People always think that Container gardening has to have special precautions in the winter. I just wanted to show you some pictures.
You notice that all around the planters is the ONLY place the snow and ice is melted!!! That just shows you how much heat the containers radiate during the winter. No special insulation is ever needed!
As long as the plants are winter hardy, they can stay in the containers, outside, all winter... without any damage.
The only thing I do is mulch the plants..... and in the spring I remove the mulch so that the soil doesn't get super hot! I reuse the mulch by putting it in a garbage bag and storing it for the summer, and re-using the same mulch in the winter