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 a Yale undergraduate in the 1980s. He visited the New Haven 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, incorporating elements of a 1972 book, Defensible Space, by architect Oscar Newman, 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: Don’t plant conifers and if you must have trees, at least keep the lower branches pruned 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 perhaps 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 preserving visibility. Residents who come out to enjoy the shade provide more “eyes on the street,” a phrase Jane Jacobs popularized in The Death and Life of Great American Cities. 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. 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, Ore., 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.