Outdoor potted plants face a unique vulnerability during cold weather because their roots lack the natural insulation provided by the surrounding earth. Unlike in-ground plants, the entire root ball of a container plant is exposed to rapidly dropping air temperatures, which can quickly lead to freezing and damage. Successfully overwintering these containers depends on understanding this sensitivity and implementing layered protection strategies. The goal is to moderate temperature swings and protect the root zone from lethal freezing.
Preparing Plants for Winter Dormancy
Preparation for winter begins well before the first expected frost to encourage natural dormancy. Stop all fertilization, typically by mid-to-late summer, or at least six to eight weeks before the cold sets in. Continuing to feed the plant promotes tender, new growth that is susceptible to frost damage and cannot harden off properly.
Any necessary late-season pruning should be minimal, focusing only on removing dead, diseased, or yellowing foliage. Heavy pruning is discouraged in the fall because it can stimulate new growth, which will be instantly damaged by freezing temperatures. Before moving the containers, inspect the plant and the top inch of soil for pests, treating them with horticultural oil if needed. Ensure the container’s drainage holes are clear, as standing water will freeze, potentially cracking the pot and damaging the roots.
Utilizing Protective Structures and Shelters
Relocating potted plants to a sheltered area provides the most significant temperature buffer against extreme cold. Placing containers against a south-facing wall is a simple strategy, as this area absorbs solar energy during the day and radiates residual heat through the night. This partial protection can be sufficient for plants that are only marginally hardy in your climate.
For more sensitive plants, moving them into an unheated garage, shed, or covered porch offers protection from wind and direct freezing. Within these spaces, the air temperature generally remains about 10 to 20 degrees Fahrenheit warmer than the outside air. The ideal temperature range for dormant plants in these shelters is between 30 and 45 degrees Fahrenheit, which prevents deep freezing but still provides the necessary cold period for dormancy. Monitor the temperature inside these structures to ensure it stays above freezing during prolonged severe cold snaps.
Insulating Containers Against Freezing
When plants must remain outside, the focus shifts to insulating the container walls to slow the rate of temperature drop around the root ball. One effective technique is “huddling,” where pots are grouped tightly together against a sheltered wall, allowing them to share thermal mass and protect the outer containers from wind exposure. Elevating the containers slightly off the cold ground, using pot feet or bricks, prevents drainage holes from freezing shut and reduces heat loss to the surface.
The container itself can be wrapped with insulating materials to create a protective air layer. Layers of plastic bubble wrap secured around the pot’s sides are effective insulators. This bubble wrap layer can be covered with burlap for an aesthetically pleasing look and extra protection. Finally, apply a two-to-four-inch layer of dry mulch, such as straw or shredded leaves, directly to the soil surface to regulate temperature and prevent damaging freeze-thaw cycles.
Winter Watering Adjustments
During winter dormancy, plants have reduced metabolic activity and require significantly less water than during the growing season. Allow the soil to dry out a few inches deep between waterings, which may only be once every few weeks for plants stored in a cool shelter. Overwatering dormant plants is a common mistake that can lead to root rot in cold, wet soil.
The plant should not be allowed to dry out completely, particularly if it is an evergreen that continues to transpire through its leaves. When watering is necessary, do it on a day when the temperature is above 40 degrees Fahrenheit. Water a few days before a hard freeze is predicted, as moist soil holds and releases heat more effectively than dry soil, helping to temper the soil temperature around the roots.