Diagnosing winterkill is a bit like going to the doctor and being told that you have "The latest virus that is going around." Winterkill seems to be a catchall phrase used when you haven't a clue as to what is wrong with a plant. In actuality, winter damage involves a whole host of problems that can affect virtually all plants.
Winter damage is divided into several broad categories, including unanticipated cold temperatures. Early frost in the fall kills the leaves on the tree or damages the fruit before it is harvested. Late frost in the spring often kills blossom buds or leaf buds or damages tender annuals and herbaceous perennials. Another type of damage occurs from the coldest winter temperatures. If they get too cold, nonhardy plants simply die. Plants that flourish and thrive in warmer areas will not tolerate our colder winter temperatures.
The only way to control this damage is to select plants adapted to our area. Temperatures below zero often damage the dormant fruit buds of peaches and other stone fruits. While vegetative buds grow the following spring, fruit buds are already dead. Consequently there is no fruit the following spring. If temperatures get even colder, leaf buds can be damaged.
Winter damage also occurs to bark and trunks. Southwest injury occurs from the alternate warming and freezing of bark tissue on the southwest side of the tree. Blackheart is a condition that occurs as extreme cold temperatures kill the cambium and inner layers of the plant. Interior wood is no longer functional for conducting water or storing food. Over time trees decline and may eventually die.
Frost cracks are large longitudinal cracks in the trunk of a tree. These large cracks are caused when wood with a high moisture content freezes, opening a large wound in the trunk.
Other kinds of winter damage result from desiccation. Broad-leaved evergreens, including euonymus, photinia and laurels, have leaves that turn brown and die as the temperatures get colder and they dry out. Many needle-type evergreens suffer winter desiccation if the soil moisture is low or in times of extreme cold when soils and trunks freeze for long periods of time.
All these different types of winterkill make determining the exact cause difficult. In fact, the only thing more difficult than the diagnosis is prevention and treatment of the damage. Damage from early fall frosts can be prevented, to some extent, by reducing water and fertilizer in the late summer and early fall.
Avoid pruning sensitive plants in the late summer. Late spring frosts are a more serious problem. Fruit growers have always faced this problem and hope and prayer are about as good as any solution. Extremely low winter temperatures cannot be avoided. Selecting cold hardy plants is one way of dealing with the problem, but choice varieties may be eliminated if selections are only on that basis.
Blackheart and frost cracks have no real control. Southwest winter damage is best controlled by wrapping or painting the trunks with white latex paint. Snow breakage and bending are controlled to some extent by proper pruning and training and by wrapping susceptible shrubs to prevent snow bending them down.
Winter desiccation is best controlled by planting sensitive plants on northern or eastern exposures so they are not exposed to bright sun during the winter. Fall watering helps and anti-transpirants may prevent some cosmetic damage.
The next time you hear winter damage as a diagnosis, realize such problems are difficult to diagnose and correct. Just like "that virus that is going around," most of the correction comes as healthy plants grow and correct the problem on their own. Prevention helps in some cases, but there are few, if any, miraculous cures.