Hi Beth, 

The issue with planting perennials in containers isn’t so much growing zone as it is the continual freezing and thawing that container plants experience during winter. The roots heave and that kills plants.

Some people have success moving containers into attached garages for winter, positioning pots against the coldest wall, watering well and insulating the sides of the pots to try to stabilize soil temperature. Being out of the sun’s rays and the wind helps.

If containers are too large to move, you could line the outside of containers with insulating hay bales to try to prevent soil temperature swings. But our winter weather is growing more erratic and this likely won’t work.

You can successfully overwinter hardy perennials in a pot by burying the pot in the ground up to its lip in the fall and pulling it when spring comes. (The pot must have a drainage hole. And some ceramic pots may crack.)

If none of these suggestions help, you could treat the perennials as annuals, replacing them each season. But the plants would never mature in a single year.

Sorry I don’t have more encouraging advice. Thanks.