Start collecting your plastic bottles, Philadelphia.

If any of the five Democratic candidates gets elected mayor and keeps his word, the city will finally have a recycling program with weekly pickup and a single bin for all a household's items.

Not stopping there, all five vow to cut city energy use, encourage "green" building, and plant enough trees to populate a small forest.

Suddenly, more than in any recent mayoral campaign, the environment is an issue.

It does not, to be sure, garner the same urgency and importance as violence or education.

It is not, as political consultant Neil Oxman puts it, "a defining issue" - not "the reason someone is voting for Candidate A over Candidate B." Oxman has worked every mayor's race since 1979; this time he is Michael Nutter's strategist.

Sure enough, when Chaka Fattah gave his opening statement at a candidate forum on the environment in February at the Academy of Natural Sciences, it did not take him long to switch gears. Within several sentences, he segued to poverty and how it should be "at the forefront of every discussion."

But such frequent mention of the environment is still a significant departure from past campaigns.

All five candidates have put out position papers - two pages each from Dwight Evans and Tom Knox, four from Bob Brady (just a week ago), nine from Nutter and 10 from Fattah.

That all five attended the academy's "sustainability" forum and a "green" forum a few weeks later at the Philadelphia Flower Show did not escape eco-activists' notice.

"I've never seen the environment mentioned as much as I have in this campaign," said Joseph Minott, head of the Clean Air Council for 25 years.

Neither have groups such as Minott's been this intent on a mayor's race before. It's the first time Clean Water Action - which once focused solely on state and federal campaigns - is endorsing a mayoral candidate. The group has met with each candidate.

So has the Pennsylvania Horticultural Society, to educate and agitate. "The issue was not who was going to be better," said J. Blaine Bonham Jr., the society's executive vice president, "but that one of them is going to be our mayor, and all of them have to be smarter about these issues."

Green is the color of the moment, mayor-wise. Last week, Mayor Michael R. Bloomberg proposed 127 ways to make New York "the first environmentally sustainable 21st-century city." On Wednesday, Mayor Street offered a plan for reducing Philadelphia's greenhouse gas emissions. He even said he and his wife now drive hybrid vehicles.

Advocates say the interest in "green" issues reflects not only a rising national focus on global warming, but that the environment is increasingly seen as linked to other issues.

Such as safety: Studies at the University of Illinois and elsewhere have shown that, all other things being equal, a green community has less crime than a barren one.

Or health: Walkable parks help reduce diabetes and obesity. Air pollution matters more "if you talk about how asthma affects 20 percent of our children," said Patrick Starr, vice president of the Pennsylvania Environmental Council.

Or the budget: The city controller's office has estimated Philadelphia could save up to $17 million a year by recycling 37.5 percent of its waste. Now, the rate is just 7 percent, one of the lowest rates in the country.

"Green" issues, too, have caught on with neighborhood-level activists.

Kristin Shipler, director of civic engagement for Congreso de Latinos Unidos, says lower-income neighborhoods typically bear the brunt of pollution and other environmental ills. She also laments, "We have green areas, but they're not safe to walk in because of glass, debris and trash."

Congreso is one of 82 civic organizations, including youth groups, unions and the NAACP, that have already done some of the next mayor's homework.

They have formed the Next Great City Coalition, which touts 10 actions it says the next mayor can take - and the city can afford - within one four-year term. Each has already been done in a "peer" city.

The candidates all say they support this plan, and one proposal in particular has gained traction: planting trees.

Philadelphia has lost 200,000 street trees in a decade, the coalition says. It has half as many as Baltimore, a third as many as Chicago's. But each $1 invested in planting trees yields a $2.80 return, the group estimates; trees ease pollution, cool buildings, check stormwater runoff and raise property values.

The candidates' response has been like a bidding war.

Nutter and Knox say they would plant 23,000 trees, as the coalition suggests. Evans promises 50,000; Fattah, 100,000 (over eight years, if he's reelected.) And Brady? "As many as we possibly can," a spokeswoman said.

"They're all falling over each other to out-tree-plant each other," said Christine Knapp, an outreach coordinator with Citizens for Pennsylvania's Future.

The commission also wants the next mayor to:

Rezone the riverfronts to preserve public access and take advantage of "blighted, vacant or underused" river properties.

Put pollution controls on older city trucks to curb soot, which aggravates asthma and other ailments. As of last week, a national asthma group ranked Philadelphia the nation's second-worst city for asthmatics.

Modernize the zoning code to guard open spaces and promote development near transportation hubs.

Generate renewable power (such as solar) that can be tapped in a crisis; construct new buildings to state-of-the-art energy and environmental standards.

Change Fairmount Park's administration from a court-appointed commission, which critics say lacks transparency; let the park reinvest income from its concessions and rentals.

Improve SEPTA stops to encourage use. Fix sewers to reduce backups and flooding.

And yes, boost recycling above that paltry 7 percent.

Said Knapp: "It's just such a no-brainer at this point."

To read our profiles of the candidates, along with their campaign promises, our reporters' blog and more, go to http://go.philly.com/mayor

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Coming Sunday

The Inquirer editorial board's endorsement of Michael Nutter.

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.