Perennials in pots are often treated as annuals, tossed out in the winter season with plans to add new ones in the spring. However, they can survive the winter with some preparation and care.
Container perennials rated one or two USDA Hardiness Zones colder than your area have a good chance of living throughout the winter outdoors. Treat them as you treat your perennials in the ground. Keep them watered until the ground (and potting mix) freezes. You may want to apply a winter mulch, and if the container is in a very exposed location, pull it into a more sheltered spot, such as against a wall.
For perennials in pots that are not hardy to your area, you have a few options. You can keep them in an unheated interior location, like a garage or a cold frame, making sure they do not dry out completely. Alternatively, you can identify warm microclimates in your garden and try one of the following techniques: Plant the container in the ground just before the soil freezes, with the rim of the pot right above or at the soil line. Cover heavily with mulch. Or remove the perennial from the container and plant it closely together with other perennials in the soil, also adding heavy mulch.
A riskier option is to tightly group all the potted perennials, add heavy mulch over the top and hope for the best. The best location will be one that’s sheltered from wind and strong sun. It may seem counterintuitive, but up against a north-facing wall or hedge is a better choice than a south-facing wall or hedge. That’s because the north wall will stay fairly consistent in temperature, while the south wall may warm during the day and cool drastically at night. This swing can harm the plants.
In addition to winter preparation, it is very important to keep in mind the container you have selected for your perennials. Can the container survive freezing temperatures, or will it crack or break? Keep in mind that as the soil in the pot freezes, it will expand. A pot that’s not very durable may break under the pressure. Fiberglass and plastic pots are least likely to break. Terra-cotta, ceramic and concrete pots might survive the winter; the thicker their walls, the better their chances.