The key to overwintering plants in containers is to prevent the soil mass from freezing and to maintain moisture in the soil throughout the winter. Freezing can be prevented by having a large soil mass in a well-insulated container or planter located in a protected area. Watering containers with needled and broadleaf evergreens such as hollies, boxwoods and ivy is essential in winter.
Most containers can be damaged by freezing and thawing conditions if the soil is left in them during winter. Large concrete and wooden planters are typically able to withstand northern Illinois winters without being cleaned out. All other planters and containers should be emptied of soil and plants and stored upside-down to prolong their useful life.