The Japanese knotweed on Philadelphia’s vacant lots can grow 10 feet high and thick enough to hide a rusty trailer. Over the last decade, the Pennsylvania Horticultural Society (PHS) has hacked through the overgrowth in thousands of these lots, replacing dumped tires and mattresses with a tended lawn, a couple of trees, and a tidy wooden fence.

The spruced-up open spaces end up making whole neighborhoods safer and healthier. “The purpose of the greening program was really to...

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