Why

Trees offer shade, shelter, habitat, beauty, and food and require little more than sun, soil, water, and a place to grow. They shade your house from the hot summer sun, reducing indoor temperatures.

Trees planted as windbreaks block cold winter winds, which lowers heating bills. Street trees help calm traffic and make walking more inviting. They provide wildlife habitat, help reverse the urban heat island effect, and pull greenhouse gases out of the air. Trees add value and beauty to their surroundings, making our neighborhoods nicer places to live.

Recommendation

Ventura County has both hot inland climates and cool coastal climates. Using trees for shading is more important in the hotter areas, while blocking cold winds is the priority in cooler areas.

• Use trees to strategically shade buildings in inland areas with high summer temperatures.
• In coastal areas where cool breezes are common and heating costs higher, use trees to block cooling winds.
• In all areas, use trees to shade large areas of pavement, and to buffer people from traffic.

Cooling Buildings (and Whole Cities)

As much as 8% of urban electricity demand is used for the extra air conditioning required to compensate for the urban heat island effect, a phenomenon where cities experience greater temperatures due to the concentration of heat-producing, heat-retaining buildings, pavement, and traffic. Greater concentrations of buildings and pavement produce greater temperature increases. Shade trees can significantly mitigate the urban heat island effect.

Trees help cool the air by providing shade, but also through evapotranspiration (the evaporation of water from leaves). Cooling from evapotranspiration can equal the cooling produced by a tree’s shade. A mature tree with a 30-foot crown transpires approximately 40 gallons of water per day.

Shade trees can make buildings up to 20 degrees cooler in the summer, and can cut summer air conditioning needs by up to 50%—a significant reduction in energy demand.

According to the publication Getting to Smart Growth, “Communities that incorporate or create landscaping (such as trees or shrubs) along right-of-ways, in town centers, along open spaces, and around other focus areas encourage walking by providing a more enticing environment.” By encouraging walking, trees also indirectly reduce vehicle travel, and the associated heat and carbon emissions generated by cars and car infrastructure.

How to Plant Trees for Cooling Shade

West and East Plantings Best. If your house needs cooling in the summer, your first priority should be planting trees to shade the house’s west or northwest walls from the afternoon summer sun. In summer and fall, when we have long days and the sun angle is very low in the sky, the sun can easily shine directly into windows, bringing unwanted heat. Cooling shade is most needed during the afternoon, when temperatures are highest and the incoming sunshine is greatest.

"During August and September, which are Ventura County's hottest months, the sun's angle is actually lower than it is on the solstice in June", says landscape architect Kathy Nolan. "When you are preparing to plant a shade tree, consider how the tree's shade will shelter your building during those late summer months."

A shade tree between the western afternoon sun and the house will help reduce the need for air conditioning. West/northwest plantings also minimize shading on southern windows, where winter sunshine is most desired. For even more shading during the longest days of the year, plant smaller deciduous or evergreen trees with lower limbs northwest of the building to provide late afternoon and early morning shade.

East walls are the second most important to shade from the morning summer sun.

Plant trees at least 10 feet from the house, but more distance may be appropriate depending upon the species’ height, width, and the invasiveness of the root system.

Shade Tree Selection

The ideal shade tree grows to about 25 to 50 feet tall and has dense foliage and a broad, spreading form. Palm trees, common in our area, are generally a poor choice for shade. The more area that can be covered with shade, the better.

Deciduous trees (those that drop their leaves in winter) are the best choice. In summer, the foliage of these trees cools buildings by blocking solar radiation. In winter, after the leaves have fallen, the sun's energy passes through trees and helps to warm buildings.

Only trees adapted to our semi-arid environment should be planted, and be very careful not to select invasive, non-native species. These species invade native habits, eliminating the animals and plants adapted to the live there; they are often more thirsty than local varieties, can spread to stream habitats choking channels and increasing flood risk, and can pose greater fire risk.

A list of invasive, non-native tree species is available on the California Invasive plant Council web site.

