Overwintering fact: Your overwintering success with container perennials is dependent on the ability to insulate the roots (containers). Plants grown in containers are exposed to colder temperatures than those grown in the ground and are considered at least two zones colder.
Consider the following techniques:
This is an old nurseryman’s technique for overwintering pots and bare root plants. Dig a trench deep enough so the tallest container is level with the soil line. Group all pots together in this trench and fill it in with leaves. Burying your pots will help insulate the roots
No time to dig? – Group pots tight together and cover them with either a thick layer of leaves or potting soil.
Knock ‘em over
Knock 'em over on their sides. Here, you are keeping your roots warmer by laying them down You will want to group them grouped together too. You can also cover them with leaves for some extra insulation. In late winter the snow melts and creates a freeze/thaw effect which kills new emerging eyes. Simply laying pots on their sides minimizes this problem.
Bring ‘em on in
You will want to bring inside anything grown in a ceramic pot, or it will truly go to pot next spring cracking into hundreds of pieces. Store containers against a wall in the garage or a cellar. The important thing to remember here is that the pots will dry out and you will need to give them a drink by January or February and monitor them thereafter.
Information provided by greatgardenplants.com