To Make Cities Safer, Add Trees

The modern American city is an urban jungle of steel and glass. It is a lonely, isolating creation in which the only plant life is the grass in the cracks of the sidewalk or the occasional tree amid a sea of asphalt. Crime also runs rampant in our cities. These two problems—the lifelessness and criminal behavior in cities—can be solved by the same initiative. We must transform these urban jungles into urban jungles.

Research has shown that bringing trees and greenery into cities not only improves the physical and mental health of their occupants but makes their communities safer. A team of researchers in Baltimore found that a 10 percent increase in trees led to a 12 percent decrease in crime. The study controlled for income, housing, age, and race, and adjusted for ruralness and density.

A study in Chicago found similar results. Frances E. Kuo and William Sullivan, codirectors of the Human-Environment Research Laboratory at the University of Illinois, studied the effects of treason crime rates at one of Chicago’s poorest housing projects, Ida B. Wells. They found that across the project’s 98 apartment buildings, those that had high levels of vegetation had 52 percent less property and violent crime than those with lower levels of vegetation. This relationship persisted after taking into account building height, vacancy rate, the number of occupied units, and the number of units per building. 

The projects’ residents felt the same way. The authors said that residents reported feeling less fear, and seeing fewer incidents of rude, aggressive, or violent behavior. A Portland study had similar findings, and a study in Cincinnati found an increase in crime after invasive beetles led to the removal of 646 ash trees. Philadelphia planted more trees, costing $5 per square meter, and cut crime by 29 percent in the poorest areas. Importantly, the Philadelphia study found that it didn’t push crime to somewhere else. It simply vanished.

Conventional methods of combating crime are expensive. The average salary of a cop is between $50,000 and $100,000 a year. The average cost to imprison someone is about $28,000 a year, but in some states, it’s higher; New York, for example, spends about $70,000, and California about $64,000. This is not to mention the revolving door that is the American prison system. Two-thirds of violent criminals are arrested again within five years of release. We’re spending tens of thousands a year on criminals who get out and commit more crimes, ones that are sometimes worse than the crimes they initially committed. Study after study shows that the presence of trees and greenery in urban areas will stop some crime before it occurs.

The question is why? Why does greenery in cities reduce crime? Trees are nature’s sculptures, drawing people outside who want to enjoy green space. With more people outside, there are more eyes on the streets. With more eyes on the streets, there is, in effect, free surveillance. Criminals are much less likely to commit crimes in the presence of eyewitnesses. 

The presence of trees in urban settings also improves residents’ mental health. Trees reduce depression, anxiety, and stress and increase people’s sense of well-being. They combat irritability and mental fatigue, both causes of aggression.

There are many ways to bring nature into the city. Planting more trees on sidewalks is one way. Rooftop gardens on existing buildings is another. Using biophilic building designs is an effective way to incorporate greenery into the built environment; Singapore is an excellent example of a biophilic city. The Japanese botanist Akira Miyawaki proposed the idea of creating mini-forests, in which you take native trees from the area and use them to create a miniature, urban forest. These forests grow 10 times faster, are 100 times more biodiverse, and absorb 40 times more carbon than conventional forests. Guerilla gardening, the graffiti of greenery, is another effective way at bringing green space into the city. Anyone anywhere can make their city a green city. All you need is a seed or a sapling and an idea.

The modern American city is a jungle of steel and glass, and overrun with crime. But it doesn’t have to be. Bringing nature into the city could reduce crime by upwards of 50 percent. It is a solution to the crime problem that does not involve the expense of hiring more police or expanding our prisons. It is better for the city’s residents, too. After all, a living, breathing city is a clean and safe city.

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