For a host of environmental and aesthetic reasons, roof-top vegetation and gardens are catching on -- though there are still many questions about how and when to apply the technique.
"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.
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.""