Gardening on the roof

The sod-house pioneers who fashioned farms from the tall-grass prairies of the American Midwest probably would have scoffed had you called them visionaries.

Yet many of the natural components they used for their dwellings, including wildflowers and grasses and thick blocks of turf-covered soil that insulated everything indoors, are the stuff of today’s popular green-roof movement.

As in those earlier times, green-roof construction now is driven as much by its environmental benefits as by adding eye appeal to otherwise drab properties.

“It’s an ecological response to urban areas,” said Edmund Snodgrass, co-author with his wife, Lucie, of “Green Roof Plants: A Resource and Planting Guide. These roof gardens help manage storm rainfall and insulate against city heat buildup, he explained.

In pioneer days, “They didn’t have a heating and air conditioning guy in the neighborhood back then,” Snodgrass said. “They had to find a way of using nature to their advantage.

“Now we’re doing the same thing, but we’re moving intentionally away from total dependence on mechanical systems.”

Builders classify modern green-roof designs as “intensive” or “extensive.”

Intensive roof gardens require a great deal of structural support for such things as trees and trails, seating and a growing medium more than 6 inches deep. They also require irrigation and frequent maintenance in showcasing trees, shrubs and flowers.

An extensive garden, meanwhile, is built on a thin layer of growing material, requires little to no maintenance and supports drought-tolerant plants capable of surviving long periods of heat or winter cold.

Snodgrass owns and operates Emory Knoll Farms Inc. near Street, Md., which he described as the first nursery in the United States dedicated solely to the propagation and sale of green roof plants.

Roof gardens are built primarily over the flat roofs of large commercial buildings and plant sites, but growing numbers of residential developers are beginning to take an interest, Snodgrass said.

The green-roof phenomenon began to spread in the 1950s when some German cities encouraged building owners to convert their tar and pebble rooftops to vegetation. Since then, Germany has developed more than 50 square miles of green roof space and adds an additional five square miles per year, according to Christian Werthmann, an assistant professor of landscape architecture at Harvard’s Graduate School of Design.

The U.S. has only a fraction of the green roof space found in Germany, but a recent study by Green Roofs for Healthy Cities determined that green roof space in this nation grew 80 percent in the past couple of years.

Chicago leads the trend among all U.S. cities, having planted 300,000 square feet of green roof space in just one year, the nonprofit industry group said. That includes putting a vegetative cover over city hall.

Now, Snodgrass said, green roofs are being seen increasingly on urban residential buildings like condominiums, which are more flat-roof. “The single-family home with a pitched roof is still a little ways off.

“People are tending to use green roofs more on additions and garages rather than on whole roofs. You can cut a door into the roof to give it some additional utility,” he said. “A roof turned garden gives your family some extra living space.”

Green roofs also can retain up to 75 percent of a 1-inch rainfall, greatly reducing the pressure storm water puts on municipal sewer systems, the society says.

The society also advises that plant selection for a roof garden varies according to geography, soil depth and type, light conditions, accessibility and personal tastes.

Some green-roof layouts are more functional than others. The roof of a hotel in Vancouver, British Columbia, for example, is planted with an assortment of kitchen herbs used by its restaurants.

“I know of one in a Boston hospital that’s built just outside of one of the cancer wards,” Snodgrass said. “ ... It makes for a great healing garden.”

Aside from providing color and beauty in generally unused settings, green roofs also can furnish cover for a variety of wildlife species.

“Green roofs won’t bring back our elk and bison herds but they will attract larvae and butterflies and other pollinators, including honeybees,” Snodgrass said. “Killdeer and many different birds also nest on these roofs.”