I have a portable greenhouse to put my potted plants in through the winter. The goal is not to start new plants nor to keep a constant temperature. Just to keep them from freezing. I put them in before the first frost and take them out in the spring. At my last house, I just hung a light in there and ran it on the nights when it was going to be below freezing and opened the vents on days when it was so warm as to create condensation. For 10 years I had very good luck with this method.

However, I have moved and the location of the greenhouse is now too far away from an electrical source to heat it with the light. I have been doing a little research today and it seems the best method would be the "heat sink" method. I was thinking of using black 33 gallon garbage cans. How many would I need to put in a 9x9 greenhouse? Somehow I'm thinking 4?? Also would it be beneficial for me to line the inside of the walls with bubblewrap?

Thanks for any help you can give me. I just don't want to lose all my tropicals over the winter :)