Researchers Kuo and Sullivan provide evidence that trees and grass around public housing sites can reduce some aggression and deter crime. Their study concluded that buildings with a high level of vegetation had 52% fewer total crimes, 48% fewer property crimes and 56% fewer lower violent crimes than the building with low-level vegetation.
Kuo, F. and W. Sullivan. 2001. Environment and crime in the inner city: Does vegetation reduce crime? Environment and Behavior, vol33 no. 3 (May), pp.343-67