The plants on most green roofs aren’t there just to look nice. They contribute to the roof’s performance and help knit the components of the assembly together. But to provide these benefits over the life of the roof—and to provide the owner with an aesthetic return on investment—the plants have to survive the strong heat, light, and wind conditions found on top of even low buildings.
That’s a tall order for most plants, and also for many designers. Selecting plants for green roofs, and the horticultural components that support them, is often a counterintuitive process that requires setting aside most of what you know about plants when they are planted at grade.
The plant palette originally chosen for the green roof at Sidwell Friends School in Washington, D.C. native flowering perennials—was in keeping with other landscape features in the school’s sustainability-focused master plan. But while these species are beautiful and, on the ground, relatively carefree, they don’t work on a basic green roof assembly such as the one on the school, which had a thin layer of mineral-based growing medium (composed of mineral aggregates) and no irrigation. In this environment, the specified plants died, leaving the medium exposed and, soon, sprouting a motley assortment of weeds. “We quickly saw that the plants couldn’t take the D.C. summers,” says plant manager and head of buildings and grounds Steve Sawyer. And Sawyer’s small grounds crew, charged with maintaining the roof as well as the rest of the campus landscape, had no maintenance plan to guide them through the situation.
The school administration brought in Furbish Co., a Baltimore-based design/build firm with considerable green roof experience, to try a different approach. The new roof “is a more conventional extensive green roof, mostly a sedum meadow with some accent plants,” says owner and founder Michael Furbish.
The Furbish team started with new growing medium; the medium in the earlier installation was extremely coarse. Working with it would have required a lot of amendment, and killing the bank of weed seeds in the system would have taken years. After laying down a new course of lightweight medium specially blended for extensive green roofs, Furbish’s team planted a mix of tough, drought-tolerant plants, mostly Sedum species. In comparison to the flowering perennials previously planted on the roof, the Sedum species conserve water in their cells, which allows them to survive long periods of drought and makes them better suited to the hot, dry, windy roof environment. In some areas, the medium was mounded a few inches higher than the typical 4-inch depth to support some flowering accent plants. Installed in spring of 2010, the plants are growing in well, despite this year’s scorching summer.
Perhaps most important to the new roof is that the Furbish team maintains all of its installations for two years. Furbish team members will conduct site visits, with more checks front-loaded during the growing season to ensure the plants are well-established. As a result, they are able to catch any problems early and swap out plants that are struggling. After each visit, the team will send reports, along with photos, to Sawyer. In addition, he is building an archive of maintenance reports for future reference.
Linda McIntyre is co-author of The Green Roof Manual: A Professional Guide to Design, Installation, and Maintenance, published in August 2010 by Timber Press.