Introduction
During the winter months, various cold-related injuries may be observed on evergreens. These cold-related injuries, also referred to as winter injuries, encompass a broad range of seasonal challenges. The following examines the types of winter injury that impact evergreens, including winter desiccation, sunscald, frost cracking, low temperature injury, spring freezes, snow and ice breakages, and root heaving.
Winter Injury on Evergreens
Winter injury on evergreens generally consists of foliar discoloration. On many evergreens, damaged foliage will turn a reddish-brown color. This causes the foliage to appear scorched. Some evergreens may turn yellow or gray. Blue spruce will turn a light shade of purple. Eventually, the foliage will be shed, creating bare patches on the affected evergreen. Despite the loss of foliage, the evergreen will often retain its buds. Healthy buds will leaf out in spring, producing a new flush of growth. This type of winter injury occurs on the western and southern exposures of the plant. Sunlight warms the dormant needles, causing them to become active. As night falls, the needles are exposed to freezing temperatures, which results in a rupturing of the cellular tissues.
Snow accumulation often prevents winter injury of the lower branches of evergreens by insulating them from drying winds, and the glaring sun. In such instances, the upper half of the evergreen will display discolored needles, while the lower half of the crown remains green. Colorado blue spruce, eastern white pine, red pine, arbovitae, yew, maples, and lindens are prone to this type of winter injury. On arbovitae and yew, discoloration of the foliage may not become apparent until May or June.
This type of winter injury may resemble several needle cast diseases. To discern whether or not an evergreen has been infected by a needle cast disease, inspect the plant for the presence of fruiting bodies on the foliage. Fruiting bodies are characteristic of many fungal diseases. If a plant has been infected, the foliage will be laden with fruiting bodies.
Winter Desiccation
Winter desiccation, also called winter drying, winter drought, or winter burn, can be observed in late winter, or early spring on evergreen plants. Broad-leaved evergreens, such as rhododendron and boxwood, exhibit browning, or total necrosis of their leaf margins. Narrow-leaved evergreens, such as arbovitae, hemlock, juniper, yew, and pine, develop slight to extensive browning of the needle tips, and a premature abscission of affected needles. Winter desiccation is most common during sunny or windy days. Plants lose an excess of water through transpiration, and are unable to replenish the moisture through their root systems or stems. This causes the plant’s foliage to dry out. Eventually, desiccated foliage dies back.
Sunscald
Sunscald primarily occurs on hardwoods, though evergreens may also be affected. Sunscald, also referred to as southwest injury, is a type of bark injury that often occurs on the southern and western exposures of smooth-barked trees, and shrubs. Maples, lindens, pines, mountain ash, and fruit trees are especially vulnerable to sunscald. Warm periods in winter can increase the cambial temperature of trees, causing them to break dormancy. As night falls, the temperature plummets, and the bark rapidly freezes. The abrupt shift in temperature can result in the formation of vertical frost cracks in the bark. Frost cracks provide infection points for insects, and disease pathogens.
Frost Cracking
Splits in the bark and wood of plants are caused by rapid drops in temperature. They may be associated with internal defects arising from a previous injury to the trunk. Defective wood does not contract as readily as healthy wood. When temperatures plunge below freezing, the defective wood attempts to contract, and becomes strained. Eventually, the outer layers of the wood crack. In winter, frost cracks can widen or narrow, depending on the temperature. Frost cracks often close in spring, and callus over in summer. Once the temperature drops in winter, the frost cracks rupture open once again. Large frost ribs sometimes form on the sides of affected trees.
Low Temperature Injury
During winter, the temperature often plummets below freezing, causing flower buds, vegetative buds, branches, stems, crowns, bark, roots, or entire plants to sustain frost injury. Young plants, and container plants are especially vulnerable to low temperature injuries. Young plants seldom develop extensive root systems prior to their first winter, and the roots of container plants are not well insulated.
Spring Freezes
As temperatures warm in spring, plants resume activity, and begin to produce new growth. Occasionally, late spring freezes may occur, resulting in damage to stems, buds, blossoms, and new shoots. Following a spring frost, succulent tissue that has been injured will appear watersoaked before withering. The damage caused by this type of winter injury resembles that of numerous blight diseases. It is possible to differentiate between frost injury, and blight diseases. Frost injuries tend to appear shortly after a hard frost has occurred, whereas blight diseases are more progressive.
Root Heaving
During winter, snow and ice loading on the upper portion of a plant can result in root heaving. Young plants, small shrubs, and container plants are especially prone to root heaving. When heaved out of the soil, plant roots are exposed to cold temperatures, drying winds, and the glaring sun. Persistent exposure to winter conditions can promote the degradation of plant roots. Extensive root damage often culminates in plant mortality.
Snow and Ice Breakage
Winter storms are often accompanied by heavy snow and ice, which accumulate on the branches of trees and shrubs. Significant snow or ice loading can cause plants to incur breakages on weak limbs.
Management of Winter Injury
- When planting, limit the selection of species vulnerable to winter injuries.
- Remove damaged and dead branches from plants to discourage invasion from disease pathogens, and insects. Eliminate branches that are structurally weak. Branches with a wide angle generally exhibit stronger attachments than those with a narrow angle.
- Ensure that plants are sufficiently watered, especially during extended periods of drought. Thoroughly drench the soil around plants every two weeks throughout the growing season. Adequate watering will improve plant vigor, and minimize the impact of winter injuries. Watering is especially important for preventing winter desiccation.
- Anti-desiccant sprays can be applied to plants in late fall and February to reduce the amount of moisture released through transpiration.
- Apply a layer of organic mulch around the base of plants to improve soil quality, moderate soil temperature, and retain soil moisture. Mulching will insulate roots from winter conditions, and help to deter frost heaving.
- When root heaving occurs, restore heaved plants quickly, and apply a combination of soil and organic mulch over the roots.
- Avoid wounding the bark of plants, especially when they are young.
- Frost cracks can be braced to prevent them from opening during subsequent winters.
- Periodically thin out trees and shrubs to promote air circulation, and to reduce the amount of snow and ice loading from winter storms.
- Cabling and bracing may be effective at preventing limb breakages due to ice or snow loading.
