Ideally I'd set up a few bioreactors and possibly some tubing and low wattage fans on solar to move the air around and warm the root zone. Polycarbonate would be a longer lasting shell but our connections with some other local nursery affords us the chance to receive offcuts from when they rewrap theirs.

I actually talked a friend into doing a walipini but his site has conditions to really maximize its usefulness. Our slope is west-facing and not as great from a structural point of view. Also my wife saw a picture from his project and is not enthusiastic about the idea of replicating it. If I can convince the town planning board to give us a variance I'll likely build a greenhouse backed against the thermal mass of the house's west face.