Parks and Recreation Commissioner Michael DiBerardinis has said he wants 300,000 more trees planted in Philadelphia by 2015, and an expansion of the city's canopy from 15 percent to 30 percent over the next two decades.

Here's a good place to start: parking lots.

Surface parking lots cover an enormous amount of land in urban areas, ranging from 18 percent in New York City to more than 80 percent in Los Angeles. Philadelphia and its suburbs have thousands of asphalted acres dedicated solely to cars.

Because of the need to provide parking for cars at every possible destination, we've created far more parking spaces than we have cars. That means most parking spaces are empty most of the time - an enormous waste of space.

Parking lots are not only an inefficient use of land; they're also ugly and bad for the environment. Among other drawbacks, these black expanses of asphalt absorb the sun's rays in the summer, making hot cities even hotter, and the storm-water runoff they generate is so harmful and costly to manage that Philadelphia plans to start taxing it in July, on the basis of impervious surface area.

But parking lots can be transformed into green spaces through the creative use of trees. Simple geometry makes this possible: The footprint of even a humongous tree seldom exceeds five square feet, even though the trunk can rise up five stories and unfurl a canopy the breadth of a house. If necessary, shade trees can be distributed throughout a parking lot without sacrificing a single parking space. (More about this later.)

Even a cash-strapped city such as Philadelphia can get started simply by adopting an ordinance to require shade trees in all new and expanded parking lots. In 2003, my town, Pottstown, adopted an ordinance for parking lots that requires one tree for every two parking spaces, evenly distributed throughout the lot. Seven years later, it's showing some tangible results.

Last month, Montgomery County Community College held a dedication ceremony for a new parking lot at its Pottstown campus - an unusual kind of observance, even in our car-crazed society. What was so special about the parking lot? Trees.

The parking lot is a brownfield reclamation project, and it has environmentally friendly features such as LED lighting and bioswales to absorb storm-water runoff, but the shade trees are its most prominent feature - 130 of them in a lot with 202 parking spaces. Thirty years from now, the college's daily assembly of hardtops will be softened by a blanket of green.

When Pottstown's parking-lot ordinance was passed, it was greeted with considerable skepticism and derision. We were accused of trying to create a forest where it doesn't belong.

But trees belong wherever people live and work. Fortunately, we got a big boost almost immediately from the Montgomery County Housing Authority, which built an exemplary parking lot in a prominent downtown location.

When a new McDonald's was proposed in town, the owner wanted more parking and fewer trees than the ordinance calls for. But we arrived at a compromise that placed the restaurant close to the street and the parking to the side and rear, with one tree for every three parking spaces.

Often, the major objection to such trees is a loss of parking spaces. That was the case with the recent expansion of a hospital parking lot in Pottstown. But when cars face each other in double rows - a typical configuration - there is a space between their front bumpers that can accommodate a five-square-foot pit for a tree. Using that design, the hospital was able to add 22 trees without sacrificing any parking spaces.

Of course, there is no more hostile environment for a tree than a parking lot. Most tree species won't get close to their natural size in one, but we've had great luck with London plane trees. People complain about their falling seed balls and peeling bark (neither of which harm cars or people), but plane trees grow big and strong, and, like a Timex, they can take a licking and keep on ticking.

Everybody loves parks, and trees are a way to create parklike environments throughout our urban areas. While the suburbs boast horizontal green spaces, cities can enjoy vertical ones through the generous use of trees.

Goethe exhorted us to encourage the beautiful, because the useful encourages itself. Trees in parking lots help us do both.

Thomas Hylton is the
author of "Save Our
Land, Save Our Towns"
and serves on Pottstown's Planning Commission and
Shade Tree Commission.
He can be reached at thomashylton@comcast.net.