Over wintering perennials, shrubs or trees in containers is never a sure thing, and what works one year, may not work the following year. When in the ground, plant roots freeze for the winter and stay frozen all winter long until the following spring when the ground thaws. When they are in a containers, they may go through several freezes and thaws over the course of the winter, which is what kills the roots - and then the plants.
For best success in over wintering perennial plants in a container, make sure to protect and insulate the roots inside the container from the sun and wind, so they stay frozen all winter. Lining the inside of your containers with Styrofoam when you plant them in the spring will help to insulate them over the winter (it will also protect roots from extreme heat in the summer too!). Extra Styrofoam can be added around the outside of the container for the winter for added protection.
The best place to store you containers over the winter would be in a north facing area of your yard.You can also try to cover or blanket them so the sun won’t reach them and heat them up on sunny winter days or place them in dark back corners of the garden that are protected from the sun by evergreens, fences etc.. If you have room and your garage is not heated, you can store them there.
Some containers will break in the winter if moisture gets inside, and expands when it freezes. These include terra cotta, porcelain, thin plastic or thin metal. Better choices include; thick plastic and metal, fiberglass, heavy stone, or wood.
Over wintering your containers is never guaranteed, even if you follow all of these tips, but if you love plants as much as we do, its worth the try!