Tree Care in Winter: Winter Injury on Hardwoods
In the tree care industry, winter injury is a general term that encompasses a slew of cold-related problems. Although more commonly observed on evergreens, winter injury can occur on deciduous plants as well. The following examines the various types of winter injury, and how they impact hardwoods throughout the season.
Winter Injury on Hardwoods
On hardwoods, winter injury can be difficult to diagnose. Symptoms of winter injury due to root and stem damage involve premature leaf expansion, or a lack of growth in spring. This is often followed by branch dieback as temperatures increase. Various other types of winter injury often plague hardwoods. These include sunscald, frost cracking, low temperature injury, spring freezes, snow and ice breakages, and root heaving.
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.
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.
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.
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.
- 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.
- During winter, place container plants in warm areas to avoid low temperature injury.
- Place a protective barrier composed of burlap over or around plants to safeguard them from heavy winds, and the winter sun. This will aid in reducing the incidence of winter desiccation and sunscald.
- 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.