Potted shrubs aren't as adaptable to tough winters as those in the ground -- the soil in the container simply can't provide the insulation in-ground soil can. Roots of plants in containers have greater exposure to below-freezing temperatures on all sides.
Cold weather also can heave plants out of the soil. This happens when temperatures fluctuate, causing the soil to freeze, thaw, and freeze again. This cycle is traumatic for roots. When heaving occurs, it leaves the plant's roots exposed to the cold weather and winter winds, which cause them to dry out, putting your plants in jeopardy.
Give Your Shrubs an Advantage
You can take a few measures that help your plants make it through a tough winter. Be aware that smaller containers freeze much faster than larger containers, so the larger the container, the better, even for dwarf shrub varieties. Young, tender plants aren't as resilient as established plants. Because fertilization and pruning results in new, tender foliage, cease doing both in midsummer to help shrubs harden off for winter. As you go into fall and winter, make sure your plants are well-watered.
Move Potted Shrubs to Unheated Shelters
An unheated garage, shed, porch, or basement can be a good place to overwinter potted shrubs, particularly those considered tender or not hardy to your Zone. Sheltered locations are good options for deciduous shrubs with branches that might be susceptible to breakage from heavy ice or snow. Keep an eye on potted shrubs throughout the winter to make sure they don't dry out. You might need to water occasionally.
When Shelter Isn't Available
If moving plants to unheated indoor areas or under a shelter isn't an option, there are a few techniques to implement outdoors. During the fall, consider transplanting the shrubs into the ground. They can be returned to the container in the spring. Another option -- if you have a garden area or raised beds where you can dig a trench -- is to bury the potted containers (up to the rim of the container) in the soil. Add straw, shredded bark mulch, or leaves around any areas of the exposed pot. If this isn't an option, look for the best outdoor area for your plants where they receive some protection. A spot on the north or east corner of your home or other structure is a place to consider.
Insulate Plants with Mulch
Remove your containers from pavement or concrete patios, which can exacerbate the extremes in the heating-and-thawing cycle. Set your containers on the ground instead. If you have several containers, group them together with the most cold-sensitive plants placed in the middle. For the more cold-sensitive shrubs, such as hydrangeas and camellias, loosely drape burlap around the plant several times. Surround each container with mulch, then add an extra layer of mulch around the outer perimeter of the grouped plants to serve as insulation. Check occasionally -- every 2 or 3 weeks -- and water as needed.
Protect with Fabric Screens
If you have small evergreens, in addition to protecting their roots by adding mulch around the container, you might want to shield them with burlap screens. This can help prevent the evergreens from sunscald. Simply drive or pound several stakes in the ground around the potted plants and staple burlap onto the stakes. Potted shrubs -- deciduous or evergreen -- also can be protected by creating a small tent using stakes, like a tepee. Drive stakes into the ground around the container and the insulating mulch, then cover the tepee structure with burlap or other fabric. Either staple the fabric to the stakes or wrap twine around it to hold the fabric in place.
If you're planning on adding new container plants to your landscape in the spring, consider plants that are hardy in two Zones colder than your area to optimize their chances of winter survival.