PHILADELPHIA — As a girl growing up here, Natalie McHugh used to play on the vast expanses of asphalt and concrete that coated the city’s schoolyards.

She didn’t question as a student why there weren’t more trees or think twice about the blacktop during her decades as an educator at Southwark Elementary School in South Philadelphia. Then, one day, as McHugh monitored recess with the school’s principal, her boss said something that made her see all that asphalt differently: “He said, ‘This is not good, Nat.’ And I was like, ‘What do you mean?’” she said.

Since then, Southwark’s schoolyard has been transformed, one of more than a dozen in Philadelphia where the Trust for Public Land, a nonprofit that helps create public parks, has done things such as plant trees, build rain gardens, and install play equipment and an outdoor classroom. Activists and parents have long worked, school by school, to turn asphalt playgrounds into islands of greenery. But as climate change sends temperatures soaring, the movement to