STREET, Maryland — A burgeoning U.S. market for "green roofs" has greenhouse owners cultivating plants that help keep out the summer heat and winter cold while also managing storm water runoff and absorbing carbon dioxide.

Chicago put a green roof on its City Hall in 2000 and since then about 150 public and private buildings have followed, including a downtown McDonald's restaurant and an Apple computer store. The construction of green roofs has been spurred in part by the city's green building and green roof policies, which apply to new public buildings, and private developments and structures that are subsidized by the city.

In addition to requirements that a certain percentage of roof space be constructed as green roofs, the city is also awarding 20 $5,000 grants to help the owners of small commercial properties and residential buildings install green roofs.

"For a city, it really helps to solve a number of our problems," said Sadhu Johnston, commissioner of Chicago's Department of Environment.

"We're finding its brings temperatures down on a roof from 160 to about 80 degrees in the summer."

That can help fight sweltering downtown temperatures that claim the lives of the elderly and sick each summer, he said.

Chicago installed its City Hall roof after Mayor Richard Daley visited Europe, where a mature green roof industry already exists. Now, the City Hall roof also has two beehives from which honey is collected for sale at auction each year, said Larry Merritt, a spokesman for the Chicago department of the environment, whose office looks down on the building.

"From my window here were on the 25th floor you look out during the summer or fall and you see this beautiful garden as opposed to your run-of-the-mill black tar roof," Merritt said.

Chicago is not alone in its zeal for living and working under a living, breathing roof.

In San Francisco, dozens of plants were tested for a 2.5-acre (1-hectare) green roof planned for the new California Academy of Sciences building, which is to be completed in late 2008. And at Ford's Rouge Dearborn Truck Plant in Dearborn, Michigan, 10-acres (4-hectare) of green roof have been installed.

While many tout the environmental benefits, such as helping reduce the so-called urban heat island effect that can make downtown areas hotter than the surrounding suburbs, proponents say they also have practical advantages. Green roofs extend the life of roofs because they protect the roof from weather extremes.

While each installation is different, studies have found green roofs can cut summer cooling needs and winter heat losses by about 25 percent, according to Green Roofs for Healthy Cities North America, a not-for-profit industry association.

Ed Snodgrass, who runs a greenhouse in Harford County, Maryland said many of his customers are businesses, governments or institutions. Few residential homeowners install green roofs because the size of their homes usually prohibits them from enjoying the economies of scale available to larger buildings, Snodgrass said.

Many of the plants Snodgrass grows in the greenhouses on his Maryland farm come from an equally challenging environment, rocky alpine slopes. The hardy succulents can grow in three inches (7.6 centimeters) of soil, go long periods without water and have thin roots that won't work their way into the roof's waterproof covering.

Snodgrass said he rejected many varieties in his search.

"There are a lot of considerations when you start putting plants on a roof. It's not as easy to maintain as your front yard," said Snodgrass, who has a book coming out this summer on the topic.

The market for green roof plants is like the plants themselves, small and growing.

Many grasses, for example, go through dry periods where they can catch on fire, not a good trait for a plant that could be placed on the top of a downtown skyscraper. Others, such as some cactuses, can withstand extended drought, but may grow too big and become too heavy for the roof. Mosses and lichens are lightweight and can resist harsh environments, but can't control stormwater runoff. Plants that could potentially escape from the rooftop and become an invasive species are also screened out.

Roofs are brutal environments for plants as anyone who has tried to grow tomatoes on their deck can attest. Plants that can thrive in such conditions must be able to withstand high and low temperatures, lack of water and poor soil.

Snodgrass describes the plants he uses as "living machines" optimized to survive in harsh conditions, soak up rainwater when it comes and look good.

Many of the varieties originally came from Panayoti Kelaidis, director of outreach for Denver Botanic Gardens, and a fan of alpine gardens, Snodgrass said.

Kelaidis, who specializes in alpine plants and has traveled to South Africa and the Andes to study various species, said he "didn't really set about trying to develop plants for green roofs."

"We in rock gardening would like to think of it more as a coincidence," Kelaidis said. "I think they are delighted the plants are turning out to be useful."

Snodgrass, who began selling green roof plants in 1999, now produces the bulk of the green roof plants for the U.S. market, particularly the East Coast, Kelaidis said.

Many of the varieties Snodgrass grows are so-called ice plants from South Africa, a biologically diverse country that produces one out of every 10 plants in the world, including a large number of high-altitude plants.

"They make very beautiful mats of evergreen foliage, extremely durable and tough particularly good in the Eastern Atlantic seaboard," Kelaidis said.

Snodgrass said his farm has five year-round employees, growing to 10 in the summer, with about $400,000 in revenue last year, a figure that has doubled ever year for the past five years.

Customers include roofers, landscape installers, and building owners and managers, he said.

Snodgrass, whose family has owned the farm for generations, said he decided to switch from traditional farming in the 1980s, first turning to sustainable agriculture before settling on green roof plants.

The nascent green roof market has given him "the opportunity to redesign the farm, try something new and let the business grow with the industry. An established business doesn't have that luxury."

Source: Associated Press