A container plant faces a harder winter than anything in the ground, and the reason is simple geometry. A root ball in the ground is wrapped in soil that never gets close to air temperature. A root ball in a pot is surrounded by air on every side and freezes right through.
The working rule is two zones hardier than the site. On a zone 5 property, anything staying outside through winter should be rated to zone 3. Plant to the site's zone instead and you have bought an annual without meaning to.
Water is the other half, and it kills more containers here than cold does. A pot dries out in days in July, and it also needs occasional water on warm winter days when the ground has thawed — a plant in a pot cannot send roots anywhere to look for moisture. Dry roots freeze harder than moist ones, so a container going into winter dry is in real trouble.
Pot size helps on both counts. The bigger the volume of soil, the slower it swings in either direction, which is why a large container is a genuinely different proposition from a small one.
