well and won't grow out of bounds. Fast-growing trees can be planted at the same time as slower-growing species to provide a temporary solution to shading problems. Such trees can be removed later as the slower-growing trees approach their mature heights. Care must be taken that these vigorous, temporary trees do not shade or otherwise compete with the slower-growing, more permanent landscape elements.
The outdoor compressor/condenser unit of the air conditioning system uses less energy when it and the surrounding area are shaded from direct sun during the entire day. A tree can shade the unit when the sun is overhead, while nearby shrubs can provide protection during the early morning and late afternoon hours. However, care must be taken not to block the conditioner's air flow ("short circuiting"). If the warm discharge air is prevented from escaping, the intake air temperature rises, causing the unit to operate less efficiently.
In winter, the sun is low in the southern sky. Southern exposures of a home in north and central California can receive free benefits of passive solar heating, provided that deciduous trees are used along the southern exposures ( Figure 4 ). Deciduous trees shed their leaves in the fall, and are bare during the coldest months of the year.

Figure 4. Summer shading and winter sun-warming deciduous trees.

In south California, where winters are short and mild, evergreen trees along the southern exposures are best because heat buildup prevention to lower air-conditioning costs during the long summer is most important.

Evergreen trees maintain their leaves throughout the year. There are two types: broad-leaf (e.g., Southern magnolia, Magnolia grandiflora; American holly, Ilex opaca), and needle-leaf (e.g., pines, Pinus spp.; cedars and junipers, Juniperus spp.). Broad-leafed evergreens provide dense shade year round, and are most useful as shade trees in the southern third of California.
By contrast, the shade cast by needle-leafed trees is sparse and more open, though pruning can, in some cases, stimulate the development of a denser canopy. When air circulation (see next section) must be balanced with a certain degree of shading, more open-canopied trees may be preferred. Tree shape also influences the amount of shade cast ( Figure 5 ).

Figure 5. The shape of a tree canopy strongly influences the shade pattern that is cast.

Trees planted close to the home begin to provide shade sooner than those planted at greater distances. The benefits of new shade trees should be obtained within 5 years. To accomplish this goal, a distance of 7 to 20 feet from tree to sidewall is recommended. Lot size and the predicted mature height of the tree directly influence this distance. Trees planted closer also shade for a longer period of time during the day, and over a greater part of the hot season. The shadow of a tree planted 10 feet from the home moves across the target surface four times slower than a tree planted 20 feet away (Parker, 1978; 1983a; & 1983b).

The correct placement of trees chosen to shade the home involves consideration of the angle of the sun's rays, the mature height and width of the tree canopy, and the height of the structure to be shaded.

Shading, which reduces the amount of radiant energy absorbed and stored by built surfaces,  Evapotranspiration, which converts liquid water in leaves to vapor, thereby cooling the air, and Wind speed reduction,                                                                                               which reduces the infiltration of outside air into interior spaces.

Shrubs and vines

Shrubs can effectively block early-morning and late-afternoon sunlight on eastern and western exposures, respectively. Small-leafed, open-branched shrubs provide shade without unduly restricting air movement for passive cooling in the spring and fall. Vegetation close to the residence also lowers the air temperature near the home, reducing the heat conducted through the walls. Espaliered shrubs, (shrubs trained to grow horizontally against a wall), can block a great deal of sunlight before it strikes and heats up the wall ( Figure 6 ).

Figure 6. Espaliered shrubs can insulate a wall from heat build-up.

Vines are especially useful for shading homes when small lot size restricts the use of shade trees. Vines are either self-supporting or twining. Self-supporting vines cling to a surface by either padlike "holdfasts" (e.g., Virginia creeper: Parthenocissus quinquefolia) or aerial roots (e.g., trumpet vine: Campsis radicans). Self-supporting vines are not recommended for wood structures because they may trap moisture which can lead to decay of the wood. On brick or concrete block homes, a fast-growing, self-supporting vine can effectively prevent the sun from heating a wall ( Figure 7 ) (Parker, 1981).

Figure 7. Twining vines block the sun from nearby windows and walls.

