Green roofs: They grow on you.

Five generations of the Snodgrass family have prospered at Emory Knoll Farms in Harford County. Its 365 acres have evolved from a 19th-century dairy operation to a crop farm for most of the 20th. Now Ed Snodgrass runs a 21st-century roofing business -- one that is environmentally friendly and has nothing to do with slate, tin, or asphalt.

Mr. Snodgrass grows plants that make roofs green. In the past eight years, he has supplied colorful, resilient and fast-growing plants to cover nearly 2.5 million square feet of rooftops across the US.

Green roofs insulate buildings in winter, cool them in summer, and prevent weather damage all year, said Snodgrass, who has 15 greenhouses at his farm in Street.

Snodgrass, who with his wife, Lucie, co-wrote "Green Roof Plants," a primer on the technology, lectures on the advantages of living rooftops and spoke at the World Green Roof Conference in London last fall.

His talks focus on managing urban problems with living systems, not mechanical ones.

The door to his office, once his grandfather's milking barn, has a sign that reads "Green roofs. They grow on you." The sweet smell of citrus, from orange and lemon trees in the greenhouse, wafts in.

The roof has been green for about three years and has never needed watering or fertilization, he says.

"It keeps the office cool in summer because the sun is not beating down on the roof," he says. "It is working for us, and not making us work on its maintenance."

About 10 years ago, after visiting several cities in Europe, where green roof technology has long been popular, he established a website, Green Roof Plants.  It helped his business expand quickly, and he expects continued growth as businesses learn that green roofs deter runoff and lower energy costs.

"I do see a time when green roofs will be required, especially when people understand their value," he says. "The Earth is built on a balance between plants and animals. If it gets out of balance, there are consequences to be paid."

In Baltimore, Emory Knoll has worked on green roofs at Loyola College, Morgan State University, the National Aquarium, the Mikulski Workforce Development Center, and the National Bohemian Building.

Snodgrass can also point to his efforts at Harford Community College and many federal buildings in Washington. The Baltimore Orioles are looking to go green at Camden Yards, he says.

A green roof is a system with several layers that begin with decking and waterproofing. The layers, which include drainage, filter, and growing medium, work together as a unit.

The roof can be built on various decking surfaces, if the right engineering, waterproofing, drainage, insulation and protective components are put in place. (Herbaceous perennials are the most desirable rooftop plants because they offer the greatest color, texture and seasonal variability; those that work best include species of phlox, dianthus, and viola.)

Why should builders invest in green roofs, which cost twice the typical roof to install?

Plants produce oxygen, reduce heat and have so many other benefits. People are moving back to the cities, and cities have to become more livable. One way to do that is with more vegetation to make cities cooler, more attractive, and more energy efficient.

Living systems also run cheaper and better than mechanical ones. Immediate savings on operational costs are realized with a better-looking, energy-conserving habitat in an urban environment.

Maintenance is also low, with maybe an occasional weeding. Ornamental plants are desirable, but that is not the primary function. Look for low ground-cover vegetation and certain physical characteristics such as plants that live long, those that provide rapid ground cover, and those that will not do harm should they fall off.

Most desirable are plants that can live on rainfall and do not require irrigation.

(Editor's note: We invite you to visit the main page of the Monitor’s gardening site , where you can find many articles, essays, and blog posts on various garden topics.)