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Environment / Sustainability

Rooftop gardens transform urban landscapes

September 24, 2010
By Paul Miller

Visit a flat roof just about anywhere, and you'll see a barren landscape of temperature extremes and water-phobic material. But emerging research at Colorado State is looking into ways of changing those sterile landscapes by creating roofs alive with green, growing plants and grasses.

Slow runoff, cool building in summer

Jennifer Bousselot, a 2010 CSU Ph.D. graduate, collaborated on a green-roof research project at the EPA Region 8 headquarters in Denver.

A roof on the west side of the Microbiology Building, in fact, hosts pine leaf penstemon, blue fescue, ice plant, sage, cactus, and other low-growing plants. Jim Klett, CSU professor of horticulture, says plants on this roof – or any other roof – help slow runoff and cool the building in the summer.

Although Klett is interested in the small test plot, he gives credit for the initial green roof idea to Jennifer Bousselot, a 2010 CSU alumna and horticulture lecturer at Iowa State University.

Effectiveness of plant species

As a Ph.D. student here, Bousselot collaborated on a green-roof research project at the EPA Region 8 headquarters in Denver. For two years starting in 2008, she studied the effectiveness of plant species and growing mediums.

Fellow collaborators evaluated how green roofs affect storm-water quality and quantity and whether plants eased the heat-island effect, a phenomenon in which metropolitan areas are significantly warmer than surrounding rural areas.

“Summertime temperatures on conventional asphalt roofs can hit 150°F or more,” she says. “Green roofs help lower the temperature of buildings and thus reduce urban air temperatures, mitigating the heat-island effect.”

Cleaner water returned to surrounding watersheds

Bousselot adds that green roofs are remarkably effective in slowing and filtering water runoff. “Cleaner water is returned to surrounding watersheds,” she says. “Green roofs also create wildlife habitat and add natural beauty to cityscapes.” At the EPA project, Bousselot found that zeolite, a commercial adsorbent she mixed into the growing medium, worked well on green roofs because of its water- and nutrient- holding abilities.

The United States is generally behind other countries in green-roof projects, Bousselot says, although more cities are embracing natural roof plots. In Dearborn, Mich., Ford Motor Co. planted sedums on its 10-acre factory roof to create one of the largest green roofs in the world.

Europe has been pioneer in green roofs

Green thumbs are working on rooftops all over the world. In Japan, rice is grown on roofs to supply small breweries, and in Vancouver, a chef ’s garden grows herbs, apples, and other fruit for diners’ plates. Europe has been a pioneer in green roofs for decades; in Basel, Switzerland, foliage is mandatory on new flat-roofed buildings. In London, rooftop wildflower meadows bloom adjacent to row houses.

With improvements in material, lower costs, and increasing public interest, a walk in the park may soon mean a stroll through rooftop gardens hundreds of feet above urban sidewalks.

Originally published in Colorado State Magazine, Fall 2010.