I think It would get used quite a bit. I would certainly take advantage of having it in the list.
I think a greenhouse tag may be appropriate, but only if it is somehow linked to sustainability, e.g. "How to make my greenhouse more sustainable?", or "How do I recycle my old greenhouse?". Greenhouses that use gas or electricity for heating are not sustainable (see also this question) so questions about that without a direct link to sustainability most likely won't fit on this site (they may be appropriate on Gardening SE).
If you create the tag, it's description should probably also mention that questions on the greenhouse-effect are not appropriate for it (and vice versa)