In a study of a Chicago public housing development, buildings with high levels of trees and greenery had 48% fewer property crimes and 56% fewer violent crimes than identical apartments surrounded by barren land. The greener the surroundings, the fewer the number of crimes that occurred, and even modest amounts of greenery were associated with lower crime rates.
Kuo, Frances E., and William C. Sullivan. (2001). Environment and crime in the inner city: Does vegetation reduce crime? Environment and Behavior. 33(3), 343-367.