Herbaceous perennials in pots — plants that die back and are dormant in winter — that have been part of your summer container displays need to be protected over the winter if they’re going to survive and bloom again next year. Here’s how to overwinter perennials in pots.
Hardy perennials have roots that sleep until next spring, which is when they begin to put on new growth. A few examples are hostas, Shasta daisies, heucheras, astilbe, lady’s mantle and daylilies. To overwinter them successfully, you need to keep the plants dormant and provide a winter environment that’s within their hardiness zone. A plant growing in the ground is more protected from severe cold (and alternate freezing and thawing) than one in a container; therefore, a plant that’s hardy to your zone usually needs extra protection if left in its container.
After a couple of killing frosts, water plants thoroughly and choose one of the following three options for overwintering:
Option 1. Leave the planted container in its current location. If the container is large and able to withstand the elements, and if the plant is at least one zone hardier than your area (i.e., if you live in Zone 5, herbaceous perennials in containers need to be hardy to Zone 4 or lower), the likelihood of successfully overwintering the plant in its pot outdoors is high. A large container holds more soil, which helps insulate roots and keeps soil temperature consistent. However, when sun hits the sides of a container, especially a dark-coloured one, alternate freezing and thawing may trick the plant into thinking it’s spring and trigger early growth, when it’s merely a warm day in February.
Option 2. Move borderline-hardy plants or those in small containers to an unheated garage or shed to increase survival odds. Because the plant is dormant, light isn’t required for photosynthesis, but do check every couple of months to make sure the soil isn’t bone-dry. Don’t overwater, however, as this could cause plants to rot or break dormancy.
When growth resumes in late winter/early spring, reintroduce the plant to normal growing conditions outdoors by gradually exposing it to the elements for increasing periods of time.
Option 3. Find an area where you can sink the plant and its pot into the ground so the roots will be better insulated. (A vegetable garden often has unused space.) Cover the plant with two to three inches (5 to 8 cm) of winter mulch, such as shredded bark or leaves. In spring, remove the mulch and lift out the container.