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Perennials in pots

Q: I live in Zone 5 and would like to plant perennials in pots. How should I care for them over the winter?

V. Woodsworth, Troy, MI

Choose containers that won't crack with winter's freezing and thawing temperatures. Photo/Illustration: Jennifer Blume

A: Gary R. Keim, a garden designer in Swarthmore, Pennsylvania, replies: The first thing you must do is to choose containers that can be left outdoors during the cold winter months. This eliminates nearly all terra-cotta and most ceramic pots since winter freezing and thawing will put cracks in these containers. Instead, pick out containers made of wood, fiberglass, reconstituted stone, plastic, or concrete—all of these are available in a dizzying array of shapes, sizes, and colors.

Second, it’s important to select perennials that will be container-hardy in your zone. This may require a bit of experimentation, but the rule of thumb for root hardiness for containers is to choose plants that are two zones hardier than the zone you live in. For instance, if a plant is hardy to Zone 3, then it will be container hardy in Zone 5. Also, keep the location of the container in mind when choosing plants. If your pot is on a sheltered patio or on the south side of the house, then the microclimate may be warm enough to allow you to cheat by one zone.

If you want to grow perennials in pots that are not container- or root-hardy in your zone, then you will have to overwinter the pots in a protected location like a dark, unheated room, garage, or basement. The area’s temperature should be as close to freezing as possible, allowing the plants to slip into dormancy and stay that way until you bring them outdoors the following spring.

Water sparingly over the winter since plants don’t need much moisture while dormant. If any spindly shoots have grown while the plants were inside, pinch them off when you move the pots outdoors in early spring. Then water the pots well and new growth will soon appear. Once they are 4 or 5 inches tall, begin feeding plants for a new season of enjoyment.

From Fine Gardening 68, pp. 74