You could make a small hoop house using PVC pipe over rebar. (I had a small one that was about three or four feet high and about six feet long. I had some hibiscus and was able to store them in this hoop with just Christmas lights for warmth. The hibiscus bloomed all winter). Just cover the hoop with plastic and set it up next to your greenhouse for some warmth. In spring, you can take it down and store the pieces in the greenhouse for next year. Below is an article about a big one that we use for the tropicals each winter. We're about to put it up again next week.
http://garden.org/ideas/view/B...
