CAN TREES STOP CRIME?
Posted by Richard Conniff on October 1, 2012
Driving the bleak, treeless streets of West Baltimore, through neighborhoods that inspired the American television series “Homicide,” and “The Wire,” Morgan Grove recites the evolutionary stages of neighborhood abandonment. First, plywood goes up over the front doors of the two- and three-story brick row houses. When that’s not enough to keep out thieves and addicts, cinder block walls fill in the entry ways. Then big spray-painted red “X”s start to appear, meaning the buildings are so dilapidated that even firemen will not enter.
Grove, an urban ecologist with the U.S. Forest Service, points out a weedy lot where a house has been demolished in mid-block. It’s like a front tooth knocked out–“The hockey player phenomenon,” he remarks–and a sign that the rest of the block will soon follow.
It’s 90 degrees at mid-afternoon, and the streets are empty of people, or any other sign of life. But then Grove turns the corner onto North Carrollton Avenue, and for one small block, it’s an oasis. London plane trees line the sidewalks and lean out toward one another, forming a green archway over the street. The houses appear to be not just occupied, but loved. At mid-block, a stand sells flavored ices, and Justine Bonner, a 74-year-old school teacher, pushes a broom to tidy up in front of the house where she has spent her entire life. The trees, says Bonner, shade the houses and filter the air. “It makes it easier to breathe,” she says, and means it literally.
But trees may also help people on this block breathe easier in the sense that they can feel a little less worried about crime. Despite urban folklore that treats all vegetation as a hiding place for muggers, car jackers, and drug dealers, new studies in three American cities—Baltimore, Portland, Oregon, and Philadelphia–suggest that the right trees in the right place can play a significant role in preventing crime and make even the worst neighborhoods feel safer.
The Baltimore study, co-authored by Grove and published in the journal Landscape and Urban Planning, covers a mixed urban and rural area of almost 700 square miles, for the most comprehensive investigation yet into the connection between trees and crime. It compares otherwise similar neighborhoods—same income level, same housing stock, same density—and shows that the ones with more trees tend to have a significantly lower crime rate. The researchers are careful not to say that trees cause lower crime rates. “It’s just an association,” says co-author Austin Troy, an associate professor at the University of Vermont. “But it’s a very strong association.” Across the entire study area, neighborhoods with 10 percent more tree canopy cover experienced 11.8 percent less crime than their comparable counterparts.
The results in Baltimore closely match the findings of another 2012 U.S. Forest Service study, using different methodologies, in Portland, Oregon. And in Philadelphia, where a program has cleaned up 4300 vacant lots and planted trees on them, a 2011 study found that it produced a substantial drop in crime, including a 7 to 8 percent decrease in gun assaults across most of the city.
The old idea that vegetation causes crime has deep roots, dating back at least 800 years, to when King Edward I required English towns to clear the trees for 200 feet on either side of main roads, as a precaution against highwaymen. This line of thinking hadn’t changed all that much by the time Grove first got interested in the topic as an undergraduate at Yale in the 1980s. He visited the New Haven (Connecticut) Police Department, where someone handed him a thick folder labeled “CPTED.” What had started out in 1971 as the title of a book, Crime Prevention Through Environmental Design, by Florida criminologist C. Ray Jeffery, had become an acronym. The essential idea was to prevent crime by eliminating hiding places and maximizing opportunities for people to see and be seen.
But that often came across as a negative message about trees: Remove trees to create wide open spaces in urban areas, never plant conifers, and if you really must have trees, at least keep the lower branches pruned up for better sightlines. Other studies in the 1980s and 90s compounded the problem by arguing that people associate dense vegetation with fear of crime, and that dense vegetation can actually encourage crime by providing hiding places and escape routes.
What got lost in the mix was a simple reality: People like trees, and they like neighborhoods with trees even better. Moreover, recent studies have shown that this isn’t just some leafy suburban ideal. The poorest inner city residents also prefer to live with trees, and they are far more likely to spend time outdoors in areas where trees provide shade and a comfortable space for socializing.
