Washington State University Extension

Garden Tips


written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published 3/21/14

Plant marketers are trying to start a new trend in gardening… planting perennials in containers. Advertisements showing both annuals and perennials planted together in pots have been evident in a variety of gardening media. 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 their roots at temperatures above the ambient air temperature. When planted in a containers, the perennials roots are subjected to colder temperatures close to the air temperature.

There are several options for overwintering perennials in container gardens. One way to protect their roots from the cold is to dig holes and sink the pots in the ground. That may be okay for a few small pots, but it would be a monumental task for me because I have numerous very large pots.

A less troublesome way to protect potted perennials is by grouping them together and placing them in a protected spot on the ground, such as in an alcove or corner on the east side of your house 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 all winter. An unheated garage is the most likely place to meet these criteria. (With the number of sizable containers I have, this would this would mean that my car would have to be parked outside all winter.)

Before storing them away in the garage, ground, or a protected spot, you must 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 down and stop so the plants can prepare themselves for winter=s cold temperatures. However, you should still water regularly enough to keep the plants from becoming drought stressed.

Before placing the plants and pots in storage, insure the plants are fully dormant by waiting for the temperature to drop below 30 degrees on several successive nights. While stored away in the garage, periodically check the potting mix. If it becomes dry, water the plants sparingly to keep the mixture slightly moist.

If you decide to follow this new trend of planting perennial in containers, select only hardy perennials. Proven Winners, a company who develops and markets annuals, perennials, and flowering shrubs, suggests when planting flowering perennials or shrubs in containers, chose ones that are hardy in our USDA 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 USDA Hardiness Zones 6a to7b, this would mean selecting perennials hardy in Zones 6a to 7b or Zones 5a to 6b. If you must leave the pots more exposed, they recommend selecting plants that are at least one to two zones colder (Zones 4a to 5b) than your region.

I am sticking with annual flowers in my container gardens. I want to park my car in the garage and I=m not digging big pits in the yard. I also like the option oftrying out different flower and color combinations every year. That=s what makes container gardening fun for me.

Published: 3/21/2014 1:14 PM

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