One school of thought has it that inner-city trees and shrubs make convenient hiding places and covered escape routes for criminals. Another, supported by an increasing body of evidence, argues that urban foliage may actually reduce crime. Where previous studies have tended to focus on individual housing blocks or, at best, neighborhoods, new research out of Temple University is among the first to examine the issue at the city-scale. TU researchers analyzed the relationship between vegetation concentration and crime for the whole of Philadelphia.

The researchers broke the city down into 363 "tracts" identified from socioeconomic census data, each containing between 100 and 8,000 people. The data, taken from the years 2005 to 2009, was also used to assess poverty and education levels in these tracts. Vegetation coverage was assessed from satellite imagery from 2005, courtesy of NASA's Landsat 7. Recorded incidents of aggravated assault, robbery, burglary, and theft were drawn from the University of Pennsylvania's Neighborhood Information System CrimeBase, also for the year 2005.

Theft, just to be clear, is the illegal act of taking something without permission. Robbery is the same, but with violence (or the threat of it). Burglary is breaking into a building in order to a commit crime (often, but not necessarily, theft or robbery.) The researchers ignored incidents of murder and rape on the grounds that their relative rarity renders the comparison senseless at tract level.

The team undertook a series of statistical analyses of the data, including multivariate ordinary least squares regression. This allowed them to test the "explanatory power" of foliage on crime while controlling for other influences on it, like population density, poverty, and education. Other techniques allowed for the control of the spatial contagion of crime.

A visual assessment of the data confirmed a negative correlation between vegetation and crime (i.e. the more vegetation in an area, the fewer incidents of crime there tend to be), and the statistics bear this out. "Notably, vegetation is significantly and negatively correlated with all of the crime variables, with the exception of thefts," the study finds. "As vegetation increases, aggravated assaults, robberies, and burglaries decrease."

Unsurprisingly, the greenest parts of the city tend to be home to wealthy and educated people. Controlling for these other potential causes, however, the researchers found that the correlation stands. "[V]egetation has a significant and negative relationship with all three types of crime, even after accounting for poverty, educational attainment, and population density," the study finds. (Note: the three crimes excluded theft due to the lack of significant correlation to begin with.)

Let brotherly love endure

By way of explanation, the researchers echo hypotheses from prior studies. There are two arguments. The first is that more vegetation promotes the use of public space, leading to more "social supervision" and surveillance. Social supervision, in essence, is the idea that people are more likely to establish beneficial relationships with positive, crime-deterring role models. Increased surveillance, meanwhile, does not imply the proliferation of CCTV. This is simply a reference to more users of a space bringing more eyes to the street. The theory: ne'er-do-wells don't like being observed when ne'er-do-welling.

The second argument is that vegetation provides a "mentally restorative effect" that can reduce violent crime. The implication is not so much that a glimpse of hydrangea will deter a violent criminal in the act, but that the presence of vegetation reduces mental fatigue and irritability, which can be the precursors to violent crime. The authors admit that such benefits are difficult to quantify, but their results would seem to support this idea. The negative correlation between vegetation and crime was stronger for incidents of robbery and aggravated assault.

Chief among the limitations of the study was the means by which vegetation was identified, the Normalized Difference Vegetation Index (NDVI). NDVI involves assessing satellite data for the quality and health of vegetation by comparing reflected light and reflected near-infrared radiation (plants absorb most of the former but little of the latter). Though the method can distinguish trees from lawns and shrubs, it cannot distinguish between specific types of vegetation, tree crown area, or the upkeep of the vegetation.

This can be a problem, as the authors suggest that overgrown abandoned lots broadcast a lack of social control that will encourage crime. Neither does the study distinguish between public and private property, which other research has found to be relevant.

There will be other contributory factors overlooked in this research, the authors point out. These may include demographic, cultural, and urban morphological factors (such as the age of a building or its proximity to a transport hub). Though nothing on this list, they suggest, prompts an obvious alternative explanation for the relationship of crime with vegetation.

The authors also point out that the study cannot rule out the possibility that vegetation is displacing crime rather than reducing it. To assess this would require monitoring changes in crime and vegetation over time. That's impossible with the data they had, but it's an avenue of future research.

This latest study adds to the mounting evidence that there is a clear negative correlation between urban vegetation and crime. What it does not do, beyond doubt, is prove causation. Though the team has attempted to control for other causal factors, there remains the distinct possibility that vegetation is a proxy and some as-yet unidentified variable is the actual cause.

Nevertheless, this is an area of research city planners and police departments would do well to monitor. Vegetation is sometimes removed in efforts to curb crime. It may yet prove that the opposite policy is more effective.

Landscape and Urban Planning, 2012. DOI: 10.1016/j.landurbplan.2012.08.006 (About DOIs).