How to Plant Trees for Wind Breaks

Blocking cold winds is extraordinarily effective at reducing heat loss. Air infiltration into buildings increases substantially with wind speed. In cold windy weather all of the air in a poorly sealed home may change two to three times per hour. Even in newer or tightly sealed homes, the entire volume of air may change every two to three hours.

Planting windbreaks to block cold winds takes more careful site analysis and planning than planting for shade because the wind is trickier to corral than the sun. When you plant a tree to block the sun you must consider where its shadow will fall. When you plant windbreak trees, you must predict what the wind will do when it hits the windbreak. For example, the wind will flow over the top, around the sides, and through any holes in the windbreak.

Windbreaks are less necessary in compact urban neighborhoods, where homes and trees already slow and redirect wind. But if you have a large property on relatively flat land, a windbreak could be helpful.

Here are some key design considerations:
Many factors affect wind direction at a given location; analyze the direction of the coldest winds at your location and plant windbreak trees to block these winds. In general, in Ventura County prevailing winter winds are most often from the southwest during the daytime and from the east at night. Multiple-leg windbreaks (like two sides of a square) can be used to give the most protection when there are variable-direction winds.

• A multilayered canopy of shrubs and evergreen trees of moderate density planted in two to five rows is the most effective windbreak design. Plant rows in zigzagging patterns.
• Ideally, windbreak trees should grow to twice the height of the
  building.
• On the leeward side (closest to the house) of the windbreak, plant low-lying shrubs to block winds at ground level.
• Wind tends to curve around the ends of a windbreak. Therefore, windbreaks should be long in relation to their height. The length of the windbreak should be at least 10 times its height at maturity.
• Although wind protection can extend downwind a distance of 10 to 15 times the windbreak height, the most effective area of a wind break is at a distance four to six times windbreak height.

Be careful: Trees that block summer breezes can reduce cooling from natural ventilation in hot weather.

Planting shrubs, bushes, and vines next to your house creates dead air spaces that can also help insulate your home both winter and summer. A dense evergreen hedge is particularly effective here. Plant so there will be at least one foot of space between full-grown plants and the wall of your home. Be mindful of the location of exhaust or intake vents, which may affect plant health or the performance of the mechanical system. An evergreen planting can also be used to shelter any door or other entry that is exposed to wind.

Windbreak Tree Selection

The ideal windbreak tree is evergreen (does not drop its leaves in winter), fast growing, visually dense, has stiff branches that do not self-prune, and has a crown that extends to the ground. According to “Tree Guidelines for Coastal Southern California Communities” , incense cedar, New Zealand Christmas tree, and Canary Island and Torrey pines are among the best windbreak trees for coastal Southern California communities.

Tree Planting Checklist

Trees for Shade

• Located so shade will fall on west/northwest or east walls of the house
• Deciduous
• 25 to 50 feet tall when mature
• Has a crown with a spreading form (more like an oak; less like a palm tree)

Windbreak Trees

• Planted upwind of area to be protected
• Evergreen
• Has a dense crown that extends to the ground

All Trees

• Not an invasive exotic species
• Low-water-using and drought-tolerant, preferably native
• Not subject to limb breakage
• Appropriately-sized for planting location (won’t outgrow the location)
• Able to withstand strong winds
• Positioned at least 10 feet (or more depending on species) from the house to prevent disturbance to the foundation from roots
• Positioned to avoid shading solar collectors or interfering with power lines
• Positioned to keep branches 10 feet away from any chimneys
• Positioned so shade, even from tree trunks, does not block south-facing windows
• A species with roots that don’t tend to spread laterally (if planted near sidewalks, driveways, or the street)
• Planted to adhere to fire department guidelines for vegetation management (if your property is next to native habitat or other high fire-threat locations)

Plant vines. Perennial vines, grown on a trellis, take just a couple years to grow as tall as the house’s walls and can work almost as well as trees for shading western and eastern walls. Annual vines can also be used; they grow quicker than perennial vines.

Convert pavement to plants. Remember the driveways with two narrow strips of pavement? All that unpaved area between the strips allowed water to soak into the ground, preventing runoff and adding to the cooling of the air. Plants, any plants (except invasive non-natives), are better than pavement (see the “Resources” section below for lists of invasive species). Wherever possible, minimize or even remove pavement, and instead plant the area with low-water-using, low-maintenance plants.

