One possible solution to the question of where Permaculture questions belong is to just duck the question entirely and put them in both places. Here's the idea, have questions that are given a certain tag on one site be mirrored on a different site, linked through the tags.
To use Sustainability and Gardening as an example, questions asked on Sustainability and tagged "gardening" could show up on Gardening under the tag "sustainable" and questions asked on Gardening and tagged "sustainable" could show up on Sustainability under the tag "gardening". Answers, tags, comments, votes everything could be mirrored across sites.
That way, the questions that really do belong on both sites could get the best possible mix of answers from the crowds of experts draw to either site. Those on the Sustainability site would benefit from the expertise of the Gardeners, but would be able to vote down answers that weren't sustainable.
This would also potentially apply to Home Improvement, Personal Finance and Money and Seasoned Advice. And possibly to sites other than Sustainable Living. Database questions could be mirrored between SO and DBA. AskUbuntu could be mirrored into an Ubuntu tag on Unix and Linux. I'm sure there are dozens of other examples of overlaps that would benefit from a little cross pollination!