A perennial rated hardy to your zone in the ground is not automatically hardy in a pot. Container roots are exposed to cold air on all sides rather than insulated by surrounding earth, which functionally drops a potted plant's cold tolerance by one to two zones.
Zones 3–5: move or bury
Sink the entire pot into the ground up to its rim, or move it into an unheated garage or shed that stays consistently just above freezing. Leaving containers fully exposed in these zones is the most common cause of winter perennial loss.
Zones 6–7: cluster and insulate
Group pots together against a sheltered wall and wrap the outer pots in burlap or bubble insulation. Clustering alone raises the effective temperature around the root balls by insulating them from each other.
Zones 8–9: minimal intervention needed
Most perennials rated for these zones need no special winter protection in a pot, though an unexpected hard freeze warrants moving containers against the house temporarily.
The watering mistake that kills more potted perennials than cold does
Container soil that freezes solid and stays bone dry all winter is far more damaging than cold alone. Water pots lightly during winter dry spells whenever the soil is not frozen — root death from desiccation is a more common cause of loss than frost damage itself.
