Residents at a Chicago public housing development living in buildings with more trees and grass reported that they knew their neighbors better, socialized with them more often, had stronger feelings of community, and felt safer and better adjusted than did residents of more barren, but otherwise identical, buildings.
Kuo, Frances E., William C. Sullivan, Rebekah Levine Coley, and Liesette Brunson. (1998). Fertile ground for community: Inner-city neighborhood common spaces. American Journal of Community Psychology, 26(6), 823-851.