The cold season presents unique challenges for perennial landscape plants. Three primary threats endanger survival: freezing temperatures, desiccation, and physical damage. Freezing temperatures cause ice crystals to form within plant cells, leading to rupture and death, especially in tissues that have not fully hardened. Desiccation, or winter burn, occurs when foliage, particularly on evergreens, loses moisture through transpiration while frozen ground prevents roots from absorbing replacement water. Plants also face physical stress from heavy snow and ice or from soil heaving, where freeze-thaw cycles push shallow-rooted plants out of the ground.
Preparing the Root Zone for Cold
Protecting the root zone is the most effective defense against the cold since roots are more susceptible to damage than above-ground tissues. Evergreens and newly planted trees require a deep watering in late autumn before the soil freezes solid. Moist soil retains heat and insulates roots better than dry soil, which freezes more deeply. Continue this watering until the ground is completely frozen, ensuring the plant has maximum water reserves to combat winter desiccation.
Applying an insulating layer of mulch stabilizes the soil temperature and prevents the damaging effects of freeze-thaw cycles that cause heaving. Use organic materials like straw, shredded leaves, or wood chips, spreading a layer approximately two to four inches deep over the root area. Pull the mulch several inches away from the plant’s crown or trunk to prevent moisture retention that can lead to rot and provide cover for rodents.
Good drainage remains a constant concern, even in dormancy, as standing water is particularly dangerous in winter. Waterlogged soil prevents oxygen from reaching the roots, and freezing ice can physically damage the root system. Ensure that planting beds are not collecting excessive runoff from downspouts or paved areas, which can lead to ice formation around the plant base.
Shielding Foliage from Wind and Sun
Above-ground plant parts, especially evergreen foliage, require protection from the drying effects of wind and sun. Harsh winter winds accelerate the rate of transpiration, drawing moisture out of the foliage even when the plant is dormant. Erecting a temporary windbreak using breathable material like burlap, secured to wooden stakes, can significantly reduce wind exposure when placed on the windward side.
Sunscald, which affects the bark of thin-skinned trees, and windburn, which damages evergreen foliage, are most common on the south and southwest exposures. Burlap screens can diffuse intense winter sunlight and prevent the rapid temperature fluctuations that cause sunscald. If wrapping smaller shrubs, ensure the burlap is tied loosely around the plant to allow for air circulation and to avoid trapping moisture that promotes fungal growth.
Heavy snow and ice loads pose a physical threat, often leading to broken branches, especially on multi-stemmed or columnar evergreens. After a significant snowfall, gently brush the snow off the branches with a soft broom, using an upward motion. Never attempt to remove ice or shake frozen branches, as the wood is brittle and susceptible to snapping.
To further minimize desiccation, anti-desiccant sprays can be applied to the foliage of broadleaf evergreens. These products form a thin, waxy film on the leaf surface, reducing the rate of water loss by up to 20 percent. Apply the spray in late fall on a dry, calm day when temperatures are above 40 degrees Fahrenheit, ensuring the plant has fully entered dormancy.
Winterizing Container and Tender Plants
Plants grown in containers are particularly vulnerable because their root systems lack the thermal buffer provided by the surrounding earth. The roots of container plants can experience temperatures 10 to 20 degrees colder than those of in-ground plants, making them prone to freezing. To increase the odds of survival, select plants rated for a hardiness zone that is two zones colder than your own.
For hardy container plants that must remain outside, grouping the pots together against a sheltered wall provides mutual insulation and protection from wind. Insulating the pots can be accomplished by wrapping them with bubble wrap or burying the pot entirely in the ground, leaving only the rim exposed. Moving dormant plants to a sheltered location, such as an unheated garage or cold frame where temperatures remain consistently between 30 and 45 degrees Fahrenheit, offers the best protection against deep freezes.
Tender summer-flowering bulbs, tubers, and corms, such as dahlias and gladiolus, cannot withstand freezing and must be lifted for storage. Dig these items up shortly after the foliage dies back following the first light frost. Cure them by allowing them to dry for several days in a warm, ventilated space to toughen the outer skin. Finally, store them in a cool, dark, and well-ventilated area, typically between 40 and 50 degrees Fahrenheit, layered in a medium like peat moss or vermiculite to prevent shriveling or rot.