Winter Protection of Trees and Shrubs

Question: Last year we lost a prized tree and some shrubs after a hard winter? What kind of winter protection can we do to prevent this from occurring again? Diana, Trenton, NJ

Answer: Diana, winter or frost injury is common in many kinds of trees and shrubs, growing over most of America where seasonal temperatures fluctuate greatly.

These injuries often weaken or wound plants, leading to later attack by insects or diseases which kill the plant. Several types of injury may be considered:

Frost Injury

Frost injury may result from freezing temperatures after plants have started growth in the spring (spring frosts) or before trees and shrubs have matured or “hardened” and entered their winter dormancy (autumn frosts).

Frost injury to deciduous plants involves mainly the leaves, blossoms, tender twigs or young fruit, which suddenly wilt and turn brownish-black. The leaves of some trees may be slit, torn and disfigured.

Setting of fruit may be reduced and twigs may die back. The needles of evergreens turn reddish-brown and may drop, leaving partially denuded plants.

Warm days in late winter or early spring stimulate succulent new growth which is easily injured by the chilly weather which follows. Trees and shrubs growing in hollows or valleys, called “frost pockets,” are often more severely injured than those on higher ground, which may show no ill effects.

Autumn frost injury is most common on young trees and shrubs with thin, smooth bark, which continue active growth late in the fall. Severe injury or death results when sudden periods of near zero weather occur in mid or late fall before plant tissues are hardened by periods of moderate cold.

A good example was the widespread damage in Minnesota, Wisconsin, Iowa, and parts of adjoining states resulting from the famous Armistice Day storm of 1940. This one storm wiped out hundreds of apple orchards.

Plants easily damaged by frost injury: apple, hackberry, pines, azalea, horse chestnut, plum, basswood (linden), Japanese dogwood, privet, boxwood, lilac, spirea, catalpa, locusts, sycamore, cherry, Norway spruce, white ash, elm, oaks (red and black), white fir, firethorn (pyracantha), peach, willow

Winter Killing

This type of injury results from low temperatures after the end of the growing season and before growth starts in the spring.

The type and severity of the injury is influenced by the species, variety, vigor and age of the plant, its degree of dormancy, soil moisture and drainage, location, natural protection, character of the root system, weather conditions and other factors.

The degree of winter injury may vary from complete killing to localized injury, such as twig blight, die back, root and bud killing, frost cracks, limb or trunk cankers, winter sunscald, crown or collar rot, and “little leaf.”

Trees susceptible to winter injury include: apple, ash, beech, catalpa, elm, maple, peach, pine, walnut, willow and Douglas fir.

Frost Cracks

Frost cracks, like other forms of winter injury, are most prevalent on the south and southwest exposures of plants. Heat from the sun’s rays warms the bark and outer wood on these sides during the day. The temperature drops suddenly during the night, creating the strain.

When warmer weather arrives frost cracks close and become sealed by a rounded ridge of callus growth called a “frost rib.” The crack, however, never closes completely, and usually reopens during following winters, thus allowing the entrance and spread of wood-rotting fungi.

Young, isolated, vigorous, thin-barked, deciduous trees growing in poorly drained locations are most susceptible to injury.

A sudden and pronounced drop in temperature may cause vertical cracks to form in the bark and wood of the trunk, which may extend to the center of the tree or even beyond.

Great strains are set up, due to unequal shrinkage between the outer and inner wood. These strains are released only by sudden separation of the wood layers, often accompanied with a sharp report.

Trees susceptible to frost cracks: basswood, elm, firs, horse chestnut,Japanese larch, maples (especially Norway and Schwedler), London plane, oaks, and sycamore.

Winter Sunscald and Frost Cankers

These appear as well-defined, scorched dead patches in the bark on the south and southwest sides of the trunk on exposed faces of limbs and in the crotches of the larger limbs.

Scalding by the sun’s rays causes overheating, drying out and killing of the inner bark and cambium. Dead bark may later curl and peel off, exposing the sapwood underneath. Cankers may form at these injured areas which are surrounded by ridges of callus growth.

Deciduous trees with smooth, thin bark are most susceptible. Such trees should be protected from both direct and reflected sunlight from nearby light-colored walls, sidewalks, concrete roads, or ice and snow.

Trees susceptible to sunscald and frost cankers: apple, beeches, birches, Douglas fir, European larch, spruces

Winter Drying of Evergreens

This common type of winter injury occurs when periods of cold weather are followed by sudden, extreme temperature in creases accompanied by warm, drying winds. These conditions cause great loss of water from the leaves which cannot be replaced by the roots either because the soil is frozen and water is unavailable to the roots, or because the wood in the stems is frozen.

The needles or leaves wilt, dry up and are browned entirely or part of the way downward from the tip (and along the outer leaf margins of broad-leaved evergreens). Young twigs and branches may die back. Injury is most common where the soil is relatively bare of snow or vegetation.

Shallow-rooted evergreens growing in exposed, warm, sunny spots are most susceptible, particularly if the summer or fall was unusually dry.

Evergreens susceptible to winter injury: arbor-vitae, juniper (or cedars), azalea, laurel, boxwood, leucothoe, firs, pines, holly, rhododendron, spruces.

Related Topics

• 12 Tips To Minimize Winter Tree Plant Injury
• How To Minimize Winter Injury To Trees And Shrubs
• Winter Tree Trimming and Pruning

Related Articles Of Interest:

• How To Minimize Winter Injury To Trees And Shrubs
• 12 Tips To Minimize Winter Tree Plant Injury
• 3 Tips On Pruning Trees and Shrubs – Why, How, When
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