Twining vines (Confederate jasmine: Trachelospermum jasminoides; painted trumpet: Clystostoma callistegioides) climb by means of stems or tendrils that require some form of support. By providing a lattice-type support or a trellis, twining vines can be used to shade walls, windows, and outdoor living spaces.
As with shade trees, only deciduous vines are recommended for southern exposures in north and central California, to allow winter sun to passively heat the home.

Wind Control

WINDS – California lies within the zone of prevailing westerlies and on the east side of the semi-permanent high pressure area of the northeast Pacific Ocean.  The basic flow in the free air above the State, therefore, is from the west or northwest during most of the year.  The several mountain chains within the State, however, are responsible for deflecting these winds and, except for the immediate coast, wind direction is likely to be more a product of local terrain than it is of prevailing circulation.

During the winter, storm tracks move further south.  Wind direction and speed are modified by migratory pressure centers.  With a strong high pressure area over the Great Basin and an intense low pressure area approaching the coast from the west, strong and sometimes damaging winds occur, usually from an easterly or southeasterly direction, especially along the coast and in the coastal mountains.  As the storms move inland the winds veer to southerly and southwesterly directions, and high wind speeds may occur anywhere within the State with the greatest velocities at high elevations.

Under a slightly different configuration of these pressure systems, winds tend to flow out of the Great Basin into the Central Valley, the Southeastern Desert Basin, and the South Coast.  Such wind situations are identified in southern California by the name “Santa Ana Wind.”  The air is typically very dry.  The winds are strong and gusty, sometimes exceed 100 MPH, particularly near the mouth of canyons oriented along the direction of airflow.  It is a situation that occasionally leads to serious fire suppression problems and often results in the temporary closing of sections of main highways to campers, trucks, and light cars.

A similar circulation pattern creates the “northers” of the Sacramento and San Joaquin Valleys.  As a result of compressional heating of air flowing out of the Great Basin this situation results in pronounced heat waves in summer.  In winter the result is usually a rather mild temperature accompanied by a dry, persistent wind that many persons find unpleasant.

The typical northwest wind of summer is reinforced by the dynamics of the thermal low pressure area located over the Central Valley and the Southeastern Desert area.  In the San Francisco Bay area there is a marked diurnal pattern in the strength of the wind even though an onshore circulation tends to continue throughout the 24-hour period.  This helps to carry locally produced air pollution products away from the Bay area, but creates problems for the regions immediately south and east of the source area.

In the Los Angeles area, however, the Basin is almost completely enclosed by mountains on the north and east.  Coupled with this is a characteristic of the air along most of the coastal area of California.  The vertical temperature structure (inversion) tends to prevent vertical mixing of the air through more than a shallow layer (1,000 to 2,000 feet deep).  The geographical configuration and the southerly location of the Basin permit a fairly regular daily reversal of wind direction—offshore at night and onshore during the day.  With the concentrated population and industry, pollution products tend to accumulate and remain within this circulation pattern.

Another local characteristic of the northwest wind alongshore is the creation of a jet effect in the vicinity of some of the more prominent headlands.  The most outstanding of these currents of air is found off of and to the south of Pt. Arguello.  Here a strong jet of air is projected southward past San Miguel and San Nicholas Islands, driving a huge eddy as much as 200 miles in diameter.  The air swings eastward near San Diego then northward and westward along the coast to rejoin the southward flowing air at the west end of the Santa Barbara Channel.  Similar but smaller eddies form in the vicinity of the Golden Gate, just south of Pt. Reyes, and south of Monterey Bay around Pt. Sur.  Wind speeds in the immediate vicinity of these major headlands can be two or three times as great as the wind flow at nearby points.

During periods of moderate to strong westerly flow at upper levels over the central part of the State, particularly during the winter and spring, the well-known “Sierra Wave” is created in the Bishop area.  Although this phenomenon is particularly useful to sail plane enthusiasts, it can also be a hazard to the unwary pilot.

