Gardens in the sky

Green roofs can keep us cool, filter pollutants and even feed us

It’s a bright, sunny day, unseasonably hot for late September. Tufts biologist Colin Orians crouches on the roof of Tisch Library, where the temperature is well over 90 degrees. He lays his palm on a container of soil: It’s sun-baked hot. The adjacent container holds a carpet of green sedum leaves, which are cool to the touch. A new garden is beginning to take hold.

Biologist Colin Orians and some members of the Green Roof Collaborative take stock of how the garden grows on the top of Tisch Library. © alonso nichols

Rooftop gardens brimming with pots of exotic plants connote cocktail party goers overlooking an urban skyline as the sun sets. But these atmospheric gardens also represent a new chapter in the urban environmental movement. Call it what you will, covering a rooftop with plants helps insulate the building below, keeping it cooler in the summer and warmer in winter.

Green roofs are hardly an invention of urban yuppies: The hanging gardens of Babylon were planted on rooftops, and after World War II, Germany made green roofs mandatory in all its major cities to prevent rainwater from washing into aging sewer systems. Germany is still considered the green roof capital of the world, with 10 percent of its buildings sporting rooftop gardens.

The practice is slowly catching hold in the United States. Chicago has 200 green-roof buildings, including City Hall, and in Dearborn, Mich., Ford Motor Co. constructed an automobile plant with a 10.4-acre green roof, the largest in the world.

In addition to serving as home-grown insulators, green roofs act as natural filters because the plants remove contaminants, such as nitrogen and phosphorous, from rainwater and carbon dioxide from the air. The rooftop plants absorb water, which then evaporates and creates a cooling effect. Where there is a critical mass of green roofs, they can actually reduce the effects of what are known as urban heat islands—metropolitan areas where expanses of asphalt and concrete make cities warmer than surrounding areas. Green roofs also keep rainwater from washing into storm drainage and sewer systems, where it is treated unnecessarily at wastewater treatment plants. They provide habitats for birds and other urban wildlife.

A player in the garden
Tufts University has joined the green-roof movement with the creation of the Tufts Green Roof Collaborative, multidisciplinary programs and research aimed at learning more about what plants grow best in the Northeast. One of the collaborative’s goals is to influence policymakers to make green roofs a linchpin of the urban environmental movement.

“I think right now we have the opportunity to become a key player in this kind of research,” said John Durant, an associate professor of civil and environmental engineering, who is one of the investigators on the project. “We’re not a state agricultural school, so we don’t have huge tracts of land. But we have the opportunity to become a player in urban-garden, green-roof research.”

This past summer, the Facilities Department hauled 20,000 pounds of soil and plants, including four kinds of sedum, Echinacea, salvia and wild strawberries, to the Tisch Library roof on the Medford/Somerville campus. The green roof experiment has brought together faculty and students from the School of Engineering, the School of Arts & Sciences and the Friedman School of Nutrition Science and Policy.

The summer’s long drought taught Colleen Butler, a Ph.D. student in biology, that hardy sedum, a succulent that stores water in its leaves, can thrive when it’s dry, and that even strawberry plants can bounce back from a drought. Butler painstakingly photographed all the plants every week, documenting how they responded to plenty of sun, but little water. Under the guidance of Orians, who is the lead investigator for the Green Roof Collaborative, Butler expects to spend the next four or five years studying green roofs for her doctoral dissertation in plant science.

She started by doing a broad survey to see what plants survive and why. Now she is studying how the cooling effect of blankets of sedum might facilitate the growth and survival of a more diverse plant community. “We are excited to build on Colleen Butler’s research to identify other projects that would be suitable for graduate and undergraduate research,” said Orians.

The green roof project began as a casual discussion among three Tufts faculty members who were fellows at the Jonathan M. Tisch College of Citizenship and Public Service. Orians had long been interested in learning more about what plants might thrive on a roof, while Durant wanted to study how urban gardens absorb water and filter pollutants. Kathleen Merrigan, the director of the Agriculture, Food and Environment Program at the Friedman School, had been interested in using part of the Rose Kennedy Greenway in Boston for urban agriculture and wondered how green roofs might be used to grow food. The three decided to collaborate, with the goal of establishing a working green roof that Merrigan hopes will influence policymakers to encourage green roof technology.

A Tufts urban garden grows in the shadow of the Boston skyline. © alonso nichols

“Imagine,” said Durant, “a grid with different kinds of plants and being able to test them by a variety of different measures: their ability to thrive, their ability to filter pollutants from the atmosphere and their ability to cool the roof. I doubt there is one plant that would do it all. We want to create a mixed garden. That would be the ideal situation. If we could eat them, all the better, but we also need to see if the food grown will be safe.”

Once the group decided to proceed, myriad problems had to be solved. After getting permission to use the Tisch Library roof, they had to figure out if it could support the weight of the plants and soil. What kind of soil mix would nurture the plants but also be lightweight? Merrigan also wanted to make sure visitors to the Tisch rooftop, including the many prospective students and their parents who stop there on admissions tours to view the Boston skyline, knew what was going on. So a sign that explains the project has been installed. “It’s also a lovely place to show some of the strengths of Tufts,” she said. “We’re very strong on environmental issues.”

Rooftop crops
Each faculty member began drawing in students to help. Merrigan is working with Jennifer Obadia, a doctoral student in nutrition who is researching what food might be grown on the roof. They planted mint and oregano from seed, but the herbs never germinated. The strawberry plants survived the drought.

“Herbs are perennials and don’t require a lot of maintenance like traditional vegetable crops,” said Obadiah. “I’m interested in actually trying food gardens on rooftops. That requires maintenance, the same way a backyard garden would. I’m curious to see how eggplants or tomatoes would thrive because they come from places with hot climates, similar to that of the rooftop environment.”

Merrigan hopes information gleaned from the Tisch green roof will help persuade state and federal policymakers to encourage their development. She envisions a green-roof corridor, perhaps along the Mystic River Watershed, a 76-mile area that encompasses 21 communities north and west of Boston. “If we go to [Boston Mayor] Thomas Menino and [U.S. Rep.] Edward Markey, D-Mass., we want to be able to share with them what we think the ramifications are and what are the policy potentials as environmental interventions,” she said.

Durant, meanwhile, is working with Daniel Brady, E10, a chemical engineering major, who is developing a system to collect and test rainwater for pollutants such as toxic metals. Another member of the team is Mara Gittleman, A09, an American studies major, who has created a website for the project: She also wrote a research paper on the history and benefits of green roofs.

Durant hopes to learn more about how plants filter pollution from rainwater and the effect this has on soil: “Depending on the kind of pollutant, plants can actually pick up chemicals with the water they absorb, and chemicals can be stored in the plants. Some pollutants, such as toxic metals, can react with cellular constituents in plants and be transformed into less-toxic forms.”

He is also well aware that urban vegetable gardens—whether on a roof or in a backyard—may pose some health risks. “Kathleen Merrigan’s problem is particularly interesting to me because I know from reading the literature that a lot of air pollutants accumulate in vegetables,” Durant said. “We theorized there might be some varieties of vegetables that can grow in urban environments and not accumulate pollutants.”

Orians said he hopes that the agricultural component of the project will help the Tufts community become more aware of how food is grown. “One thing our society misses is where things come from,” he said. “The more you can get people involved in agricultural production, the better.”

Marjorie Howard is a senior writer in Tufts’ Office of Publications. She can be reached at This story ran in the November 2007 issue of the Tufts Journal.