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American Society of Landscape Architects

 

June 2008 Issue

State of the Art
As green roofs build a track record in North America, what fundamentals apply?

By Linda McIntyre

State of the Art

While green roofs have been commonplace in Europe for decades, they have only recently emerged from oddity status here in North America, becoming downright fashionable in some quarters. But green roofs will only transcend the current moment of LEED-certified green hype if the benefits they offer—stormwater management, energy efficiency, long life, aesthetics—live up to the promise.

As the first generation of green roofs starts to mature, what are those who have been monitoring and studying them finding? Do they work as well in the more extreme North American climate as in temperate Western Europe? How far can you push the aesthetic envelope? Landscape Architecture looked at the evidence and spoke with experts in the field and the academy to see what basic facts can be gleaned from the ever-growing data. Their findings might help you make informed decisions on your own green roof projects.

Plants

What should I plant on a green roof?

While more and more designers are using native plants at grade, research and experience continue to confirm that sedums are usually the best choice for the majority of plants on extensive (thin profile) green roofs. Other plants can be integrated, ideally in more sheltered areas, to provide seasonal interest. Horticulture research at Penn State University and Michigan State University indicates that sedums establish easily and stand up to the extreme conditions on a rooftop, providing relatively consistent coverage. Though some individual plants might go dormant during dry spells or winter weather, they will rebound quickly. Sedums are also attractive to pollinators, and because they need little coddling, roofs planted with sedum are less likely to contribute to water quality problems by discharging runoff with high nutrient loads.

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