A home loses a greater amount of heat on a cold, windy day than on an equally cold but still day. About 1/3 of the heat lost is transferred through the ceilings and walls (conduction). Wind increases heat loss from the outside surfaces of those same walls and from the roof by sweeping the warm air away (convection). Cold-air infiltration through spaces around windows and doors also increases reliance on costly home-heating systems powered by fossil fuels. Windbreaks and foundation plantings can substantially reduce the heat-robbing action of winter winds.

Windbreaks

In California, windbreaks situated on the north, northwest and, to a lesser extent, northeast exposures of the home can provide significant energy savings during the winter heating season. The height and foliage density of trees used in windbreaks directly influence their effectiveness as wind barriers. Evergreen trees with dense canopies provide the most complete protection. However, extremely dense or solid windbreaks tend to concentrate their effects over a much shorter distance than those of moderate texture. A multilayered canopy of shrubs and trees of moderate density planted in 2 to 5 rows is the most effective windbreak design, but even a single row of trees provides some windbreak action ( Figure 8 ). 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 occurs within a distance of 5 times the height of the windbreak.

Figure 8. Windbreaks and foundation plantings will weaken the effects of cold winter winds.

Foundation shrubs

A dense planting of shrubs close to the north and northeastern walls of the home creates a "dead-air" space that has insulating properties ( Figure 8 ). By reducing air movement in the immediate vicinity of the walls, this dead-air space also decreases secondary heat loss by cold air infiltration through cracks and window spaces. Dense evergreen shrubs (e.g., Podocarpus nagi, Pittosporum tobira) closely spaced together provide this type of protection effectively. These same shrubs keep north sidewalls cool in the summer via transpiration and shading during early morning and late afternoon.

Summer

Wind Pattern Shifts

The reason for warmer ocean temperatures, and thus warmer and more humid landmass air temperatures in July were a result of two important events.

1.) Prevailing northwesterly ocean surface winds, which generally flow southeast along the coasts of Oregon and California to Point Conception have been weak and even taken on at times a more southerly / southwesterly direction off the coast of central and northern California. Once the winds round Point Conception, they have abated almost entirely.

2.) The high pressure system responsible for the desert southwest's summer monsoon. (Discussed in the next section.)

Ordinarily, the northwesterly winds are responsible for a great deal of upwelling along the California coast. Colder ocean water below the surface is pulled to the coastline on wind-generated surf and works it’s way southeast. With this influence abated, as was the case in July, the shallower ocean depths retain solar energy.

Consider that in July 2005 --a typical wind pattern year along the California coast-- the water temperature 121 miles off the coast of San Diego was 57 degrees (10 degrees cooler than in July 2006) and at San Clemente Pier the water was 69 degrees, or 9 degrees cooler than present day. Northern California ocean water in July typically runs in the low to mid-50s, with high 40s not unusual in July, but even those water temperatures are running one to three degrees above average. As of press time however northern California's weather patterns have adjusted back to what would be considered normal for July with fog and clouds along the coast and air temperatures barely reaching 65 degrees in San Francisco midday.

Cause and Effect

The northwesterly wind pattern usually rounds Point Conception and enters the coastal waters of southern California and heads toward the central Baja Peninsula. The wind can sometimes be deflected by the channel islands in which case spins into what is called a Catalina eddy or micro surface low.

As desert heat builds inland in excess of 100 degrees, the Pacific-cooled air (some 30 and 40 degrees cooler) is pulled inland across southern California. With the cooler air in motion, the coastline is kept oftentimes cloudy and the sun is blocked from heating surface water to a great extent. This pattern is most noticeable in May and June, but usually decreases in July as the Pacific warms slightly to varying degrees of between 66 and 71 degrees, and the southwestern monsoon season is in full swing.

In early June 2006 the monsoonal high pressure system in the four-corners region established itself early. Mid-level moisture streamed into Tucson and Phoenix, Arizona, and Las Vegas, Nevada, in the first few days of June and all three cities were slammed with thundershowers, flash flooding, and wind damage. The monsoon season could be said to have arrived one month early, but moisture subsided for the most part until later in the month and new record high temperatures were noted instead of unusually high moisture levels.

The event could have been written off as unusual, however the pattern has been steadfast now for nearly two months.

