Preparing perennial plants for the dormant season is a necessary process known as winterizing. This practice protects the perennial’s crown, the point where the stem meets the root system just below the soil surface. Winterization guards against repeated freeze-thaw cycles, which can cause frost heaving and expose delicate roots. It also prevents desiccation, or drying out, which occurs when roots cannot absorb water from frozen ground but the stems lose moisture to cold, dry air.
Timing the Winterization Process
The precise moment to begin winterizing depends on local climate, but the window opens only after the first hard frost has occurred. This frost is a natural signal that causes the plant’s foliage to die back, triggering the necessary shift into dormancy. Attempting to force the process earlier can disrupt the plant’s natural hardening-off mechanisms, making it more vulnerable to winter damage.
The preparation must be completed before the soil freezes completely. A final, deep watering of the perennial beds is important just before this freeze occurs. Hydrating the root zone ensures the plant has sufficient moisture reserves to withstand desiccation when frozen soil prevents water uptake.
Pruning and Garden Cleanup
Once the foliage has collapsed after the hard frost, the physical work of pruning and cleaning the garden can begin. For many common garden perennials, such as hostas, daylilies, and peonies, it is beneficial to cut the dead stems and leaves back to within a few inches of the soil line. Removing this spent material prevents the spread of fungal diseases like powdery mildew or botrytis blight that can overwinter in the decaying matter.
This cleanup is also effective against pests, as many insects lay eggs or seek shelter in hollow stems and decaying plant material. Carefully removing and disposing of all diseased material away from the garden bed ensures pathogens are not present near the dormant root crown, reducing the inoculum source for the following spring.
However, not all perennials benefit from a fall cutback; some species offer structure and food sources during the winter months. Ornamental grasses and plants with rigid, attractive seed heads, such as sedum ‘Autumn Joy’ and coneflowers, should be left standing. These structures provide visual interest in the snowy landscape and offer shelter or food for overwintering birds and beneficial insects.
Semi-evergreen varieties, including hellebores and certain types of coral bells, should also be left intact. These plants retain some foliage throughout the winter, and pruning them back in the fall can unnecessarily stress the plant. The remaining foliage can provide a small degree of insulation to the crown, and any necessary cleanup can be deferred until new growth begins in early spring.
Applying Protective Layers
The final stage of winterization involves applying an insulating layer to protect the soil from severe temperature swings. This protective layer, known as winter mulch, stabilizes the temperature of the root zone by keeping the ground consistently cold. It must be applied only after the top layer of soil has frozen slightly, ensuring the plant is fully dormant and rodents have not yet established nests.
Suitable materials for this application include shredded leaves, clean straw, pine needles, or wood chips, all of which provide loft and trap air effectively. Avoid using heavy, undecomposed materials, which can compact and hold too much moisture, potentially leading to crown rot. The mulch must be loose and breathable to prevent moisture buildup around the plant tissue.
The layer of winter mulch should be applied four to six inches deep over the entire root zone, extending slightly beyond the plant’s drip line. Do not cover the plant’s crown directly with a dense layer of mulch, as this can smother the plant and encourage decay.
Protecting Container Perennials
Perennials grown in above-ground containers require different protective measures because their root systems are fully exposed to freezing air temperatures. The soil mass in a pot freezes much faster and deeper than in the ground, offering little buffer against cold. Containerized plants are often one or two zones less hardy than those planted directly in the garden soil.
For maximum protection, smaller containers can be moved to an unheated but protected location, such as a cool cellar, shed, or unheated garage, where temperatures remain just above freezing. If moving the containers is not feasible, the entire pot can be buried in the ground in an empty garden bed, mimicking the insulation of the natural soil. This method effectively protects the root ball by surrounding it with stable earth temperatures.
Alternatively, larger containers can be grouped together against a sheltered wall and wrapped to insulate the pot walls. Materials like burlap, bubble wrap, or even several layers of thick plastic can be used to create a thermal barrier around the container itself. Wrapping the container minimizes the rate of temperature change within the root ball, significantly reducing the risk of freeze damage and ensuring a healthy spring emergence.