Marketers are trying to start a new trend in gardening: planting perennials in containers. Advertisements showing annuals and perennials planted together in pots have been evident. However, I am not sure this is a trend that gardeners in our area will want to try.
Locally, we are usually not concerned about hardy garden perennials surviving the winter, even after the cold temperatures we experienced this past winter. When planted in the garden, the soil provides insulation, keeping roots at temperatures above the ambient air temperature. When planted in containers, the perennial roots are subjected to colder temperatures close to the air temperature.
There are options for overwintering perennials in container gardens. One way to protect roots is to dig holes and sink the pots in the ground. That may be OK for a few small pots, but it would be a monumental task for me because I have large pots.
A less troublesome way to protect potted perennials is by grouping and placing them in a protected spot on the ground, such as in an alcove or corner and mulching them with compost or straw.
Perhaps the best option is to move the potted perennials into an unheated structure where the temperature will stay cool but above freezing. An unheated garage is the most likely place to meet these criteria. (With the number of sizable containers I have, this would mean parking my car outside all winter.)
Before storing, prepare containerized perennials for winter. This is done by not fertilizing or heavily watering the plants in late summer and fall. You want growth to slow and stop so the plants can prepare for winter's cold temperatures. However, still water regularly to keep the plants from becoming drought stressed.
Before placing the plants and pots in storage, ensure the plants are fully dormant by waiting for the temperature to drop below 30 degrees on successive nights. While stored, periodically check the potting mix. If it is dry, water sparingly to keep the mixture slightly moist.
If you decide to follow this trend, select only hardy perennials. Proven Winners, a company that develops and markets annuals, perennials and flowering shrubs suggests choosing ones that are hardy in our United States Department of Agriculture Hardiness Zone or one zone colder if you will be overwintering them in an unheated garage or burying the pots in the ground. Since we live in Hardiness Zones 6a to 7b, select perennials hardy in Zones 6a to 7b or Zones 5a to 6b. If you must leave the pots more exposed, the USDA recommends plants that are at least one to two zones colder than your region.
I am sticking with annual flowers in my containers. I want to park my car in the garage and I'm not digging big pits in the yard. I also like the option of trying flower and color combinations each year. That's what makes container gardening fun for me.
-- Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.