During June the monsoon system flirted with southern California at the surface, but pushed further west at the mid levels, out and over the Pacific, further disrupting coastal eddy and marine layer influences as the northwesterly flow abated in northern California. Mid and low level wind flows across southern California shifted to an offshore flow, giving the region above normal temperatures -- but average to below average dew points (30s and 40s.) This pattern abated slightly for the first week of July at the surface, but continued at the mid levels as tropical moisture increased into the system from the gulfs of both Mexico and Baja California.

Humidity levels increased in early July, but backed off in the second week. Forecasters continued to predict the four corners high pressure system would move east, returning southern California's pattern to a more typical west-to-southwest flow aloft. During July 2006 however --with the pattern unchanged-- the ocean wind patterns off southern California provided a new twist.

Around 10 July 2006 coastal winds along Baja California started to head offshore --west/northwest-- in response to numerous easterly waves leaving old Mexico.

The four-corners high-pressure system reinforced off shore gradients in Baja and southern California -- keeping cloudless skies with nocturnal offshore winds at the 1,000-2,000 foot level that prevented any marine influence from developing.

Foothill regions in Santa Barbara, Ventura, Los Angeles, Orange, and San Diego counties experienced sundowner winds, which pushed midnight and pre-dawn temperatures above the 90-degree mark on northeasterly winds of 30 miles per hour. Montecito, California, at elevation 1,500 feet recorded five consecutive days and nights in early July with air tempertatures at or above 85 degrees, and yet the recording station is only 2 miles from the cool Pacific Ocean in Santa Barbara county.

In response to cloudless skies and lack of northwesterly surface winds, the Pacific Ocean had the full effect of solar heating from late June through all of July. With an ocean water temperature average having warmed to 75 degrees by mid-July, it was taking very little solar energy to warm the air into the 80s even at the beach. The air mass grew increasingly moist, which held nighttime temperatures above 70 degrees.

On 28 July the air mass in southern California had grown saturated enough that there was little difference between temperatures along the coast and those up against the foothills -- traditionally 20 degrees warmer in July. By late afternoon higher elevations were shrouded in low cloud cover, dew points were in the 60s/70s at a time of day usually with dew points in the 30s.

After sunset, clouds formed everywhere on the 28th and in some cases produced light showers notably in San Diego county's coastal region as temperatures met dew points -- in the 70s.

Given the same conditions under what would be considered 'normal' temperatures --with the ocean at 65 degrees and landmass air temperatures of about the same or cooler-- the interaction of these patterns would have passed unnoticed.

Low clouds and fog creeping inland is a mainstay pattern in southern California, but not with temperatures of 10 degrees above average and dew points in the 70s. Such measurements are indicative of weather patterns in Hawaii.

In California, summer breezes prevail from the south and southeast. In north California, breezes during the "dog days" of July and August originate from the south or southwest; in south California they largely remain southeasterly (Black, 1999). How best to use plants to interact with summer air movement is largely determined by the means with which the home is cooled.

Homes Cooled by Natural Ventilation

For a home in which air conditioning is used only minimally, trees and shrubs should be strategically situated to channel cooling breezes toward the windows ( Figure 9 ).

Figure 9. For passively cooled homes, trees and shrubs should be positioned to direct breezes toward windows.

Low-branching trees should be avoided on the southeastern and/or southwestern exposures or low branches should be removed ( Figure 10 ). Plants used to shade windows from the sun should be far enough away not to restrict air movement. Shrubs near the windows can be positioned to further funnel moving air into the house ( Figure 9 ). If shrubs are to provide low shade for exposures facing prevailing summer winds, use species that have small leaves and an open branching pattern (e.g., glossy abelia: Abelia X grandiflora in northern California; thryallis: Galphimia glauca in south and central California). Winter wind barriers on the north and northwest sides of the home also deflect cooling breezes from the south back toward the house in the summer ( Figure 8 ).

Figure 10. To allow air movement, low-branching trees should be avoided for passively cooled homes.

