Tree-lined urban streets, grass, shrubs and other vegetation do more than just make a city look nice. Well-maintained greenery is also associated with lower levels of aggravated assault, robbery and burglary according to a new study from Temple University published last month in the journal Landscape and Urban Planning. The study strongly contradicts the long-held belief that planting too many trees and other vegetation in city settings makes crime worse.

"There is a longstanding principle, particularly in urban planning, that you don't want a high level of vegetation, because it abets crime by either shielding the criminal activity or allowing the criminal to escape," the study's co-author, Temple associate professor of geography and urban studies Jeremy Mennis, said in a prepared release. "Well-maintained greenery, however, can have a suppressive effect on crime."

The authors used satellite imaging to see which neighborhoods had the most greenery and then compared that data with information on crime and socioeconomic factors. After they put all of those numbers together they found that greener neighborhoods had less crime. The authors found that this effect wasn't just in the rich parts of town, which tend to be greener and more vigilant, but in all economic and educational strata.

So why were the crime rates lower? The researchers say that two factors appear to be at play, although their research did not set out to prove either relationship. First, greener neighborhoods encourage both social interaction and a community effort to keep public spaces well-maintained. This meant that people in greener communities were more active and vigilant, providing fewer opportunities for crime. Second, the researchers say the greener landscapes had a calming effort. This again contradicts many urban planners, who say that people feel uncomfortable around trees and bushes that could provide hiding spaces for waiting criminals.

A similar study (pdf) published last year in the same journal came to similar conclusions. That study, conducted in Baltimore, found that a 10 percent increase in tree canopy levels correlated with a 12 percent decrease in crime. The authors, from the University of Vermont and the USDA Forest Service, also found that the benefits were not affected by a neighborhood's economics.

However, none of the benefits of urban vegetation existed in areas where the greenery was not well-maintained. "Wild vegetation in a vacant lot suggests there is very little social control over an area, and that may encourage criminal activity," Mennis told ScienceOmega. "It is suggestive of a place where people aren't paying attention; where neighbors are not coming together or looking out for their environment or each other."

Mennis said urban vegetation provides other benefits, such as stormwater management. He suggests that urban planners now have extra incentive to increase levels of vegetation within their cities.

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