How can I over-winter unplanted perennials still in their containers?

FAQ
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.

Perennials in containers must be protected from cold temperatures over winter to insure they will survive.

Container grown perennials should not be left outdoors above ground over the winter months. The roots of most perennials are far less cold hardy than their aboveground stems. Container grown plants are growing in relatively small amounts of potting soil. The temperature of the potting mix may drop into the single digits if container grown plants are left above ground in winter. Single digit temperatures may damage or destroy the root systems of plants.  

Bury the Container in the Ground

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. Multiple plants can be placed pot-to-pot in the ground to minimize the amount of digging needed. 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.

Watering

Before the soil freezes in the fall and after it thaws in the spring, provide water when needed.  When the soil is frozen, supplemental irrigation is not necessary.  Keep in mind that drainage will be different if watering containers buried in soil.  Containerized plants buried in soil will not drain as readily and will likely need less supplemental irrigation than they need sitting above ground. Avoid over-watering plants which will promote root, crown, and foliar diseases.

Adding a Blanket of Mulch

An extra layer of mulch can help further insulate perennials and protect their root systems and crowns.  Apply a 3-to-4 inch thick layer of winter mulch after the top layer of soil has begun to freeze, but before your mulch pile freezes preventing you from being able to dig and spread the mulch.  This is typically shortly after the first hard frost.  By putting mulch down at this time, you will help stabilize the temperature of the soil. Applying mulch too soon may delay freezing and encourage heaving and thawing or prevent plants from going fully dormant.  Remove mulch in early spring as soon as it thaws enough to pull it off - typically around mid-March in much of Iowa.

Protect Plants from Animals

Be sure to protect above ground portions of the plants from hungry rabbits, deer, mice, and other animal pests.  Surround plants with chicken wire fencing pinned to the ground and tall enough to prevent access to the plant after a heavy snow.  If possible, create a dome or lid to cover the sides and top of the plant to prevent animal browsing.   

Overwinter in a Garage or Other Building

Container grown perennials can also be over-wintered by placing them in a moderately cold location (temperatures from 20 to 45 degrees Fahrenheit) over the winter months. The cold temperatures will keep the plants dormant until spring. Soil moisture must be monitored carefully in this situation.  Check moisture levels of the soil often during the winter and irrigate if necessary. In an unheated structure, water may be needed as often as once every two weeks if temperatures are above freezing. Avoid over-watering plants.  An attached, unheated garage is often a suitable over-wintering location, if you are confident the temperatures can stay consistently in the 20 to 45 degrees Fahrenheit temperature range.  Many unheated structures can vary more widely in temperature than this.  It is important to monitor and adjust temperatures inside the structure, if needed.