deciduous vines on southwest facing fence (180548 bytes)

Beat the Heat with Landscape Plants

By Denny Schrock, Colorado State University Cooperative Extension agent, horticulture

Writing the check to pay summer electric bills is a good reminder to look at natural air conditioning possibilities in the yard.

Searing summer sun builds up heat in unshaded areas and puts a strain on electric air conditioners and swamp coolers. Add green plants and you increase the comfort level and provide shade and evaporative cooling.

Results can be dramatic. Models constructed in Logan, Utah showed a 98 percent reduction in summer cooling costs by shading the entire house. In Sacramento, California, shade trees reduced attic temperatures 20 to 40 degrees - equivalent to the cooling effect of several room-sized air conditioners. In Mesa, Arizona almost $220 in annual cooling cost savings were realized from trees planted on the east and west sides of a house. Individual results will vary depending on home design and orientation, but significant energy savings are possible with proper landscaping.

Start by noting shade tree placement in the yard. Most summer heat gain in the home occurs in the morning and afternoon when the sun is low enough in the sky to shine directly through windows. A tree in full leaf can block 70 to 90 percent of solar radiation. Shade trees on the east side of the house prevent heat build-up in the morning, while those on the west and southwest side shield from intense afternoon rays.

While trees planted directly south of the house can provide additional shade and cooling, for year-round energy efficiency, it is best to leave the south side of buildings unshaded. Even after a deciduous tree loses its leaves, the remaining branches may cut out up to 60 percent of the sun's rays. Solar heating through unshaded south walls during the cold winter months more than offsets summer cooling costs.

An energy-efficient shade tree alternative are roof overhangs designed to shade south windows at mid-day during the summer. They also allow light in during the winter when the sun is lower in the sky. For shading south walls through the summer only, cover the home with deciduous vines that lose their leaves in fall. Or use annual vines that die back to the ground. An 8 F difference between shaded and unshaded surfaces equals a 30 percent increase in insulation value for the shaded wall.

Decks, patios, and other outdoor living areas benefit from shade trees placed on their south and west sides. Without shade, these outdoor rooms often become unusable heat traps. Keeping lower limbs pruned high adds to the benefit of shade trees. With high ground clearance, breezes funnel beneath trees, picking up speed and creating natural ventilation. Faster air movement increases evaporation near the tree, adding to the cooling effect.

Driveways and rock mulch areas can be heat trouble spots. An unshaded concrete driveway can be as much as 35 warmer than surrounding lawn. Organic mulches such as shredded bark and wood chips will be cooler than rock or gravel mulches. For an even greater cooling effect, consider using a water-efficient turfgrass such as buffalograss.

Shade outdoor air conditioner condensers with trees or large shrubs. Prune low branches so they don't obstruct air flow to the condenser. Condensers with restricted air flow or those exposed to heat from the sun do not run efficiently.

Take the following steps to a cooler, more comfortable energy-efficient landscape.

  • Plant large deciduous shade trees on the east, west and southwest sides of the house. Avoid planting them directly south of the house where they will block winter sun.

  • Plant deciduous vines to climb on trellises and provide extra shade on the south and west sides of the house.

  • Shade outdoor activity areas by planting trees on their south and west sides. Vine-covered arbors also are effective sun barriers.

  • Shade air conditioning condensers for more efficient operation.

  • Plant trees and shrubs that will act as wind tunnels to channel breezes into the house. Keep lower limbs pruned up to allow breezes to pass underneath shade trees.

Following these guidelines will make living spaces more comfortable and utility bills more bearable.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010