It is clear that green spaces are good for us:
Futures Forum: "Green spaces in town and cities create immediate and long lasting improvements in people's wellbeing."
This study from Exeter University from a couple of years ago backs this notion up with a huge amount of data:
Would you be happier living in a greener urban area? | ECEHH
Would You Be Happier Living in a Greener Urban Area? A Fixed-Effects Analysis of Panel Data