Have you lost a prized tree or shrub after a hard winter? Perhaps you can do something now to prevent this from occurring again.
Winter or frost injury is common in many kinds of trees and shrubs growing over most of mid-America, where seasonal temperatures fluctuate greatly.
These injuries often weaken or wound plants, leading to a later attack by insects or diseases that kill the plant. Several types of injury may be considered:
Frost Injury
Frost injury may result from freezing temperatures after plants have started growing in the spring (spring frosts) or before trees and shrubs have matured or “hardened” and entered their winter dormancy (autumn frosts).
Frost’s 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. The 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 following chilly weather.
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
Autumn frost injury is most common on young trees and shrubs with thin, smooth bark, which continues 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 are influenced by the species, variety, vigor, and age of the plant, its degree of dormancy, soil moisture and drainage, location, natural protection, the 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. The 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 strain. When warmer weather arrives, frost cracks close and becomes sealed by a rounded ridge of callus growth called a “frost rib.”
The crack, however, never closes completely and usually reopens during the 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.
Due to unequal shrinkage between the outer and inner wood, great strains are set up. These strains are released only by sudden separation of the wood layers, often accompanied by a sharp report.
Trees susceptible to frost cracks:
- Basswood
- Elm
- Firs
- Horse chestnut
- Japanese larch
- Maples (especially Norway and Schwedler)
- London plane
- Oaks
- 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 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 in these injured areas, 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 frost cankers:
- Apple
- Beeches
- Birches
- Douglas fir
- European larch
- London plane
- Maples
- Mountain ash
- Pines
- Spruces
Winter Drying of Evergreens
This common type of winter injury occurs when periods of cold weather are followed by sudden, extreme temperature increases accompanied by warm, drying winds.
These conditions cause a 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 partially down from the tip (and along the outer leaf margins of broad-leaved evergreens).
Young twigs and branches may die back. Injury is most common when 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 is unusually dry.
Evergreens susceptible to winter injury:
- Arbor-vitae
- Azalea
- Boxwood
- Firs
- Holly
- Juniper (or cedars)
- Laurel
- Leucothoe
- Pines
- Rhododendron
- Spruces
How to Help Prevent Winter Injury
Many types of injury are out of man’s control. However, several precautions and preventive measures can be taken:
1. Grow species and varieties of plants that are winter hardy and are adapted to your particular area. Check with your state extension service for recommendations regarding what types of trees and shrubs to plant. Avoid planting susceptible plants except in protected locations and well-drained soils.
2. Avoid practices that encourage susceptible, tender growth late in the season, such as large summer applications of a high-nitrogen-containing fertilizer, heavy pruning, and excess watering.
3. Fertilize the soil around injured deciduous trees in the early spring to increase vigor. It is best first to have soil tests made and then follow the recommendations made from these tests.
As a general rule, a total of two to three pounds of commercial fertilizer (For example, 12-12-12, 10-10-10, 12-6-4, 10-8-6, 10-8-4, 10-6-4, 10-3-3, and 8-5-3) for every inch diameter of the trunk at chest height is sufficient.
Small trees (less than six inches in diameter) should usually receive only half of this amount.
Apply the fertilizer in a series of crow or punch bar holes in a circle around the outermost ends of the branches. The holes should be 12 to 24 inches deep and about two feet apart.
Feeding needles and compressed-air drills or soil augers are usually used by commercial arborists.
Larger trees need several concentric circles of holes, with the outermost circle underneath the branch tips.
After putting the fertilizer in the holes, water it well with a hose for about three days. The holes may be left open or filled with sand, pea gravel, peat moss, or rich topsoil.
Small evergreens may be injured by commercial fertilizers. Any fertilizer used should probably be a slow-acting, organic type such as soybean or cottonseed meal. The fertilizer may be hoed or watered into the soil thoroughly.
Broad-leaved evergreens, azaleas, rhododendrons, Andromeda, laurel, and the like require acid soil.
For these plants, liberal quantities of acid peat moss or rotted oak-leaf mold into the soil and add more as a mulch.
The soils in many areas of mid-America are too basic for these plants to grow unless the soil is artificially acidified.
For shrubby-type pines, spruces, and junipers, apply about one-half to one pound of commercial fertilizer per plant twice a year, in early spring and again in late spring.
Large specimen evergreens usually require two to two and one-half pounds per inch of trunk diameter. The fertilizer is placed in holes 12″ to 15″ inches deep under the tips of the branches in early spring or in the fall.
Fertilization tends to increase the amount and depth of root growth, hence helping reduce the possibility of severe winter injury.
4. Improve the air and water drainage in heavy, wet soils by shallow spading of the soil under the tree. Susceptible trees always suffer less winter injury where the soil is well-drained.
5. Thoroughly soak the soil out under the branches of trees at two to three-week intervals during dry weather, using long pieces of plastic or canvas “soaker” hose or a watering lance. Soak the soil thoroughly before freezing weather sets in. Evergreens, especially, should not go into the winter in a dried-out condition.
6. To prevent deep freezing of evergreens, apply an early fall mulch of oak-leaf mold, straw, sawdust, or acid peat moss. Mulching also aids in water conservation during dry periods.
7. Protect susceptible evergreens from drying winter winds by planting them in a protected location or, where practical, erecting a canvas, burlap, or other types of screen.
8. Control destructive insects and diseases.
9. Prune trees and shrubs judiciously to eliminate crowded branches and increase the water supply to the remainder of the plant. Cut out and burn all dead or damaged wood after growth commences in the spring.
10. On sunscald-susceptible (or frost crack-susceptible) trees, paint the trunk with whitewash in the fall; shade the bark with boards, lath, screen, or wrap with burlap, nursery or building paper, cloth, or aluminum foil.
11. Remove dead and decayed bark and wood from sunscald cankers or frost cracks.
The surface of the wound should be smoothed, and enough living bark should be removed from around each wound to give a somewhat egg-shaped form, with the pointed ends running lengthwise off the trunk.
Healing proceeds most rapidly if wounds are cut to this general shape. The wound should then be covered with tree paint to keep out infection and promote rapid healing.
12. Various wax and plastic materials are available in many areas for spraying evergreens. These preparations, when sprayed on valuable shrubs, protect against excessive winter drying.
If available, following the manufacturer’s directions, you might try one on a small scale this fall. If good results are obtained, you can spray more plants next year.
44659 by Malcolm C. Shurtleff