There’s good news for gardeners living in colder climates. Perennials hardy for zones 2 and 3 can be overwintered in pots (outside) when placed in a sheltered spot in the yard.
Perennials should be potted and allowed to establish themselves for several weeks prior to overwintering. Pot most plants in early October to establish the roots by early November. The better the root system, the better the chance the plant will survive the winter. Dying plant debris should be removed prior to overwintering to prevent botrytis infection. Once the pots are placed in a sheltered area, surround them with piles of leaves to act as mulch and to provide insulation and protect the roots.
Since the roots of potted plants are confined and can’t go searching for what they need, the proper container, soil and drainage is key. Be generous when considering the size of the container – keep in mind the mature size of the plant.
In regions with mild to moderate winters, many perennials will thrive in containers with minimal winter protection. Cold climates, however, create challenges. Even plants hardy in your zone can suffer root damage if the soil in the pot freezes solid. It is critical to the plant’s survival to prevent the freeze-thaw cycle. Problems generally occur when the plant is over- or under-watered. In the fall, water perennials well, but let the foliage dry prior to covering with mulch. While the plants should not go through the winter “dry”, be careful not to over-water them and check soil moisture periodically.
Soil temperatures should range from -1 C to 1 C for most perennials. The key to overwintering plants is to keep them cold and alive but not actively growing. However, temperatures cooler than -1 C may kill some sensitive plants, so it’s wise to verify temperatures with a soil thermometer.
It’s also important to remember that unlike plants in the garden, plants in containers cannot take advantage of temperature buffering provided by the soil. Winter protection of container plants is essential to their survival. The degree of protection required depends on the root hardiness of the species being grown.
Hostas are particularly forgiving and can withstand the winter if left in the ground (and insulated with mulch) or placed in pots and left outside, again as long as they are insulated and protected from the elements. Sedums can also be overwintered in pots placed in sheltered areas.