In cold-winter climate areas, many container-grown perennials, trees, and shrubs can’t be left out in the elements — even if the same plants growing in the ground are perfectly hardy. When you choose perennials for containers, you need to consider their climate adaptability.
Check locally to find out exactly which plants survive outdoors all year where you live and remember that plants that are hardy in the ground in a certain climate may not be hardy in a container. (Add roughly two climate zones.)
The hardiness zones refer to their hardiness when planted in the ground. There is no firm data for container plants.
Gardeners in cold climates take steps to overwinter their container plants to shelter them until the milder temperatures of spring arrive. Tropical plants need to be brought indoors into room temperatures; treat these as houseplants over the winter. Plants from temperate regions (where the plants normally go dormant in winter), on the other hand, need the down time induced by cold weather. For these plants, the goal of overwintering isn’t to keep plants warm but rather to keep them from getting too cold.
Individual gardeners have their own favorite method of overwintering container plants that need a cold-induced dormant period. You can try the following techniques to overwinter cold-sensitive plants that aren’t hardy to your region. Not all of these suggestions are simple, and none of them are foolproof.
After plants are dormant (meaning herbaceous plants have died back and woody deciduous plants have dropped their leaves), water them one last time and place the pots in an insulated garage or cool basement. Look for a spot that will stay in the range of 32 to 45 degrees Fahrenheit. Check the plants occasionally, and water them if the soil dries out. Move the plants back outdoors in spring.
If the storage area is likely to drop below freezing, place the containers in a large cardboard box and fill it with hay, packing peanuts, or anything else that will provide insulation.
For small shrubs and herbaceous perennials, wait until the first hard frost. Then bury each plant, still in its pot, in the middle of your mulch pile, tilting the pot to make sure water from rain and melting snow will drain off. When the weather warms in spring, move the plants back to their regular spots.
For small to medium-sized plants, wait until they’ve entered dormancy, and then create a tall cylinder around the container and plant (using chicken wire, wire fencing, or hardware cloth). Fill it with chopped leaves, straw, or bark mulch. This will prevent the soil and plant from freezing and thawing in winter.
In colder zones, you can use the “tip and bury” method on a deciduous tree that is normally hardy only to zone 7 (such as a fig tree): Tie the branches of the plant together after it loses its leaves, lay it on its side in a trench that is 14 inches deep and wide enough to hold the entire plant (including the pot). Cover it with burlap, and then cover the plant — container and all — with soil. When the soil starts to thaw out in April or later, dig out the plant, stand it up, start watering, and see whether it responds.
Even in regions where winters are chilly but not severe, hardy plants benefit from some protection. Group container plants together in a protected area out of strong winds and bright sunshine. Or, after plants have entered dormancy, wrap pots with bubble wrap to help insulate soil against freeze/thaw cycles.
If you live in a cold climate, remember that winter temperatures can affect your containers as well as your plants. If left outside, terra-cotta, concrete, and ceramic pots with soil in them can crack when moist soil expands as it freezes; move these pots into protected areas for the winter.