On a warm and windy day in September, I visited Philadelphia to observe the sites that P.H.S. had remediated. Keith Green, a P.H.S. employee with a salt-and-pepper beard, picked me up in his blue Ford pickup truck, and told me that we’d begin by driving to West Philadelphia, where P.H.S. maintains 2.3 million square feet of vacant land. Green, who grew up in a part of Philadelphia that’s so gray they called it “the concrete city,” started working at P.H.S. twenty-one years ago, first as an intern and then on community-garden projects. “I never thought I’d be doing this for so long,” he told me. “But I found my niche when we started fixing up abandoned property.”

As we drove, Green told me about one of his first jobs. “The city asked us to clean up a two-block area in North Philadelphia where there was a flea infestation. We got there, and it was like the entire area had turned into a jungle. Weeds, tall grass, messed-up trees. People were using it as a dumping ground.” The team ended up treating a hundred and twenty-five empty lots. “It was a horrible job, but when we finished you could tell that the neighborhood was going to be different,” he said. “And people were so happy. I’d have kids running up to my truck, yelling, ‘Mr. Keith! Mr. Keith! Can you come back tomorrow?’ They treated me like I was Mister Softee.”

Green drove slowly up Fortieth Street, on the west side of the city. “You’re gonna want to keep your eyes open,” he said. The area looked a lot like Englewood and North Lawndale, neighborhoods I’d studied in Chicago, where row houses and apartment buildings, some empty, some well-kept, sat adjacent to large, open lots that were thick with weeds, debris, and six-foot-high grass. “See that?” He pulled over at a corner lot with a low-lying wooden fence, two benches, trimmed trees, and a neatly cut lawn. “That’s one of our treated sites. You can tell because it’s so well maintained.”

We got out, walked through the pocket park and over to a vacant house and large lot a few steps away. There, the grass had grown both high and wide, so that it came through the sidewalk and out to the curb. “Now this—this is a disaster,” Green said. “It’s probably got an owner who wouldn’t let us work here, or someone we couldn’t track down. If you live here, you’ve got to deal with all the problems this attracts into the neighborhood: pests, insects, garbage, crime. And you know it’s gonna make it hard for new development to work here. People see that and they want to run.”

We crossed the narrow street to look at another property. Loretta, a woman in her late twenties, out for some exercise, was walking briskly toward us. I paused and asked her if she lived there. “No,” she replied. “But I walk around this neighborhood all the time.”

“Have you noticed all the little parks with small fences?” I asked.

“Not really.” She looked around, took them in. “They’re nice, though.”

“What about the abandoned lots with all the weeds and garbage?”

“Um, yeah,” Loretta answered, cracking a little smile. “Why do you think I’m walking on the other side of the street?” She paused for a beat, then looked over at the lot. “Those places are scary. You don’t know what’s going on in that mess, who’s around. There’s a lot of places like that around here, and I just try to keep away.”

Green and I headed up the road again before turning onto Westminster Street. He pointed to a large remediated lot that residents had converted into a community park, with picnic tables and a small garden. “A guy who owns a store a few blocks away helped fix up this block,” Green explained. “He just wanted the neighborhood to look nice, to get more people out on the sidewalks and gardens. We see a lot of that. If we maintain things, residents go a little further, and put in what they like.”

We crossed over to a set of three row houses that had pocket parks on either side. As we approached, a man with gray hair, sunglasses, and a wooden cane was sitting on a picnic table and talking on a flip phone. He stood up, nodded, and introduced himself as Micky. Green asked if the park made the neighborhood better. “Oh, you know it does,” he replied. He pointed to the front porch of the row house next door, where a woman named Joyce, in sandals and a white T-shirt, was relaxing on a rocking chair. “Ask her, she knows.”

Joyce was nodding. “I’ve been staying here ten, twelve years now. Those lots were bad when I first got here. Drugs and all that. Kids up to no good. People would let their dogs run all around them, too—oh, did it smell!” She grimaced and shivered a little. “But they fixed it up pretty soon after I got here. Put them tables in, big umbrellas, too. Kids started coming around. We got the garden going. Before, everybody would avoid this block. It was ugly, and dangerous, ’cause you didn’t know who was gonna jump out of those bushes. Now it’s a lot better.”

Green and his colleagues at P.H.S. suspected that fixing up the empty lots and buildings was improving Philadelphia’s poor neighborhoods, but they weren’t certain. Branas and MacDonald had a more specific hypothesis: that remediation would reduce violent crime nearby. “It’s not simply that they are signs of disorder,” Branas told me. “It’s that the places themselves create opportunities for gun violence; they take what might just be a poor neighborhood and make it poor and dangerous.”

The reasons are straightforward. Abandoned houses are good places for people involved in crime to hide when on the run. They’re also good places to store firearms. Untended lots are notoriously useful for drug dealing—in part because most law-abiding residents avoid them, and in part because dealers can hide their products in the weeds and tall grass if the police drive by. For communities, and for the police, they are hard places to monitor and control.