Seeking out the shade is something we’re all likely to do more of here in the Northwest as temperatures inexorably rise over the course of this century. Compared to the last three decades of the 20th century, average temperatures are forecasted to rise between 2 and a sizzling 8.5 degrees Fahrenheit by the years 2041–70 warns the Oregon Climate Change Resource Institute. Unless greenhouse gas emissions worldwide start dropping soon, expect more heat waves like the one that gripped Europe in the summer of 2003. Daytime temperatures soared above 100 degrees for over a week in many places, resulting in some 70,000 heat-related deaths.
The breakneck growth of cities is worsening the urban heat island effect where pavement and buildings absorb the sun’s heat during the day and radiate it back at night. Couple that phenomenon with waste heat generated by cars, lawnmowers, computers, and air conditioners, and city dwellers swelter more than their counterparts in the cool, green countryside, especially at night. Trees help reduce the urban heat island effect in two critical ways. First, they block sunlight from warming surfaces. A shaded house can be 8 to 10 degrees cooler inside than if it bakes in the sun. Shaded streets can be several degrees cooler than nearby sunlit areas. Trees also release water vapor into the air—lots of it. Evaporating moisture absorbs energy, which cools the air. The cooling effect is most pronounced in areas rich in big shade trees, such as parks and older residential neighborhoods.
Skyrocketing land values and a building boom in the Pacific Northwest means space for large trees is too often begrudged. Tree-filled, walkable neighborhoods are irresistible to buyers and developers alike. But small homes on large lots—and any big shade trees—are typically demolished for new construction. Even if spared during development, new homeowners eager for a sunny patch in which to plant tomatoes or an herb garden may still fell mature trees. Even if replacement trees are planted, they almost invariably grow less than 30 feet tall, too small to shade an entire yard. The result is that in fast-developing neighborhoods the reservoir of cool air created by interlocking big trees in adjoining yards is diminishing. It gives an unpleasant new meaning to the term “hot real estate market.”
But there’s hope for our urban tree canopy. Between 2000 and 2010, an enthusiastic push to plant all available street tree spaces in Portland helped increase the urban canopy from 27.3 percent of the city’s land area to just shy of 30 percent. Public and private partnerships have helped fill highway margins, like those along Interstate 205 in east Portland, with substantial numbers of new trees. Once barren school grounds, some asphalted over to cut down on upkeep, are green again with new trees provided by Portland’s Learning Landscapes program operated by the Parks and Recreation Bureau. The city also encourages homeowners to purchase street trees at bargain prices subsidized in part by fees charged to developers for cutting down trees. Not only does this new forest help mitigate Portland’s heat island effect, the shade makes asphalt streets last longer, and the trees intercept and slow stormwater runoff that would otherwise have to be treated in the city’s sewer system.
Water: The secret ingredient
Key to successfully increasing Portland’s canopy has been a commitment to water trees for at least two summers after planting. For people on the typically soggy west side of the Cascades, it can be hard to fathom that trees can literally die of thirst in our brief summers. But they do. With warmer weather in our future, the non-profit, tree-planting organization Friends of Trees believes watering—at least 15 gallons weekly to start—should be extended to at least three years and as many as five depending on the species. Watering frequency can taper off as trees mature, although even established non-native trees appreciate being irrigated during hot spells.
Some trees do a better job of shading us than others. Palms may shout subtropical chic, but most don’t cast much more shade than a streetlamp. Columnar trees or those shaped like a pyramid offer less shade than trees that spread their limbs wide, such as beeches, chestnuts, and many oaks. To buffer your property against the heat of the sun, keep three things in mind:
1. Plant deciduous shade trees where they will deliver the greatest cooling benefits. Sunshine falling on the west and south side of a building is usually the most intense. Site trees to shade these surfaces in summer. Come autumn, the leaves drop, allowing in sunlight to brighten our gloomy winter.
2. Consider canopy shape and placement. To shade your home at mid-day, either plant a broad canopy that shades the roof and invest in covered gutters to prevent leaf accumulation, or plant trees half the distance from your house that their canopy spreads. For example, if a tree grows 40 feet wide, plant it 20 to 25 feet from your house. Your roof will be exposed at mid-day but the tree will shade out the worst of the hot afternoon sun and your gutters will remain clear.
3. Don’t forget the driveway. A paved driveway absorbs heat during the day and then keeps your surroundings uncomfortably warm at night. Reduce its heat load by planting trees nearby, which will also keep your car shaded and cool. Plant trees at least several feet from the pavement’s edge to allow room for root growth. In narrow planting spaces, select trees with fibrous rather than woody roots. Large shrubs with a tree-like habit, like crape myrtle cultivars such as ‘Natchez’, ‘Muskogee’, and ‘Arapaho’, are also less likely to lift and damage pavement.
A Cast of Shady Characters
Medium shade trees to 35 feet tall, suitable for one-story houses and smaller lots:
• Cladrastis kentukea—Yellowwood has a broad, rounded canopy with long compound leaves and intensely fragrant wisteria-like white flowers; may not flower until tree is 10 years old.
• Maclura pomifera ‘White Shield’—Osage orange is a tough, heat- and drought-tolerant Midwest native with glossy green leaves that turn butter-yellow in autumn; ‘White Shield’ is a fruitless male cultivar.
• Nyssa sylvatica—Black tupelo is pyramidal when young, developing a spreading crown with age and has good red to orange fall color.
• Pistacia chinensis—Chinese pistache is tough as nails with flaming orange or red fall color.
Large, broad shade trees over 50 feet tall
• Catalpa speciosa—Northern catalpa is a flowering tree with a rounded canopy and large, heart-shaped leaves.
• Gymnocladus dioica—Kentucky coffeetree is a large tree with compound leaves to 3 feet long and casts a dappled shade; ‘Espresso’ is a male cultivar without the castanet-like hard seedpods, which contain toxins. Drought tolerant once established.
• Quercus garryana—Oregon white oak is a West Coast native that requires no water once established. The picturesquely spreading limbs are strong and the roots deep, making the 50- to 90-foot trees very windfirm.
• Quercus frainetto—Italian or Hungarian oak is drought tolerant and has large, deep green leaves up to 8 inches long that look great all summer.
• Quercus shumardii—Shumard oak is native from the Plains to the Atlantic. This red oak relative can have good red
or orange fall color; buy in autumn to be certain.
• Styphnolobium japonicum syn. Sophora japonica—Japanese pagoda tree casts a light shade and flowers beautifully where summers are long and hot; choose canker-resistant cultivars.
• Taxodium distichum—Bald cypress is a deciduous conifer with a pyramidal growth habit suitable for narrow spaces. Medium-green needles turn orange before dropping in the fall.