Preparing perennial plants for winter is necessary to ensure they survive the cold months and return vigorously in the spring. A perennial lives for more than two years, with herbaceous types dying back to the ground each fall while their root systems persist underground. The primary goal of winter preparation is to protect the root crown, the junction between the stem and roots, from the destructive cycle of freezing and thawing. This fluctuation causes the soil to expand and contract, known as “heaving,” which can physically lift the plant out of the ground, exposing and damaging its roots.
Fall Pruning and Garden Cleanup
Physical preparation begins once the foliage has died back, typically after the first hard frost. For most herbaceous perennials, cutting the dead stems back to within a few inches of the ground is appropriate. Timing is important, as the plant reclaims nutrients from the dying leaves before dormancy. Removing spent foliage helps prevent diseases, such as powdery mildew on plants like Bee Balm and Garden Phlox, from overwintering and reinfecting new growth in the spring.
Leaving some plant material can be beneficial, and not all perennials should be cut back in the fall. Plants like Sedum, ornamental grasses, and Coneflowers offer visual interest and texture during the winter months. The standing stems and seed heads also provide food and shelter for beneficial insects and birds. For plants that keep their foliage for insulation, such as ferns and Heuchera, or those prone to disease, delay cutting back until early spring.
Hydration and Soil Preparation
Deep watering late in the season helps protect perennials from desiccation, often called “winter burn,” caused by cold, dry winds. Even when dormant, roots require moisture, and dry soil allows frost to penetrate deeper, leading to damage. Water deeply and thoroughly before the ground freezes solid to ensure the root zone is saturated.
Moist soil holds heat more effectively than dry soil, providing a thermal buffer that reduces the shock of temperature drops. This late-season watering should be done when air temperatures are above 40 degrees Fahrenheit, allowing the water to soak in before potential freezing occurs. Ensure the garden bed has adequate drainage, as standing water during thawing periods can quickly lead to root rot.
Applying Protective Mulch
Mulch is an insulating blanket applied to the soil surface to moderate temperature fluctuations and is the most effective defense against frost heaving. The timing of this application is crucial; wait until the ground is frozen hard, typically after several hard frosts, not before. Applying mulch too early can trap warmth, preventing the plants from fully entering dormancy and making them susceptible to cold damage.
Suitable organic materials for winter mulch include shredded leaves, straw, pine needles, or wood chips, which create air spaces that enhance insulation. The purpose is not to keep the ground warm, but to keep it consistently cold, minimizing the repeated freeze-thaw cycles. Spread a layer of mulch, ideally three to four inches deep, over the plant crowns, ensuring the material is kept slightly away from the plant stems to prevent rot and discourage rodent nesting.
Winterizing Container Plants and Tender Varieties
Perennials growing in containers are significantly more vulnerable to winter damage because their roots lack the insulating protection of the surrounding soil. The roots in a pot are exposed to much colder temperatures and are susceptible to freeze-thaw cycles that can crack the container. One method is to group the pots together in a protected, unheated space, such as a garage or cold frame, where temperatures remain cool but above freezing.
For hardier container perennials, the pots can be buried directly into the ground or a garden bed just before the soil freezes, with the rim at or slightly above the soil line. Cover the buried pots with a heavy layer of mulch. Borderline hardy varieties, those at the edge of their survival zone, may require specialized protection, such as wrapping the exterior of the container with insulating material like burlap or frost cloth to buffer the roots from temperature swings.