Shade paved areas. When you must have pavement, shading it will help to keep the immediate area cooler and help the pavement last longer.

Shade the A/C. A shaded air conditioner will need to work less and so use less energy. If your air conditioner is on the ground, plant a small tree near it so it is shaded during the hottest months of the year. Keep the fan or air-flow free of leaves or vegetation, and prune branches to allow several feet of clearance.

Build a fence. If space is limited or if you want wind protection right away, a fence on the windward side of a house can provide a barrier against cold winter winds.

More Than Temperature Control

Trees perform many important ecological functions.
These functions have direct economic and social value as well as environmental value. Because they provide so many services, trees can be thought of as a “conservation bargain”—a small investment that yields large returns.

Reduce greenhouse gases. When trees and vegetation reduce energy use, they also reduce carbon dioxide emissions—the primary greenhouse gas—from power plants. In addition, trees pull carbon dioxide out of the air and store it in their biomass, offering another opportunity to help combat global warming.

Flood protection. Impervious surfaces such as roads, parking lots, and rooftops increase runoff by preventing water from soaking into the soil. As the amount of runoff increases, so does the threat of flooding. Trees intercept rainwater—on their leaves, branches, and trunks—and temporarily store the water, helping to slow the flow of water to flood channels.

Enhanced water supply. By slowing floodwaters, as well as the overland movement of water, trees allow more water to seep into the ground. Water that could have been quickly lost to the ocean is instead held by the soil. This water recharges underground aquifers and also slowly releases into streams, supplying the base-flow waters that feed streams long into the dry season.

Clean water. Trees, along with shrubs and groundcovers, help slow the overland movement of water, causing sediment and pollutants to be deposited on the land before they can reach the stream.

Soil stabilization. Trees have extensive root systems that help to stabilize and hold soil in place. Additionally, trees help to remove sediment by slowing down runoff from the surrounding area, allowing the sediment to settle out before entering streams.

Air cleaning. Trees clean the air by absorbing odors and pollutant gases such as nitrogen oxides, ammonia, sulfur oxide, and ozone, and filter particulates out of the air by trapping them on their leaves and bark. Trees that shade asphalt surfaces and parked vehicles also reduce emission of hydrocarbons that evaporate from gasoline or diesel leaking from fuel tanks and worn hoses.

Critical wildlife habitat. Trees provide essential breeding, nesting, feeding, and refuge habitats for many forms of birds, mammals, and insects. Trees also contribute organic matter such as leaves and fallen wood, which provide food and shelter for many animal species.

Food. Tangerines, oranges, avocados, apples … trees are quite important as a human food source also.

Quality of life. Trees and vegetation can help reduce noise, a highly valued trait in urban areas. Trees can mask concrete walls, parking lots, power lines, and poles. They also provide shade from harmful ultraviolet radiation, particularly in playgrounds, schoolyards, and picnic areas. In addition, trees and vegetation generally improve the appearance of neighborhoods; several studies have shown that home values are higher on tree-lined streets.

Cost and Cost Effectiveness

Trees provide so many benefits that it is hard to put a dollar amount on their value. Energy savings offered from trees will be different for every situation, depending on such things as summer temperatures, exposure to cold winds, and amount of home insulation. The costs associated with planting and maintaining trees include installation, staking, pruning, irrigation, fertilizing, mulching, pest control, and litter cleanup. Studies of the benefits of trees have found that net benefits of trees increase with tree size: The larger the size of a mature tree, the greater the benefits over costs.

In order to maximize the cost-effectiveness of tree planting, be sure to select low-maintenance species. If a tree requires a lot of mechanical pruning, the energy used by the chainsaw (not to mention the greenhouse gases emitted) can negate any benefits.

Plant low-water-using species adapted to the local climate to reduce water costs and promote the trees’ long-term health. Careful placement of trees can avoid the need for costly pruning, tree removal, or sidewalk repair.