Homes Cooled by Air Conditioning

During the 4 to 8 months of California's uncomfortably warm temperatures, some residents find it impossible to stay cool without air conditioning, regardless of the energy investment. Wind movement around the home during the cooling season substantially raises the energy costs for air conditioning by increasing the infiltration of hot, outside air around windows, doors and through other cracks in the building. Studies of air-conditioned homes in California have determined that heat gain by infiltration is actually greater than gain by conduction and radiation through walls and windows.

Figure 11. For air-conditioned homes, trees and shrubs should be positioned to direct breezes away from homes.

Shrubs and trees should be positioned around the air-conditioned home to divert the prevailing southern breezes away from the house, the exact opposite of what would be desired for a passively cooled home ( Figure 11 ). A multilayered 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 house. In this way, a "wind ramp" can be created that will channel the breezes over 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 reduces or eliminates warm air infiltration. Deciduous shrubs, or open-branched species should be used on the south side to allow the sun to passively heat those exposures in winter in north and central California.
During the mild transitional months of fall and early spring, natural ventilation is desirable, even in homes that are 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 does 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 ( Figure 12 ).

Figure 12. For air-conditioned homes, low-branching trees placed by windows provide potection against air movement.

Cooling Effects of Transpiration

Plants release water through pores in their leaves. The evaporative loss of this water is called transpiration. As hot air passes over the surface of the leaves, the moisture absorbs some of the heat and evaporates. The air surrounding the leaf surface is thus cooled by the process. This interaction is called evaporative cooling, which can lower air temperatures surrounding vegetation by as much as 9°F (5°C).

The greater the leaf area in the landscape, the greater the cooling effects of transpiration. The use of plants for shade and wind control instead of structural features such as fences and arbors thus provides an additional benefit toward maintaining thermal comfort during California's long summer. Air temperatures near shade trees and foundation shrubs are considerably lower than open areas, resulting in lower heat gains through nearby walls or windows. If summer breezes are channeled through and across vegetation, their cooling capacity in non- weather is increased.
To maximize the effects of evaporative cooling, increase the amount of plant cover around the home. Use turf grasses and ground covers to their fullest potential in the landscape as alternatives to paved surfaces such as asphalt or concrete. Many ground covers require less maintenance than turf grasses. Because fertilizers, water, pesticides and mowing are energy intensive, ground covers are energy-saving alternatives to large lawns. Suggested plant materials and guidelines for using ground covers are in the following publications:

Summary

In order to reduce energy needs for summer cooling:

1. Maximize the use of ground covers and turf grasses; limit the amount of dark, paved areas.
2. Maximize shading on the roof by the overhead canopy of trees (making sure that solar panels are not shaded).
3. Maximize shading in the summer on east, west and south walls with shade trees. Shadows on the south, southeast, and southwest exposures are shorter than shadows cast on west and east exposures. Position trees and shrubs accordingly.
4. Use trellises in combination with ornamental vines for eastern, western and southern exposures.
5. If air conditioning is used only minimally, maximize the beneficial effects of prevailing cooling winds.

• Select and prune trees to allow air flow to the house.
• Avoid dense plantings away from the house that would act as a windbreak to cooling breezes
• Avoid the use of solid foundation plantings that create a dead-air space on the south side.

6. If you rely on air conditioning:

• Shade the outside compressor unit from direct sun.
• Select trees to block wind-driven air flow into the house through breaks in outside walls, such as windows, doors, and other openings.
• Situate a windbreak at some distance from the house to divert or block prevailing winds from the home.
• Reduce warm-air infiltration with a solid foundation planting that creates a dead-air space near the house in the face of prevailing winds.

7. To minimize energy needs for winter heating:

• Use deciduous trees, shrubs and vines on south, southeast and southwest locations, except in south California.
• Maximize the use of evergreen plant materials in foundation plantings to create an insulating dead-air space along the northern exposures of the house.
• Create a windbreak for the north and northwestern exposures of the home using evergreen trees and shrubs in a multi-layered canopy, preferably in 2- to 5-foot rows.

With careful attention to these guidelines, the California homeowner can improve the value of his or her home with landscaping and realize substantial savings in energy and for the environment throughout the year.