Perennials can be great subjects for garden containers that at one time were reserved for annual plants only. Perennials bring a new note to pots, expanding our options and contributing different types of interest from spring to fall, through foliage, flower and seed head. In recent years, plant breeders have been selecting perennials (and shrubs) for more compact size, making them even more suited to container culture and display.
In cold climates, the onset of fall brings the question: What do I do with my potted perennials for the winter? A major benefit of a perennial plant is its ability to contribute to the garden for more than one growing season. Potted perennials can offer that same advantage. Just follow these guidelines:
Check the hardiness. If your potted perennial is rated hardy to one or two zones colder than where you garden, it has the best chance for making it through the winter outdoors. For example, if you garden in USDA Zone 6, a perennial hardy to at least Zone 4 (two zones colder) has the best chance of surviving your winter. The soil in the pot will not stay as warm as the soil in the ground, because it possesses less mass. It will also freeze sooner and thaw more easily than the ground, and it may freeze and thaw several times during the winter, putting stress on the plant. So a potted perennial must be bone-hardy in your area and colder to survive in a container with no special treatment.
Offer protection. Potted perennials rated to your hardiness zone or just one zone colder can be left outdoors, but they need some protection. Move the pot to a sheltered location, such as right next to a structure (house, shed, evergreen hedge), preferably facing north, where it will be out of the wind and not subject to the thawing power of the sun, as it would be facing south. If you have several potted perennials, cluster the pots together. Cover the pots with shredded leaves, straw or other organic material.
Bury the pots or bring them inside. If you’re concerned about your potted perennials’ hardiness, or they’re choice specimens you’re loathe to risk, consider sinking their pots into the ground before it freezes. Simply dig a hole and plant the perennial pot and all. This way the soil in the pot will benefit from the temperature-stabilizing mass of the ground soil. Alternatively, bring the pot into an unheated, not-too-bright interior space, such as a garage or cold cellar. (It needn’t be completely dark.) Lightly water the plant occasionally over the winter, just enough to make sure that the soil doesn’t completely dry out.
Be sure to consider how the container will weather the winter, too. A frost-proof or freeze-proof pot may be just fine, but containers not designated as such might crack, chip or otherwise show some wear come spring.