The city's newest green roof is also, in all likelihood, its smallest and oddest - a cheerful puff of plant life atop a bus-stop shelter at 15th and Market Streets.
At all of 60 square feet - hardly as big as a living-room rug - it might not stop more than a few buckets full of rain from entering the city's aged storm-water system.
But its mission remains ambitious: To show the crush of passersby at one of the city's busiest intersections the value of absorbing rainwater and stalling its flow.
"We basically see this project as an opportunity to inspire homeowners," said Tiffany Ledesma Groll, the project's coordinator. "A lot of the green roofs that are really fabulous and gorgeous are on high-rises" - invisible to most. "We thought it would be fun to bring it down to eye level."
As the latest installation underscores, green roofs are booming in the region.
In Philadelphia alone, 52 green roofs have sprung up, totaling 10.6 acres by the count of the Water Department, which is tracking them as part of its effort to amass data on storm water.
Storm water is a historic problem for Philadelphia because most of the city's underground pipes combine rainwater and sewage. During storms, the system becomes overwhelmed and water polluted with raw sewage and road dirt overflows into streams and basements.
"Every acre manages or eliminates roughly a million gallons of storm-water runoff a year in the city," said Chris Crockett, acting deputy commissioner of environmental services for the Water Department.
The roofs installed so far manage enough storm water every year to fill a swimming pool the size of City Hall's block with 5.5 feet of water, Crockett said.
The roofs also are seen as a way to improve the efficiency of buildings by keeping them cooler in summer and warmer in winter.
Green roofs top sites including the main Philadelphia Free Library, the Peco building, the Friends Center, Drexel University dorms, the Curtis Institute of Music, the Penn Charter Performing Arts Center, the new Kensington High School for the Creative and Performing Arts, the Urban Outfitters corporate headquarters, and numerous private homes.
At St. Joseph's University, researchers have planted the top of the science building with four types of green roofs, including more than 20 plant species. They also have installed sensors so they can collect data on water absorption, temperature differentials, and more.
An additional 34 green roofs, totaling about 11 acres, have been proposed.
Officials in 2009 said Philadelphia was second only to Chicago in the number of green roofs, but the ranking is squishy. "It's hard to get true numbers," Crockett said. It depends on who's counting.
Suburban green roofs are less assiduously tracked, but a national database shows green roofs in Pennsylvania and New Jersey atop government buildings, medical centers, college buildings, condos, private homes, and a mall.
These include Swarthmore College residence halls, the Colorcon global headquarters in Harleysville, and a pavilion at the Elmwood Park Zoo in Norristown.
Philadelphia's newest green roof increases the city tally only minimally - by .0014 acres - to be sure.
And surely the big focus is on the recent state approval for the Water Department's $2 billion storm-water plan, which has been called both innovative and a game-changer.
Nevertheless, for the moment, anyway, it's this little jot of color atop the bus shelter that has officials grinning.
"This is a baby step" in the integration of green infrastructure into our everyday cityscape, Crockett said. "Big dreams start with little ideas."
Officials plan to formally unveil it Wednesday as a tiny trophy for their annual assessment of the city's Greenworks plan. Proposed by the Nutter administration in 2009, it details actions to make the city greener.
So on Tuesday, workers from Roofmeadow, a Philadelphia green-roofs company that is doing the installation pro bono, were busy topping the shelter with its mini-oasis.
Off came the roof, to be replaced by slanted sheets of aluminum that have gutters for excess water to flow through. Next came layers of fabric and barrier cloth, and then three inches of soil.
Finally, they planted a fuzz of pink dianthus, multicolored portulaca, and sedum.
Roofmeadow also has committed to maintaining the roof for its first two years.
Charlie Miller, president and founder of Roofmeadow, said the roof "will give people a sense of familiarity . . . how it contributes to the quality of life."
His son, Ari, designed the installation so it could be replicated as a "kit" for other bus-stop roofs in the city.
"It's fun. It's cool. It's the right thing to do," said Rina Cutler, deputy mayor for transportation and utilities, who can see the bus-stop roof from her office but nevertheless stopped by Tuesday for a look.
Katherine Gajewski, the city's director of sustainability, said the new little roof was "a great example of the creative approaches we can take to demonstrate sustainability to Philadelphians. And have them interact with it in their daily lives, even at unlikely places like 15th and Market."
Signage will direct people to a website - www.phillywatersheds.org/greenbusshelter - with more information about things residents can do.
The site at 15th and Market is, to officials' knowledge, the only green-roofed bus shelter in the nation. San Francisco used to have one, but it was dismantled when the structure itself was replaced.
In a perfect world, Roofmeadow's Miller said, the new bus-stop roof wouldn't be surprising. It would merely be one of myriad sorts of green roofs.
He's not even convinced this roof is the smallest in the city. "I'll bet there's one on somebody's back deck somewhere," he said.
Either way, he said, the bus-stop roof "will certainly be the most visible green roof in the city."