In simpler times, container gardening was small-scale landscaping using flowering annuals. Enjoy their colour for one season and go with something new the next.
But shifts toward food production and mixed container planting perennials with annuals have changed all that, creating the need for over-wintering.
“Many more people have limited or no garden space (and) as a result are doing their food gardening in containers,” said Marianne Ophardt, an extension horticulturist with Washington State University’s Benton County office. “Cultivating small (perennial) fruit like raspberries, strawberries and blueberries is one way for these gardeners to grow their own fruit, and it’s fun.”
Ignore the rules about hardiness zones if you’re trying to over-winter perennials in pots. All bets are off when containers are exposed to dehydrating winds and seasonal freeze-dry cycles.
“When we put woody or perennial plants in pots above the soil, they have lost the insulation provided by the soil and are exposed to potentially tissue-killing temperatures,” Ophardt said. “As a general rule, the least hardy parts of woody plants are the roots.”
The best way to help plants survive the winter is to select the right varieties to begin with, she said. Buy the most cold-tolerant varieties you can find.
“I pick the hardiest available rated with a USDA zone that is two zones colder than my zone,” Ophardt said.
Some plants genetically are more perennial than others. Woody plants like shrubs, for example, usually are more cold-hardy than are soft-stemmed herbaceous perennials.
Many techniques have evolved to help potted plants survive the winter. Several of the most common:
Use bigger containers. “Larger root masses and soil volumes are less susceptible to winter injury,” said Hannah Mathers, an Ohio State University professor who specializes in commercial nursery and landscape extension.
Dig them into the ground. “Dig a pit and cover with poly (plastic sheeting) and straw to over-winter some smaller containers,” Mathers said. “Trapping the heat from the earth is an ideal way to over-winter plants.”
Group them. Less troublesome that digging can be “grouping and placing them in a protected spot on the ground, such as an alcove or corner, and mulching them with compost or straw,” Ophardt said.
Bring them indoors. “Anyplace cool but that will stay above freezing,” Ophardt said. “Most often an unheated garage meets this criterion.”
Potted plants should be prepared if they’re to be over-wintered in unheated structures, Mathers said.
“Watering should be reduced in late September and early October to help the plant acclimate,” she said. “Fertilizer should be stopped in early September to reduce lush growth and again allow for acclimation.”
Over-wintering perennials gives you a running start for the next growing season, especially if you’re a fan of mixed container gardening. It supplies nearly filled containers that need only a few annuals to complete.
“I myself this year did several mixed planters with small shrubs, small conifer trees and herbaceous perennials mixed with annuals,” Mathers said. “I will be over-wintering these in an unheated greenhouse. Some of the larger pots with conifers I will keep outside for winter colour.”