Wildflower is published quarterly by the Wildflower Center. Its content is national in scope with articles about the conservation and use of native plants as well as news from the Wildflower Center. A subscription is provided to Wildflower Center members as a benefit of membership.
Raising the Roof Adapted from an article by Julie Bonnin - Fall 2006The centuries-old practice of green roofs is experiencing a revival of sorts in North America. Over the past decade several high-profile green roof projects -- from Chicago's City Hall to a Ford truck assembly plant in Michigan to the Gap's business offices in California and Utah's Church of Jesus Christ of Latter-Day Saints headquarters -- have helped put the practice on the map here in the United States.
The advantages of green roofs -- especially those that reflect the regional character and plant heritage of a particular place -- are too important to ignore. Green roofs have been shown to reduce the urban heat island effect -- a rise in temperature due to the loss of green space that can make temperatures as much as 10 degrees higher in cities than in surrounding regions -- as well as the impact of storm water runoff. Their shading and cooling properties can increase the energy efficiency of buildings and the lifespan of the roof. Or, as happened atop Chicago's City Hall, green roofs can re-create a habitat that draws insects and dozens of kinds of migrating birds.
That U.S. green roof square footage grew 80 percent from 2004 to 2005 is partly a reflection of public policy initiatives that encourage green building, says Steven Peck, president of Toronto-based Green Roofs for Healthy Cities.
Projects at the Wildflower Center are among those helping to bring more information about how to build green roofs with native vegetation to the public and private sectors. For a green roof demonstration project atop an Austin, Texas, Starbucks, Wildflower Center environmental designer Heather Venhaus chose a plant mix hardy enough to endure Central Texas' climate for the 8,000-square-foot rectangular roof. Blooming plants like agarita, rock rose, skeleton leaf, goldeneye and crossvine are in the foreground, and Mexican feather grass runs in bands throughout the garden. The broader picture, as concerns about global warming heat up, is a need for building practices that don't just minimize environmental impact, but that are also "restorative," according to Peck. These are practices that "give back more than they take over the course of their life cycle," he says. Green roofs are among those that do.
While a green roof can contribute to softening the built environment, aesthetics are often secondary to survival when it comes to choosing a native plant palette for your region. Not just any indigenous plant will do. Since most green roofs offer a shallow growing medium, those that require deep soils are not going to be happy, notes the Wildflower Center's Heather Venhaus.
Few generalities apply from one region to the next. Prairie grasses on a Michigan State University test roof, used to being anchored by root systems that can grow up to seven- feet deep, did not survive their rooftop debut. Texas grasses, adapted to spread roots horizontally in shallow soils, are a natural farther south. Sedums, the favorite green roof plant in Europe, are native to many parts of the U.S. and have thrived without irrigation on green roofs in the Midwest, Northeast and some parts of the Southeast. In any part of the country, perennial plants or those with a root system that will remain intact should be used to prevent erosion. Carefully research the plant you have in mind, study its growth requirements and select accordingly.