My neighbourhood 'green' ratings seem to vary with the season, so I'm guessing that some of it might be related to the season in which plants spawn and some to heating and cooling. As I didn't want to go out and meet a bunch of Sims, I cheated my way to replacing the NAPs I didn't like with those I did, and then I turned my neighbourhood community space into a gardening space. The last turned out to be the wrong thing to do, as my neighbourhood is now permanently on 'Neutral', and the community space appears to be the culprit.
Therefore, I've deduced that now I've turned the community space into a garden, my Sim will need to plant some stuff there, and maybe do a little maintenance and upgrading of the solar panels and dew collectors. I think I'll be trying that today.