Friday is officially Arbor Day, but every day is Arbor Day for Treevitalize Pittsburgh and Tree Pittsburgh, which are working hard to grow a green canopy over the metropolitan area. More than 13,000 trees have been planted in Allegheny County since 2008.
Nearly all the new trees have been planted in parks and along streets, the traditional location for shade trees in densely populated cities. But there's another vitally important urban habitat for trees: parking lots.
Surface parking lots cover an enormous amount of land in urban areas, from 18 percent in New York City to more than 80 percent in downtown Los Angeles. Allegheny County has countless asphalt acres dedicated solely to cars. Because we must provide parking for cars at every possible destination, we've created far more parking spaces than we have cars. That means most parking stalls are empty most of the time -- an enormous waste of space.
Parking lots not only consume land, they're ugly and they're bad for the environment. Among other drawbacks, their dark expanses absorb the sun's rays in the summer, making hot cities even hotter. They generate a tremendous amount of storm water runoff, which is ecologically harmful and costly to manage.
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 5 square feet, but its trunk can rise 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 like Pittsburgh can get started simply by adopting an ordinance to require ample shade trees in all new and expanded parking lots. In 2003, my town of Pottstown adopted an ordinance for parking lots requiring one tree for every two parking spaces, evenly distributed throughout the lot. Nearly a decade later, it's showing tangible results.
Two years ago, for example, our local community college built a parking lot as a brownfield reclamation project next to a new environmental education center. The lot has LED lighting and bioswales to absorb storm water runoff, but its most prominent green feature is shade trees -- 130 of them, in a lot with spaces for 202 cars. Thirty years from now, the college's daily assembly of hardtops will be softened by a blanket of green.
When our ordinance was first 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 our county housing authority, which built an exemplary parking lot in a prominent downtown location.
When a new McDonald's restaurant was proposed, the owner wanted more parking and fewer trees than our ordinance called for. But we compromised on a plan that placed the restaurant -- distinctively designed -- close to the street with the parking to the side and rear, with one tree for every three parking spaces.
Often, the major objection is a loss of parking spaces. That was the case with the recent expansion of our hospital's parking lot. But when cars face each other in a double row -- a typical configuration -- there is an open space where four front car fenders come together. That corner can accommodate a 5-foot-square pit for a tree without intruding on the area where people enter and exit their cars. Using that design, our 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 planes grow big and strong -- and like the Timex watch, they can take a licking and keep on ticking.
Everybody loves parks, and trees are a way to create a park-like environment in formerly asphalted wastelands. While the suburbs boast horizontal green spaces, urban areas can enjoy vertical ones with 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, who writes for the Post-Gazette occasionally, is the author of "Save Our Land, Save Our Towns" and a former chairman of the Pottstown Planning Commission and the Pottstown Shade Tree Commission (firstname.lastname@example.org).