Driving around the city, you see so much noncontiguous verdant space that the Green Acres theme song comes to mind. So do sheep.

The city has so much abandoned property and so many lots that no one's quite sure of the number. In 1999, a survey found 31,000 vacant lots, 26,000 vacant houses, and 2,500 commercial vacancies.

A decade later, the city's Redevelopment Authority (RDA) estimates 40,000 abandoned properties and vacant lots, almost 70 percent of them privately held. But the Pennsylvania Horticultural Society says there are 40,000 vacant lots alone, a quarter of which it manages, 23 acres in total.

Green space can be wonderful, but too much makes neighborhoods less safe, without generating an additional cent of revenue to the city, which ultimately foots the bill of neglect. Abandoned properties are disasters waiting to fall when one out of six owners doesn't bother paying taxes.

"The city has to get its act together about vacant land," says Terry Gillen, executive director of the RDA, which controls 3,500 properties from the disbanded Neighborhood Transformation Initiative and lists them on its Web site, hoping to entice sales. Last week, an NTI audit found that millions in bond funds were mismanaged.

Flint, Mich., runs a land bank that seizes control of abandoned properties after one year and foreclosed properties after two. (There's special dispensation for owner-occupied hardship cases.) State Rep. John Taylor (R., Phila.) has introduced a bill establishing a land bank to take moribund property from neglectful owners.

"Philadelphia has to have the will, direction, and resources to go through the process of bringing in an inventory, taking over properties and disposing of them. Up until now, there's been no plan," Taylor says. "We can provide tools for the municipalities, but we can't make them act."

Philadelphia hasn't opted to shrink intentionally like Flint. Evidence suggests that for the first time in a half century - good news alert! - the city may not lose population this year.

One of Philadelphia's great assets is affordability, especially with housing. The trick is to sell abandoned properties not to more absentee scofflaws but to people who will invest in and improve them.

"When a city is in decline, its institutions reflect the culture of decline. The city doesn't know how to think about growth," says Jeremy Nowak of the Reinvestment Fund, a nonprofit neighborhood-revitalization program. "And local politicians would rather control power than facilitate growth and change. True entrepreneurial growth, business or civic, takes power out of the hands of the local political class."

Sure enough, the NTI program still has $30 million left for development. Rather than being allocated to one agency, it will be divided 11 times among Council members and the mayor.

Nowak knows about abandoned neighborhoods. "You know that block of Strawberry Mansion where there's only one house standing, at 31st and Montgomery?" he asks. "That's the house I grew up in." If neighborhoods aren't revitalized, it's impossible to stimulate commercial development. We're not talking dog boutiques or gastropubs. We're talking grocery stores.

"We know, in the long run, that lots that are greened up greatly increase property values," says the horticultural society's Blaine Bonham. "But I'm a proponent of keeping cities dense, not suburban. At some point they should be developed to revitalize the urban fabric. The city is at a critical juncture."

The consensus seems clear: Out of all these vacancies can come great opportunities for jobs, revenue, growth, and, perhaps most of all, neighborhoods.