I fell in love with my West Philadelphia neighborhood in a single afternoon. On a whim, I took a walk from the heart of Center City out to 50th and Baltimore, and spent hours weaving in and out of the largely residential blocks along the way.

I couldn't get over how green everything was.

It was still undeniably the city - with the trolley and the African grocery stores, the sirens and the rowhouses - but the trees lining the residential streets lent the neighborhood a serene quality utterly missing from many other sections of the city.

Less than a month later, my wife and I had bought a West Philly home on a block shaded by a series of majestic, 80-year-old London plane trees, with canopies so wide they covered the entire width of the street.

The appeal of trees in city settings is so simple and obvious that it would hardly seem to warrant mentioning. And yet, for many years, Philadelphia and other cities managed to overlook the psychic, environmental, and economic benefits that a healthy urban forest could provide.

Trees planted in past generations aged and died and were not replaced. So, bit by bit, the city has grown less verdant. The sidewalks have become a bit hotter. And more storm water has flowed uncaptured to the sewers, carrying pollutants that the city's overtaxed water-treatment plants must filter out.

That indifference to the near-magical abilities of urban trees is fading away fast, both in city government and among Philadelphia residents.

Two years ago, the Nutter administration set for itself the almost outlandish goal of planting 300,000 trees by 2015. So far, about 70,000 saplings have been planted.

"Are we going to get to that goal? I don't know," acknowledged Michael DiBerardinis, a deputy mayor and chief of the city's Parks and Recreation Department. "I do know we're going to make a good effort."

The trouble is that planting a classic street tree - one that gracefully takes the edge off urban life - is an expensive proposition. The trees have to be reasonably mature at planting, if they're to survive. And then there's the matter of cutting a pit into the sidewalk. All told, it can cost $400 or $500 a pop. Given that the city's annual tree-planting budget is tiny, there are really funds only for a few thousand street trees a year.

This seems like an area where the city should spend a bit more, even in these lean times. Tree-planting is the bluest of blue-chip investments, a low-risk, high-reward proposition.

Nothing makes this clearer than PhillyTreeMap.org, a captivating, crowd-sourced inventory of Philadelphia's trees and potential planting sites put together by Azavea, a local data firm.

The site relies on the city's arbor enthusiasts - and there are many - to input the trees' vitals (species and trunk diameter, for instance), the better to assess where the urban canopy is healthy and where it needs work.

What makes PhillyTreeMap.org so gratifying is its instant calculation - based on federal research - of the annual dollar value of a given tree.

Those massive London planes outside my house? The one I measured yields a return of about $387 a year. The shade it creates cuts down on neighbors' cooling bills ($300), removes about six pounds of air pollutants ($24) and 722 pounds of carbon dioxide ($2.41) each year, and intercepts 6,212 gallons of storm water ($61). And that doesn't even take into account the well-documented boost to property values that mature trees provide.

"We saw a way to mitigate the city's lack of resources a bit by harnessing volunteer citizen energy to help build a higher-quality inventory of the city's trees," said Azavea CEO Robert Cheetham.

The company built the platform - which includes a companion iPhone app released Thursday - at no cost to the City of Philadelphia, but has managed to turn a profit nonetheless by selling it to other towns that are also trying to get a handle on their urban forests.

"Our hope is that OpenTreeMap makes it easier for the city to make the case for investing in trees. It makes it easier to show how big a benefit you get over a five-, 10-, 20-year timeline," said Cheetham.

One relatively inexpensive way to a greener city is a new initiative that gives free trees to residents, for planting on their own property. The next round of free trees is available in October. Residents can request a tree from TreePhilly.org.

"This is a tangible, durable way for residents to improve the city for the long term," said DiBerardinis, and "make a major contribution to the future."