Winter protection for outdoor plants prevents cellular damage caused by extreme cold and environmental stressors. The primary purpose of covering plants is to mitigate freeze damage, which occurs when water within plant cells expands and ruptures the cell walls. Proper covering also guards against desiccation, commonly known as winter burn, which happens when harsh winter sun and wind pull moisture from leaves and needles while the frozen ground prevents roots from replenishing the water supply. Establishing these protections before the most severe weather arrives significantly increases the likelihood of plant survival through the cold months.
Timing and Pre-Winter Plant Preparation
Protection should generally be applied after the first few light frosts have occurred but before the ground freezes solid, typically between late October and mid-November, depending on the climate. Allowing plants to experience a light frost cycle is beneficial because it signals the plant to fully enter a state of dormancy, which increases its cold hardiness.
A deep watering regimen before the onset of freezing temperatures is effective for all plants, especially evergreens. Moist soil retains heat better than dry soil, providing a thermal blanket that helps insulate the roots and prevent a fatal freeze-thaw cycle. Pruning should be limited to removing only dead, diseased, or broken material, as heavy pruning encourages tender new growth vulnerable to cold injury.
Choosing the Appropriate Covering Materials
Selecting the right material balances insulation with the need for air exchange. Breathable materials like burlap and spun-bonded row covers are preferred for wrapping shrubs, as they allow air to circulate and prevent the build-up of damaging moisture that can lead to fungal diseases. These fabrics also filter intense winter sunlight, which is a major cause of desiccation in broadleaf evergreens.
Mulch, such as shredded leaves or straw, primarily insulates the soil and the plant crown. A layer of organic mulch helps to stabilize the soil temperature and minimizes the heaving that occurs from repeated freezing and thawing, which physically damages shallow root systems. Plastic sheeting should be used with caution and never draped directly over foliage; if employed, it must be supported by a frame and vented, as it traps moisture and heat, leading to condensation and tissue burn.
Methods for Securing and Insulating Plants
The application method should be tailored to the plant type to ensure efficacy and prevent physical harm.
Mounding for Perennials and Roses
For in-ground perennials and roses, the technique of “mounding” is applied after the ground has frozen. Pile approximately eight to twelve inches of shredded bark, compost, or soil directly over the plant’s crown. This deep layer insulates the most vulnerable part of the plant, the graft union or growing point, from temperature extremes.
Protecting Shrubs and Small Trees
Protecting shrubs and small trees requires creating a supportive structure to hold the covering material away from the plant itself. Drive three or four sturdy stakes into the ground around the plant and wrap the chosen material, like burlap, around the stakes to form an open cylinder or cage. This frame prevents the weight of snow or ice from crushing branches and ensures the cover does not press against the foliage, which would negate the insulating air layer.
Insulating Container Plants
Container plants are highly susceptible to cold because their roots lack the insulation of the surrounding earth. To protect them, group containers tightly together against a sheltered wall, then wrap the entire cluster of pots with an insulating material such as bubble wrap or thick burlap, securing it with twine. For maximum root protection, a four-inch layer of straw or wood chips should be applied to the soil surface inside the pot, or the entire container can be placed inside a larger, insulated box filled with leaves.
When to Remove Winter Protection
The removal of winter protection must be done gradually to allow plants time to acclimate to increasing light and changing temperatures. Covers should remain in place until the threat of sustained hard freezes has passed, generally when nighttime temperatures remain reliably above 28°F. Removing the protection too early subjects tender new growth to late frosts and the shock of direct sunlight.
It is best to remove covers on a cloudy or overcast day to prevent immediate sun-scald and scorching of foliage that has been shaded all winter. Once the covers are off, inspect plants for any signs of pest activity or mold. Remove excess mounding material from the crowns of perennials and roses to allow the soil to warm up and prevent the development of crown rot as spring moisture increases.