Young, newly planted, or marginally hardy trees are vulnerable during winter. Unlike established trees, they lack the natural defenses to withstand freezing temperatures, strong winds, and fluctuating daylight. Winterizing efforts mitigate these environmental challenges to ensure the trees emerge healthy for spring growth. Protection involves strategic measures to maintain stable soil temperatures, guard the bark, and prevent moisture loss.
Insulating the Root Zone
The root system is often the most vulnerable part of a young tree because shallow roots are easily damaged by repeated freezing and thawing cycles. Ground insulation moderates soil temperatures, preventing rapid swings that stress fine root hairs. Stable soil temperature also allows roots to remain viable and absorb water during brief winter thaws.
Organic mulch acts as a crucial insulating blanket for the root zone. An ideal layer, such as aged wood chips, shredded bark, or straw, should be spread two to four inches deep. This layer retains residual soil heat and moisture, essential for root survival.
Use the “mulch doughnut” technique, spreading mulch outward from the trunk while leaving several inches clear against the bark. Piling mulch directly against the trunk (“volcano mulching”) traps moisture, creating an environment for pests and fungal diseases that cause bark decay. The mulch ring should extend to the tree’s dripline, or a few feet in all directions for smaller trees, to cover the active root zone.
Shielding the Trunk
Thin-skinned, young trees are susceptible to sun scald, which typically occurs on the south or southwest side of the trunk during winter. On bright, cold days, dark bark absorbs solar radiation, warming the underlying cambium layer. When the temperature rapidly drops back to freezing, the activated cells rupture, leading to long, vertical cracks in the bark.
Applying a protective, light-colored tree wrap in late fall reflects sunlight and maintains a stable bark temperature, preventing the rapid freeze-thaw cycle. Wraps are typically crepe paper or breathable plastic, spiraled from the base up to the lowest main branches. The wrap must be removed in early spring, usually around April, to prevent trapped moisture from harboring insects and fungal pathogens.
Protection must also guard against hungry winter animals, as rodents and deer feed on soft, young bark when food is scarce. Install a physical barrier, such as hardware cloth or a rigid plastic tree guard, around the base of the trunk to prevent gnawing damage. The guard should extend a few inches into the soil and be tall enough to protect the bark above the expected snow line.
Preventing Winter Desiccation
Winter desiccation, or “winter burn” in evergreens, is a physiological drought where water loss from foliage exceeds root uptake. Evergreens lose water through needles and leaves due to dry winds and sun exposure, but roots cannot absorb replacement water because the soil is frozen. This moisture imbalance causes foliage browning and eventual death, often becoming apparent only in spring.
The most effective preventative measure is deep, thorough watering in late fall before the ground freezes solid. This practice, known as dormant watering, fully hydrates the tree’s tissues, maximizing internal water reserves for the dry winter. Watering should be slow and deep to penetrate the entire root ball.
For vulnerable evergreens, such as arborvitae, boxwoods, and rhododendrons, an anti-desiccant spray can be applied to the foliage. These liquid waxes form a thin, protective coating on the leaves or needles, slowing the rate of water loss by up to 20%. The spray must be applied on a mild, dry day above 40°F after the tree is fully dormant, usually in late November or December.
Protecting the Canopy from Wind and Sun Scald
The above-ground structure, especially the foliage of broadleaf and coniferous evergreens, requires protection from winter wind and intense solar radiation. Strong winds accelerate desiccation by stripping away the boundary layer of humid air around the leaves, significantly increasing water loss. Wind exposure can also physically damage branches and cause needle browning.
To mitigate wind damage, construct temporary physical barriers on the side of the prevailing winter winds. Use stakes or posts to support a screen of burlap or woven snow fencing positioned several feet from the tree. This creates a windbreak that diffuses the air current. The barrier should be taller than the plant but should not completely enclose the top, allowing for air circulation and preventing heat buildup.
For smaller, sensitive evergreens like boxwood or upright junipers, loosely wrapping the entire plant in burlap provides a dual benefit. This wrapping protects the foliage from drying winds and shades the leaves from direct winter sunlight, preventing sun scald damage. Secure the burlap loosely with twine, ensuring it does not compress branches or restrict air flow, which minimizes potential fungal issues.