If a tree, shrub, or 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 plants should not be left outdoors above ground over the winter months. The roots of most trees, shrubs, and perennials are far less cold hardy than their aboveground stems or trunks. 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.
Trees, shrubs, and perennials still in their containers must be protected from the cold temperatures of winter
Bury the Container
One of the most reliable ways for home gardeners to over-winter container-grown plants 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.
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 that are 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.
Add a Blanket of Straw or Mulch
Further insulate plants with a layer of straw or pine needles. After plants have gone dormant, place 6 to 12 inches of straw, pine needles, or other loose mulch over the containerized plants you've buried in the ground. Avoid using leaves as they often compact too much and can smother perennials and other pants. Gently tie together the branches of woody shrubs so they won't be damaged when you pack insulating material around them. Make a cylinder around the outside of the plant with chicken wire or other types of garden fencing. Make the cage tall enough to enclose the entire plant. Fasten the wire fencing to a stake with wire or staples to add support. Fill the cage with straw working carefully so no branches are broken in the process. It can be helpful to place the straw around the plant before adding the wire cylinder. To insure the straw does not blow away, wrap the outside of the cage with burlap or shade cloth and secure it with twine. Remove the straw in early spring after the extreme temperatures of winter have passed but before the plant breaks dormancy - typically around mid-March in Iowa.
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 organic mulch such as woodchips 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 heavy snow. If possible, create a dome or lid to cover the sides and top of the plant to prevent animal browsing. Tree wrap or plastic tree tubes can be used on trees to prevent rabbits and other rodents from stripping bark.
Overwinter in an Unheated Garage or Other Building
Container-grown trees, shrubs, and 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 such as an unheated structure. 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 to three weeks if temperatures are above freezing. Avoid over-watering plants. An attached, unheated garage or a three-season porch is often a suitable over-wintering location if you are confident the temperatures can stay consistently in the 20 to 45°F temperature range. Many unheated structures can vary more widely in temperature than this getting both warmer than 45°F on sunny winter days and much colder than 20°F during the night. It is important to monitor and adjust temperatures inside the structure if needed.