I awoke this morning nestled beneath my warm blanket and began to think about what I was going to write about for this week’s gardening article. Then it came to me – my warm blanket protecting me from the cold air could be compared to the blanket of snow that protects perennials during the winter months. Snow in the winter is what wood chips are to the summer….think protective mulch.
In the past annuals were the majority of plant materials being used in containers. These days small trees to shrubs to perennials flowers are being used. Perennials in pots need protection because their root system is basically above ground. This causes a problem as the roots are then subject to cold injury. If the same plants were planted in the soil, their root system would have the benefit of the soil to help insulate and protect the roots from cold injury.
With any container that you want to overwinter, first make sure that the plant has gone dormant. Wait for the temperatures to go into the 20s for several nights and make sure the soil in the container is moist. Plants in moist soil tend to overwinter better than those that are dry.
Snow can act as an insulating barrier for perennials in containers as the freezing weather can damage roots in thin pots. I like to group my potted perennials together along an area of my house and then keep them covered in snow as it is available. Group them together, preferably on the east or north side of a building, then mulch them heavily with straw or hardwood leaves. If you use a live Christmas tree for the holidays, cut off the boughs and lay them on top of and around the potted perennials to give them extra protection after the holidays are over. Avoid placing the pots on a raised location such as a deck as the exposure to the cold air from below can be detrimental.
During late fall, before the ground has frozen, potted perennials can be placed into holes dug in the ground. The soil acts as an insulator to protect the roots. Just remember to remove them in the early spring. If left in too long the roots can begin to grow so much that they will grow through the drainage holes and anchor themselves to the ground. I have placed pots in a protected area from wind, turned them upside down to keep the pots from cracking, then covered them with a combination of mulch, straw, leaves and pine boughs with much success. Once the weather has warmed I start checking my overturned pots for signs of growth.
Potted perennials can also be stored in a garage or outbuilding, however it has to be a cool location or they will not overwinter properly. Always keep in mind that the tissues of some plants, especially trees and shrubs, are more susceptible to cold weather damage in their youth or the first couple of years after transplantation. It is only when they have reached a certain level of maturity that they are then fully hardy.