There’s nothing tricky about growing plants in containers as long as you offer them good soil, food and water. But because a plant growing in the ground is more protected from severe cold (and alternate freezing and thawing) than one in a container, the real challenge is overwintering those potted specimens so they can adorn your landscape next spring.
As with any game plan, you need to know your players. Some plants are easier to overwinter than others, while some are more trouble than they’re worth. How cold your winters are and to what lengths you’re willing to go to protect your containers are also factors. Here are methods I recommend for saving your plants for next season.
Hardy and borderline-hardy herbaceous perennials
These plants usually die back and are dormant in winter, while their roots sleep until it’s time for new growth next spring. Examples include hostas, shasta daisies, heuchera, astilbe, lady’s mantle and daylilies. The goal is to maintain dormancy and provide a winter environment that’s within their hardiness zone.
After a couple of light frosts, water plants well and choose one of the following storage options:
Leave as is. If the pot is large and the plant is at least one hardiness zone below 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 pot outdoors is high. A large container holds more soil, which helps insulate roots and keeps soil temperatures 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.
Store borderline-hardy plants or those in small containers in an unheated garage or shed. Because the plants are dormant, light isn’t required, but check periodically to make sure the soil isn’t bone-dry. When growth resumes in late winter/early spring, reintroduce plants to normal growing conditions outdoors by gradually exposing containers to the elements for increasing periods of time.
Find an area, such as the vegetable garden, where you can sink the pot in the ground up to its rim so roots will be better insulated. Cover the entire plant with about five centimetres of winter mulch, such as shredded bark or leaves. In spring, remove mulch and lift out your container.