Q. I grew two small evergreens in containers this summer. Can they remain outside in winter?
A. Expecting a hardy woody plant to survive an unpredictable Midwestern winter in a container is risky business. When planted in the ground, an evergreen’s vulnerable roots are insulated from frigid temperatures. When planted in a container, the roots are now above ground, exposed on all sides to temperatures than can drop well below zero. Plants are not "hardened" below ground, and the cold tolerance of roots is between 10 and 15 degrees. Temperatures colder than that will freeze tissue and kill the roots. There are several methods for overwintering small, hardy woody plants. If you have an unheated garage, stairwell or basement space where the temperature remains consistently between 20 and 30 degrees, you can store the plants there once there have been several hard freezes.
You can also transplant your small evergreens directly into the garden or bury them in their pots into the garden in early September. Choose a sunny location where they will be sheltered from strong winds. Water the plants well now, throughout autumn and even during winter thaws. Another option is to wait for several freezes and then move the containers close to the house where they will be protected from direct sun and wind. Water well and wrap the plants and containers in a chicken wire cage or in several layers of bubble wrap stuffed with enough leaves to completely cover the small trees. Make sure the containers are not resting on cement or stone since those surfaces will quickly conduct cold to the plants. As the weather warms in spring, gradually remove the protection, resume normal watering and then move the plants back to their preferred locations.