The turnaround in thinking about trees and crime began with a 2001 study of public housing projects in Chicago, comparing buildings that had trees close by with others that were surrounded by pavement. Researchers Frances Kuo and William Sullivan at the University of Illinois in Urbana-Champaign found that buildings with more vegetation had 52 percent fewer total crimes and 56 percent fewer violent crimes. The researchers proposed a straightforward logic: Dense vegetation may promote crime by facilitating concealment. But that implies the opposite is also true: Widely-spaced, high-canopy trees and grassy areas may discourage crime by enhancing visibility: Residents who come out to enjoy the shade provide more eyes on the street. Being outdoors also builds stronger neighborhood social networks, which tend to make criminals feel unsafe. “One of the classic suspects in environmental criminology,” Kuo and Sullivan concluded, “does not always promote crime.”
All three new studies identify circumstances where vegetation can go wrong. In Baltimore, it’s often in the overgrown border zones between residential and industrial areas. There and in Philadelphia, weedy vacant lots provide a convenient place for criminals to stash guns and drugs, out of their actual possession when police pass by and yet close enough to be handy at all other times. In Portland, Oregon, USFS researcher, Geoffrey Donovan was puzzled that, for trees in yards, 42 feet in height seemed to be the critical dividing line. “It was a head scratcher,” says Donovan. There was less crime when a tree was 43 feet tall, and more at 41 feet. “Is this a view-obstruction thing? Finally I sent a student out to measure.” It turned out that when a tree gets to be 42 feet tall, the bottom of its canopy tends to just clear the tops of the first floor windows, meaning clear sight lines from both the house and the street.
More important, all three studies also showed how trees can serve as a sort of soft policing tool. Philadelphia’s LandCare program, for instance, doesn’t just clean up vacant lots, but also seeds them and keeps them mown, plants what are often the only trees in the neighborhood, and installs a knee-high fence as a sort of territorial marker at the perimeter. The result, when University of Pennsylvania epidemiologist Charles Branas compared vacant lots that had been greened with similar lots that hadn’t, was a marked decrease in almost all forms of crime, probably because there was no longer any good place to hide guns or drugs. Moreover, says Branas, the data suggest that crime doesn’t simply move around the corner when green happens. “It’s a net decrease.” He plans to follow up next spring with a full-scale experiment, randomly assigning about 200 lots each to the greening treatment, to no treatment at all, or to a treatment in which people remove the trash and then turn up at regular intervals to maintain cleanliness. The aim is to address the persistent doubts of greening skeptics: “Is it really a placebo effect? That is, is it really the greening?” asks Branas. “Or is it the fact that people show up once a month to do work?”
An ample body of research suggests that it’s the greening. Studies have demonstrated that having trees and grass in the neighborhood reduces stress and anxiety, encourages exercise, and generally makes people more civil. But the effect of well-maintained trees also fits the “broken windows theory” proposed in 1982 by social scientist James Q. Wilson. It suggests that broken windows are an invitation to criminals because they convey the message that no one cares about the neighborhood. Grove adds that empty sidewalk tree pits say the same thing.
Street trees send the opposite message, and in the Portland study they were always associated with lower crime rates. Both the Portland and Baltimore studies also found that properly maintained trees in public parks have a dramatic effect on crime rates.
All three studies boil down to a few simple rules: 1. Wherever you have a tree, make it look nice, even if it’s just a maple sapling that’s sprung up on a vacant lot. 2. Plant your public parks with tall trees. 3. Plant street trees. 4. Get residents involved in the effort and have them meet their neighbors. 5. Plant yard trees far enough from the house, and prune the lower branches, so they don’t block sightlines.
Will it stop crime? “If you’ve got $200 and you want to prevent crime, buy a burglar alarm, not a tree,” says USFS researcher Geoffrey Donovan. “That’s what I always tell people.” But trees are multi-taskers, especially on city streets. The list of benefits attributed to them includes moderating rainwater runoff and the attendant flood problems, reducing heating and cooling costs, increasing property values, and encouraging people to relax and enjoy their lives.
“Will a burglar alarm shade you on a summer day?” Donovan adds. “Is it going to improve your mental health, or even your physical health? It’s not like you can buy a tree and then not lock your doors. But they do provide this wider range of benefits that’s worth considering.”