Nature grade?

Hello out there. I just started playing City Folk again for the first time in a while, partly because my brother wanted to see me catch a shark again and partly to get me used to Animal Crossing before I buy the new game. Now when you go to the Town Hall/Post Office, there's the option of asking about the towns nature. The people complain, saying it gets a D+  or C-, that there isn't enough green/plants/I can't really remember the right term, but everywhere I go there's tons of trees and flowers. How do I get it to go higher, when there's plenty of what they're asking for?