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Features | The Urban Gardener

Trees can function like a natural air conditioner

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Trees function as a natural recycling machine. Tree leaf surfaces take in moisture from the air (in the form of carbon dioxide) and heat from the sun to make food, a process called photosynthesis. This process, translated to a homeowner, not only means improved air quality around the home, but trees in your landscape can act as a natural air conditioner.

Heat from hot air passing over and under leaf surfaces is partially absorbed by evaporation of surface moisture. This process cools the air surrounding leaves and lowers air temperatures by as much as 9 degrees Fahrenheit. The more leaf surface in an area, the cooler surrounding air temperatures.

To take full advantage of trees’ cooling functions, use them to shade west-facing walls and windows to avoid heat buildup from afternoon sun. And strategically placed tall trees will eventually shade the roof from midday heat.

Trees should be planted no closer than 15 feet from the foundation of a house to avoid damage from root systems. Home gardens located close to power lines should not grow trees or shrubs that will reach higher than 30 feet tall at maturity. You don’t want your shade trees to interfere with safe, reliable electrical service.

Before planting, a soil test should be conducted so that you are knowledgeable about nutrient management and pH balance. This service is available to home gardeners through the Cooperative Extension Serv-ice at the University of Hawaii-Manoa campus. A pH test kit also can be picked up at local garden stores. It is vital to the health of your tree that pH levels of 5.5 to 7.0 are reached before planting.

The next step is to check the soil for drainage. Dig a hole at least a foot deep in the area you plan to plant your tree. Fill the hole with water. If the water drains slower than 1 inch per hour, drainage is poor and amendments will be needed. Mixing six to eight inches of good compost (not mulch) in with the local soil will increase drainage. The idea is to amend the entire planting area in your yard at the same time. Plant roots will outgrow the planting hole required for a tree in a short time.

Once the planting site is ready, dig a hole two times the diameter of the tree container. The depth is determined by the soil level inside the pot. When placed inside the hole, the plant’s original soil level should be level with the top of the ground. Burying the plant too deep will create rot at the base of the trunk.

It’s best to avoid fertilizing until the tree has had time to establish itself in its new home. The compost will provide a few nutrients, and some fertilizer will still be present from nursery cultivation.

Lastly, mulch is a must in the tropics. Apply two to three inches of mulch to cover the newly amended soil.

This and much more home gardening information is available in the UH CTAHR publication "Planting a Tree." Visit and type "planting a tree" in the search box; or call the UH Master Gardener help line at 453-6055 on Oahu; 244-3242, ext. 228, on Maui; 274-3471 on Kauai; 981-5199 in East Hawaii; and 322-4892 in West Hawaii.

Jayme Grzebik is an urban horticulturist with the University of Hawaii at Manoa Cooperative Extension Service, part of the College of Tropical Agriculture and Human Resources. E-mail her at


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