How many mayors does it take to change a lightbulb?

Mayor Nutter and five other current and former mayors didn't exactly address that question yesterday, but they did take the stage together at the opening session of a conference at the Philadelphia Convention Center to talk about changing their cities through economic redevelopment.

The three-day conference, dubbed Brownfields 2011, is intended to promote so-called brownfield sites - defined by the Environmental Protection Agency as properties whose redevelopment or reuse may be complicated by hazardous materials.

Nutter and Connecticut Gov. Dan Malloy (former mayor of Stamford, Conn.) and the mayors of Lansing, Mich.; East Palo Alto, Calif.; Flint, Mich., and Youngstown, Ohio - discussed how their cities were renovating brownfields for revitalization.

Nutter highlighted examples of Philadelphia's brownfield use, including commercial development in the Navy Yard after the Navy closed some facilities, and growing food commercially in Kensington on raised beds instead of in the ground.

"I focused on both the green economy and brownfields out of both an imperative and an opportunity," Nutter told the gathering of more than 100 people.

Malloy mentioned his involvement as mayor in one of the largest brownfield redevelopments in New England. He learned that with brownfield reuse "it's a long period of time to get payoff" but that they're important in improving cities.

Youngstown Mayor Jay Williams said redeveloping brownfields is the reason his city, hurt by a decline in steel manufacturing, has "survived as a community."

Nutter, meanwhile, said it is important to "be creative" and to "play to your strengths."

"I'd like to think if there was no economic crisis, we'd be fairly smart," Nutter said. "But in an economic crisis, you have to get near genius levels."

Brownfields 2011 is sponsored by the EPA and the International City/County Management Association. About 7,000 people are expected to attend the conference, said Danielle Goodwin, of the Washington, D.C.-based association.