WINTER INJURY ON TREES
Winter injury is a frequent occurrence on trees in Minnesota. Winter injury is a broad category that encompasses frost cracks, root or stem damage, winter browning, die back, and even plant death. Although usually more obvious on conifers, all trees are susceptible to varying degrees of winter injury. The frequency and severity of winter injury are predicated on a combination of planting site, tree species, tree health and ultimately the severity of the winter in question.
Winter Injury on Conifers
One of the most obvious symptoms of winter damage is the appearance of red-brown foliage on evergreen trees and shrubs such as Colorado blue spruce, eastern white pine, red pine and yew. This injury is usually due to freezing following a spell of warmer weather. Care must be taken to make certain that the symptoms of winter damage are not confused with the symptoms of some of the needle cast diseases. Remember to make certain that none of the signs of needle cast diseases (black fruiting bodies on the needles or twigs) are present! One procedure that may help you distinguish winter injury from disease is to examine where the injury occurs. Often, winter injury begins where the snowline ends. This results in the upper half of the tree having reddish-brown needles, while the lower half (which was protected by the snow) remains green (Fig. 1). The snow cover prevents winter injury of young conifers by providing shelter from drying winds and from the glare of the sun.
Another diagnostic feature of winter injury is that it is most severe on the southern and western sides of the trees. Sunlight from southern and western exposures warms needles, breaking dormancy. At sunset, the stress of rapidly cooling needles coupled with the rapid formation of ice crystals often results in tissue death. It must be stressed that this type of injury is limited to southern and western exposures. When an entire tree develops reddish or purplish-brown foliage, drought stress should be suspected instead. Finally, the reddish browning of evergreens such as arborvitae and yew may not show up until May or June.
Regardless of what type of evergreen you are dealing with, wait to prune affected branches until bud break occurs. Although numerous needles may be lost, the buds are usually protected and will leaf out come springtime. By patiently waiting until bud break occurs, you can more accurately determine where the damage ends. Prune out the infected portions of the plant, removing only dead branches.
Winter injury on Hardwoods
Compared to conifers, winter injury on hardwoods is more difficult to diagnose. With hardwoods, the symptoms of winter injury due to root and stem damage include early leafing out, or no leafing at all. This is followed by shoot death when warm weather develops. You can avoid this problem if you use only zone-hardy trees and shrubs, and maintain them with proper watering, fertilizing and mulching. It is also important to remember to plant them in the proper site. Finally, to prevent winter injury in the future, generously mulch them with wood chips to help protect roots and prevent cold injuries. This type of injury is more common during severely cold winters without adequate snow cover.
Another type of winter injury that primarily affects (but is not limited to) hardwoods is sunscald (Figure 3.). Sunscald and bark cracks occur mostly on the south and southwest sides of smooth-barked trees and shrubs. Maples and other thin-barked trees seem to be particularly vulnerable, followed by apple and other fruit trees, linden, and mountain ash. The cambial temperature of south to southwest facing trees can reach into the 60-degree range while the shaded portion remains at freezing (32° F). (The cambium is the a thin, formative layer beneath the bark of the tree that gives rise to new cells and is responsible for secondary growth.) This heating results in the tree losing its dormancy, which is followed by lethal freezing when the sun sets. Sunscald, coupled with drought, can result in vertical frost cracks and death of the cambium. Frost cracks also provide an infection court for decay and canker pathogens. Prevention is the best method for contending with sunscald. If possible, provide shade by strategically placing other plants or structures on the south sides of thin barked trees and shrubs. Tree wrapping with reflective or light-colored material may be effective in preventing sunscald and bark cracking. The current recommendation for trees that are newly planted is that they should be wrapped for at least two winters. Thin-barked species should be wrapped for several years or until mature bark is established. Regardless of how many winters you wrap your trees, care must be taken to remove the wrapping in the spring. Moisture that collects between the bark and wrapping may provide an infection court for disease when the weather warms up.
Management of Winter Injury
The primary factor limiting the northern range of plant species is winter injury. Many popular trees such as red and white pines, maples, and lindens, along with the non-native Colorado blue spruce and mountain ash, are more vulnerable to winter injury than the more boreal (arctic) species such as aspen, black spruce, and jack pine. Avoid planting those species or cultivars that are unusually susceptible to winter damage. Although occasional winter damage is a fact of life for most trees, winter damage that occurs consistently will weaken trees and predispose them to potential insect pests and disease. Adequate watering, fertilizing and mulching to improve tree vigor protects the tree from winter damage or minimizes the impact of such damage when it occurs. Winter damage is unsightly, but it need not be fatal. With the proper protection and a little patience, your trees will recover their healthy green after the last red needles fall.
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