Question:
How can I over-winter unplanted perennials still in their containers?Answer:
If a perennial cannot be planted in its final spot before winter arrives, you will need to take steps to make sure it survives the winter while still in the container.
Container-grown perennials should not be left outdoors above ground over the winter months as the unprotected crowns and root systems will be damaged by the cold winter temperatures.
An excellent way for home gardeners to over-winter container-grown perennials is to dig holes in a garden area and set the pots in the ground. After the plants have been placed in the holes, place soil around the pots as if planting them. Soil is a good insulator and will protect the plant’s roots from extreme cold.
Plants overwintered in containers should be heeled into the ground after they go dormant but before the soil freezes. This is typically within a few weeks after the first frost in the fall. Pull containers out of the soil in spring when the extreme temperatures of winter have passed and the soil thaws, allowing you to dig and remove the containers. This is typically several weeks before the last frost date for your area.
Container-grown perennials can also be overwintered by placing them in an attached, unheated garage or other structure if you are confident the temperatures can stay consistently between 20 to 45°F. Many unheated structures can vary more widely in temperature than this. It is important to monitor and adjust temperatures inside the structure if needed.
More detailed information about overwintering plants in containers can be found in this article: Overwintering Unplanted Trees, Shrubs and Perennials
