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 enhance property values,” says Charles Branas, an epidemiologist at the University of Pennsylvania’s Perelman School of Medicine, but local police may benefit as much as realtors. According to his recent research, “Gun crimes in particular are affected once you clean and green these lots.”
The inspiration for the study was a discovery Branas made in 2008: “Poverty and unemployment seemed to pale in comparison” to the effect of abandoned buildings and vacant lots on aggravated assault, he says. So he and colleagues decided to use the PHS program as an experiment. They compared changes in crime statistics and health survey responses in the areas around 4,000 greened lots to changes in those statistics in the areas around 13,000, or three times as many, randomly chosen ungreened lots. All over the city, improving vacant lots decreased the number of gun assaults. In some parts of the city, it also reduced vandalism, and it made residents exercise more and feel less stress.
“We see it happening,” says Robert Grossman, director of the PHS Philadelphia Green Program. “The neighborhoods where we’ve done a lot of this work are really transformed.” The vacant lots host weddings and barbecues, instead of drug dealers and prostitutes, he says.
The best explanation for the decrease in gun crime is purely physical—there’s nowhere to hide illegal firearms anymore. But other research suggests that the effect might be broader. In inner-city Chicago, for example, trees around public housing reduce violence and theft. “If the neighborhood is nicer and people are drawn out to use it more, there are going to be more eyes on the street,” explains Frances E. Kuo, that study’s author and director of the University of Illinois at Urbana-Champaign’s Landscape and Human Health Laboratory. “The more green, the less crime.”
Gardening as a public health intervention has a lot of potential. “We would love for people to begin thinking about investing in these greening strategies for enhancing health and safety, as opposed to always going to strategies that are reactive, like more policing” or lifestyle modification, Branas says. Having worked for years on biobehavioral programs for high-risk youth that “have had really impressive impact, but only for small groups of people,” Branas now hopes to emulate the mass-impact public health campaigns of 100 years ago. Like chlorinating the water, “greening would be less expensive, and would touch more people for longer periods of time, than other programs.”
Charles C. Branas, Rose A. Cheney, John M. MacDonald, Vicky W. Tam, Tara D. Jackson, and Thomas R. Ten Have, “A Difference-in-Differences Analysis of Health, Safety, and Greening Vacant Urban Space,” American Journal of Epidemiology, 174, 2011.