Winter injury describes a wide range of conditions, mostly environmental and mechanical, that can affect landscape plants, especially in areas where winters are harsh. Cold temperatures, high winds and bright winter sun, independently or in combination, can lead to damage mostly characterized as excessive drying. Sunscald, frost cracks and heaving of newly planted stock are also the result of temperature differentials. Finally, accumulations of ice and snow can break branches and snap stems.
Evergreens are especially susceptible to desiccation, especially at times when foliage is transpiring, but the roots and stem are frozen, and so transpired moisture is not replaced. Dry fall conditions, coupled with highly exposed plantings will exacerbate drying. Sunny weather with cold temperatures can only make matters worse. Adequate moisture immediately preceding ground freeze will help alleviate early onset of drying injury, and mulch (at least a couple of inches on top of the rootzone) will control evaporation and insulate roots. Some landscape managers swear by the use of antidesiccant sprays, while others think they don’t seem to last very long.
Burlap wraps are probably the most effective way to protect evergreen shrubs, because they significantly decrease exposure, especially to wind and sun. These measures, however, are usually reserved for new plantings to give them a season or two to become established, and then they are on their own. Going to such lengths to protect a client’s landscape each year is probably over the top, and plants should be moved to more naturally protected locations. Better yet, convince the client to plant more winter-hardy species in exposed areas.
The exception to this rule is for plantings that are exposed to plow spray, when the snowplow cruises by at 30 miles per hour throwing usually salt-laden slush up to 25 feet. In situations such as this, a burlap barrier, or something stronger, is necessary to protect plantings. The only alternative, of course, is to avoid planting within 25 feet of a road that requires regular winter plowing. This is especially true for salt-sensitive species in areas where chloride is commonly used as a deicing agent.
Frost heaving is often common with newly planted stock, especially in clay soils. Alternate freeze-thaw cycles eventually push a burlap-wrapped ball up, sometimes right out of the ground. Usually heaving is not quite so severe, but it is enough to push the roots up a few inches or more, exposing them to drying conditions. It is easy to reset the planting in the spring, but often not before the plant has been injured from exposed roots.
Generally, the finer the soil, the more difficult it is to prevent frost heaving in areas that experience alternate freezing and thawing. The only solution is to plant in the spring, using practices that will encourage roots to extend beyond the root-ball and into surrounding soil. Mulching will help to insulate the ground and to protect exposed roots until the sapling can be replanted next spring. Nevertheless, in northern climates, try to avoid fall planting in heavy soils, or be prepared to mulch heaved plantings during the winter months to protect exposed roots.
Often, a snow pack that develops in late November and lasts throughout the winter will keep soils from freezing, or limit the depth of freezing to only a few inches. When this happens, frost heaving is rare, regardless of soil type (and the incidence of desiccation is also much less, except in areas where desiccation is caused by sodium chloride that has migrated from roads to leaf surfaces as microscopic airborne particles).
Frost cracks and sunscald are caused by rapid changes in temperature. Scalding is most common with thin-barked species and results in death of cambium tissues, usually on the warm side of the tree (the south and southwest sides, but this can vary if the stem is near a highly reflective surface). Tree wraps that are applied in November should be removed in April to avoid the possibility of injuring the cork cambium and to discourage insects and diseases that might also use the wrap for protection.
Frost crack, as the name implies, results in a vertical fissure on the main stem. They tend to form on sunny, winter days that quickly transition into bitterly cold nights. The differential is too much for the exposed stem to withstand, and a crack forms (actually, it is the outer layers contracting more rapidly than the inner layers that cause a vertical crack in the bark to form, usually on the warmest side of the stem). Frost cracks are more common in stems that harbor internal defects, and they almost never completely callous over—once a frost crack, always a frost crack.
Probably the most common form of winter injury in trees results from the accumulated weight of ice and snow. Even though tree branches are quite strong (right-angle branching is inherently stronger than branches that form at more acute angles), they begin to break after accumulating 40 or more times their weight in ice and snow. Of course, some species are stronger than others. Depending on conditions, every tree is susceptible to branch breaking. Accumula-tions of ice are usually far more damaging than snow because of higher density (a .25-inch layer of ice is the equivalent of nearly 3 inches of snow that the tree can easily shed in even the slightest wind).
Ice storms are most often the result of fairly warm, moisture-laden air colliding with and riding over a stationary bubble of very cold air. Precipitation falls as rain that is supercooled by the time it hits freezing cold surfaces on the ground, where it instantly changes from liquid to solid. The one thing about most ice storms is that they don’t last for more than a few hours. When they do last longer, ice accumulations are sufficient to cause branches to break, and even stems of saplings to snap.
The aftermath of an ice storm is far worse than the long-term effects of branches breaking in tree crowns. In fact, during the ice storm of January 1998 in the northeastern states, some researchers suggested that the crown thinning caused by broken branches actually might have had an overall positive effect on some trees. Cleaning up broken crowns in a timely fashion was the key. Since the damage took place during the winter months, when there are virtually no infectious inoculums in the air, exposed wood surfaces (that had been properly trimmed) had ample time to dry before serving as an infection court.
Unfortunately, there is little that anyone can do to prepare for an ice storm, but valuable landscape trees can be pruned to remove acute branch angles, and cabling can be used to support vulnerable portions of the crown. With cabling, the trick is to center the weight over the main stem as much as possible, and to balance the crown so the tree can carry the extra weight without tipping or snapping.
Since the proximate cause of most winter injury is due to climatic conditions, the actions of arborists are limited to selecting winter-hardy species, planting in configurations or in landscape locations that offer protection from climatic extremes, and the use of barriers such as burlap and stem wrap in situations that require a little extra protection.
The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.