On July 13, Washington, D.C. hit its 17th consecutive day above 90 degrees. The following week was no better with a heat advisory in effect from Maine to South Carolina. Casey Trees, a D.C. based nonprofit committed to restoring, enhancing, and protecting the tree canopy of the nation’s capital, says air temperatures in cities, particularly after sunset, can be as much as 22˚ F warmer than the air in neighboring, less developed regions. This urban heat island effect, as landscapers know, is driven by asphalt, concrete, and other hardscapes that retain and amplify heat. But more than just being uncomfortable, excessive heat is a serious health risk that often disproportionately affects those in underserved neighborhoods with less tree canopy.
But it’s not just humans who feel the ill effects. According to Casey, urban heat islands have a wide-ranging and deadly set of impacts on the environment:
- Urban heat islands increase energy demand for cooling. During extreme heat events, which are exacerbated by urban heat islands, the demand for cooling can overload systems and may result in brownouts or blackouts to avoid power outages.
- Companies that supply electricity typically rely on fossil fuel power plants to meet much of this demand, which in turn leads to an increase in air pollutants and greenhouse gas emissions.
- High pavement and rooftop surface temperatures can heat stormwater runoff. This heated stormwater drains into storm sewers and can raise water temperatures as it is released into waterways, which can be deadly for aquatic animals and organisms.
Made In The Shade
As many know, strategically located large canopy trees can do much to mitigate the effects of urban heat islands. But what types of trees make the best choices? Here are five recommendations from Casey (check your USDA Plant Hardiness Zone):
Overcup Oak. With its straight trunk and round top, the Overcup Oak is a relatively unknown small to medium-sized shade tree that fits well into many small yards. The Overcup Oak has brilliant reddish or gray-brown bark and displays leathery dark green leaves in summer. The overcup gets its name from its shaggy, ornate acorn cap – and the acorns are highly valued by wildlife. This tree needs full sun and will reach approximately 30 feet in height in 40 years. Zones 5 to 9.
American Beech. The American beech is an iconic, sturdy shade tree with smooth, blue-hued bark. It produces small (beech) nuts in a showy pod that are valued by wildlife and visually unique. It’s dark green leaves turn a beautiful yellow in fall. A slow grower, American Beech usually reaches 50’ tall and 30’ wide in about 50 years. Beech does well in shade or partial shade, but can also be planted in full sun. Zones 4 to 9.
Southern Magnolia. This elegant tree has large fragrant flowers that bloom from March to June, striking seed pods that drop bright red seeds in the fall, and bold, beautiful leaves. Southern magnolias are a much sought-after, durable evergreen tree. Southern magnolias reach approximately 50 feet in height in about 50 years. It thrives in full sun or partial shade. Zones 6 t0 10.
Catalpa. A showy medium-sized shade tree is often known as “cigar trees,” Catalpas are typically planted for its springtime show of large, white, orchid-shaped flowers from which develop slender, long green seedpods that darken and drop in fall. At home in most soils and moisture regimes, the Catalpa is a tough urban tree. Catalpas will thrive in full sun to partial shade and may attain 40′ in height in 30 to 50 years. Zones 4 to 8.
Black Willow. The black willow has elongated green leaves and dark brown to black, deeply furrowed bark. The tiny yellowish-green flowers that appear in catkins in the spring provide nectar for bees and other pollinators. Growing best in full sun to part shade, the Black Willow can grow to a height of 50 feet in an urban setting. Tolerates wet sites. Zones 2 to 8.
Of course, just as trees take care of us, we need to take care of trees during periods of prolonged heat spells—by watering. For clients without irrigation systems, make sure to emphasize the importance of proper watering, particularly for those trees planted within the past one to three years. Watering tips and more can be found on the Casey Trees site.
This article was adapted from blog posts written by Jona Elwell, communications specialist with Casey Trees. (Article photos from Casey Trees. Cover photo: ISTOCK.)
Though River Birch was considered a great tree for yards. Also, Big-leaf Magnolia.
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