Preventing Winter Burn on Evergreen Landscape Plants

Pennsylvania's climate zones, ranging from USDA Plant Hardiness Zone 5 in northern mountainous regions to Zone 7 in southeastern areas, present varied challenges for landscape plants. Winter burn, also known as winter desiccation, represents a significant threat to landscape plants throughout Pennsylvania. Damage from winter burn can lead to leaf browning, to branch loss, or even to the demise of the entire plant. Understanding the mechanisms behind winter burn and implementing science-based prevention strategies can help Pennsylvania gardeners protect their valuable plant investments throughout the harsh winter months.

Understanding the Mechanisms of Winter Burn

Winter burn is a physiological disorder that affects both evergreen and semi-evergreen species. These plants continue to transpire (release moisture) through their leaves and needles throughout the winter. Common broadleaf evergreen plants include azalea, boxwood, rhododendron, mountain laurel, and holly. Common needled evergreens include fir, spruce, pine, yew, and hemlock. Also at risk are evergreen plants with scale-like leaves, such as arborvitae, juniper, and false cypress.

Desiccation occurs when frozen ground conditions prevent root water uptake while winter sun, wind, and temperature fluctuations continue to draw moisture from plant tissues. The frozen roots are basically unable to resupply the leaves with the water they need. This fundamental imbalance between water loss and water uptake creates the conditions that lead to tissue damage, browning, and plant stress. The most severe damage will be on tissue farthest from the veins, such as leaf edges, leaf tips, and tops of plants. Damage is usually most severe on the side of the plant exposed to sun and wind. Yet, entire branches and sometimes even entire plants can die from winter burn.

Evergreen shrubs and trees do not go dormant during the winter and have evolved to be able to withstand winter cold to varying degrees. For example, rhododendrons curl their leaves inward and downward when the temperature drops below 20°F. This adaptation, known as thermonasty, occurs at a cellular level when a plant hormone (abscisic acid) is activated by the cold. The leaf tips curl and the entire leaf droops. This curling helps protect the evergreen leaves in several ways. Moisture loss is reduced from the stomata (the microscopic pores that are mostly on the undersides of the leaves). Shade is created, protecting the leaves from sun damage. Curled leaves also thaw more slowly than flat leaves, helping to prevent damage from the freeze-thaw cycles. The degree of curling is fascinating as it can act as a natural thermometer, with leaves curling more tightly as temperatures drop. Another adaptation occurs in needled evergreens when their needles develop thick and waxy coatings. This coating prevents some water evaporation and also acts as insulation.

Yet, with climate change, winter weather is more variable and storms seem to be harsher. My own Zone 6b garden has experienced a few Zone 4b wind storms in the past several years. The combination of temperature fluctuations, harsh wind exposure, and inconsistent snow cover creates conditions particularly conducive to the development of winter burn. Choosing a plant adapted to your garden’s Hardiness Zone is no guarantee that it will avoid the damage of winter desiccation.

Strategic Plant Placement and Selection

Prevention begins with thoughtful plant selection as well as strategic placement in the landscape. "Right plant. Right place." It is important to understand the microclimates within your own property. Exposure to wind is a key factor, as these maladies may be prevented by protecting the plant from the wind or shading the plant. Plants positioned in locations with natural windbreaks, such as against buildings or behind established plantings, experience significantly less water loss during winter months.

Native and adapted plant species may demonstrate superior winter hardiness compared to non-native alternatives. Pennsylvania's native evergreen perennials and woody plants have evolved mechanisms to survive local winter conditions.

Plants that are hardier than your garden's zone will generally survive winter desiccation more easily.

Mulching and Root Protection

Proper mulching is an effective preventive measure against winter burn. Most perennials love to have a blanket of snow in place from December through March. So do evergreens. Snow acts as a great insulator. When covering the soil, snow protects plants' roots and shelters the foliage from strong winter winds and sun. Lacking a layer of insulating snow, the plants' best friend is a layer of mulch.

The insulating properties of mulch serve multiple functions in winter burn prevention. By maintaining soil temperatures above freezing for longer periods, mulch extends the time during which root systems can continue absorbing water. Then, once the ground is frozen, mulch keeps the soil from experiencing the freeze/thaw cycle, providing a buffer against rapid temperature fluctuations that can stress plant root systems. Additionally, mulch reduces the evaporation of moisture in the soil. This is yet another reason to "leave the leaves" — nature's mulch — where they have fallen. Cut branches from Christmas trees can provide a nice insulating layer, too.

Physical Protection Methods

Direct physical protection offers another approach to preventing plant damage from winter burn. Burlap screens and temporary shade structures can be helpful. These protection methods prove particularly valuable for newly established plants that have not yet developed extensive root systems or hardened tissues. It is also valuable for those plants whose hardiness zone matches the garden’s own hardiness zone.

To create a physical barrier, pound 4- to 5-foot-tall stakes into the ground about 2 feet from the drip line of the plant. Either surround the plant or create a wall or arc on the sides exposed to the winds, usually the south and west sides. Attach burlap or canvas to the poles. Leave the top open. Do not fill with leaves as this encourages rodent activity and eating of bark. Remove promptly in the spring when temperatures are above 28°F.

Proper Pruning and Plant Health Management

The timing of pruning activities significantly influences plant susceptibility to winter injury. Avoid pruning in late summer. Pruning stimulates new growth. This tender new growth may lack the physiological hardening necessary to survive winter conditions, making the plant more vulnerable to both winter burn and cold injury. The same is true for fertilization. Avoid fertilizing in the fall, as this will encourage new growth that will be more susceptible to winter burn.

Spring recovery practices also contribute to long-term plant health. Prune away any damaged parts of the plant in the spring before new growth starts. This helps the plant recover from winter burn damage. Proper removal of damaged tissue prevents disease development and allows plants to direct energy toward healthy new growth.

Plant new evergreens in late summer (before October) to allow the roots to grow and establish before the ground freezes.

Water Management and Fall Preparation

Adequate soil moisture entering the winter season provides plants with essential resources for surviving periods when root water uptake becomes limited. Deep, infrequent watering during fall dry periods helps establish soil moisture reserves that plants can access during brief winter thaws. However, avoid oversaturation, as poor drainage during winter can lead to root rots and death if the soil is saturated and frozen.

Conclusion

Preventing winter burn on evergreen plants requires an integrated approach. Proper plant selection, strategic placement, protective mulching, physical barriers, and appropriate cultural and maintenance practices are all valuable strategies. It is more effective to be proactive and prevent winter burn than to be reactive and treat the plant after damage is done. By understanding the physiological mechanisms behind winter burn and implementing these protection strategies, Pennsylvania gardeners can maintain healthy, attractive landscapes throughout even the most challenging winter conditions. As climate patterns continue to shift, these fundamental principles of winter plant protection will be essential tools for successful landscape management across Pennsylvania.