tt-157 BY DON MEEHAN

SERIES ON WINTER INJURY OF LANDSCAPE PLANTS - Part 2 - Types of Injury

Plants in the Pacific Northwest may suffer injury due to extremely cold temperatures or other types of winter weather. These past few winters have been especially severe. This column will be devoted to a four-part series on winter injury of landscape plants. The first part covered "How Winter Injury Occurs". Today's column is Part 2 of the series. If you missed the first part you can call the Island County/WSU Extension Office at 679-7327 to request a copy. This information has been provided by Sharon J. Collman, WSU County Extension Agent, and Ray Maleike, WSU Extension Horticulturist. PART 2: TYPES OF WINTER INJURY COLD TEMPERATURE INJURY Twig or branch death may occur even though the leaves still look healthy. Gradually, the leaves will discolor and fall. A cut into the branch through the bark reveals discolored tissue. Some plants or branches will appear to have escaped winter cold temperature injury only to fade and die later in spring or summer. It may be that the branch tissue was killed or that the roots, especially of container plants, were killed but the leaves don't "show" it yet. There is often a tremendous variation in the hardiness of different plant parts on the same plant. Generally, stems are hardier than vegetative or leaf buds. The leaf buds are hardier than their flower buds. All these parts are usually hardier than root tissue. Buds will die or be damaged if the tissue is not genetically able to withstand cold temperatures or if the buds deacclimate. Some floret buds or tissues may be killed while others remain healthy. This results in deformed or distorted growth, especially on early flowering plants. On some cultivars of more tender rhododendrons, there may be partial to complete flower bud kill from cold temperatures. In some cases, there may not be a full complement of flowers on these plants. Roots in an above ground container may freeze during below freezing temperatures, killing the roots. Because in many cases the stem tissue may be as much as 60 - 70 degrees hardier than the roots, the top of the plant is not damaged by freezing temperatures. In this case, the plant will leaf out in the spring and then, for no apparent reason, wither and die. Check for dead roots to see if this type of injury has occurred. SUN SCALD ON LEAVES This occurs during periods of severe cold or extended cold weather, combined with bright sunshine. The leaves of some broad-leaved evergreen plants can heat up to 50 to 60 degrees F. during these sunlit days. This causes the water status in the leaves to change. It is a complex process which involves a very rapid deacclimation. When the sun sets, this deacclimated leaf tissue freezes rapidly. Ice crystals form in the individual cells within the leaf, rupturing and killing these cells. The effect is death of leaf tissue, particularly those leaves oriented toward the afternoon sun. The leaves on the outside of the plant, and especially those on the south and southwest side will be most affected. Damage is often most severe on leaf tissue that is perpendicular to the sun. Sometimes the outline of an overhanging leaf will leave a green "shadow" on the sunburned leaf. Plants vary in their susceptibility to sun scald. SUN SCALD OF BARK (Southwest Winter Injury) Caused by similar environmental conditions, sun scald occurs on sunny days in winter when the bark of a tree is warmed by the sun, especially on the southwest side of the trunk. The bark and cambium tissue deacclimate and are not able to reacclimate quickly enough when the sun sets and the temperature drops abruptly. The result is damage or death of tissue. The bark often cracks open or it may separate from the tree without splitting. Sun scald is more prevalent on stressed, recently transplanted, smooth-barked, or thin-barked trees. BARK SPLITTING (Frost Collars) AND FROST CANKERS Bark splitting at the crown of the plant, where roots and stem meet, is not uncommon. It is caused by cold temperatures near the soil surface, and is usually a problem where there is no protection by mulch or snow cover. After thawing, the dead bark dries, splits, and separates from the wood, girdling the crown. This prevents the plants from transporting water and food from the leaves to the roots and the plant will die. In the spring the twigs and leaves above may appear alive and green, but the plant is "clinically dead". Sometimes, instead of the bark splitting, it adheres to the wood instead of cracking and as it dries it forms a sunken area or canker. Bark splitting and frost cankers may be found at the crown and on the trunk. FROST CRACKS Frost cracks are long splits up and down the trunk in the wood of stems or the main trunk of the tree. These cracks may be shallow or deep fissures. There are two schools of thought on the cause. The standard theory is that the cells on the south to west side of the trunk are warmed by the sun allowing water to move back into the "winterized cells". As the temperature drops drastically after sundown, the water freezes suddenly. A different theory is proposed by Dr. Alex Shigo. He indicates that all cracks result from wounds, or otherwise weakened or stressed tissue such as that of recently transplanted plants. Any wound makes the tree more vulnerable to infection and the tree reacts by setting boundaries to stop decay or invading microorganisms. This boundary setting sets up stresses and any number of events such as natural growth stresses, sudden heat or sudden cold, wind, etc. can result in a crack. Two cracks on opposite sides of a tree are usually fatal. WINTER BURN OR LEAF SCORCH Wind and sun, alone or in combination, can cause damage to evergreens because they cause the plants to transpire or lose water through their leaves. The water can't be replaced because the roots do not function in cold or frozen soil. The leaves turn brown in varying degrees of severity, starting with brown edges or needle tips and progressing between the veins or down the needles. The most severe damage will be on tissue farthest away from the veins such as leaf edges, leaf tips, and tops of plants. Damage is usually most severe on the side of the plant exposed to sun and air flow. The most acute damage often shows up on the south and west side of the plant, which is exposed to the sun. LEAF DROP Leaf drop occurs in some plants as a normal protective reaction or as a result of high winds that just rip them off. The twigs and branches may be very healthy. Nandina and evergreen privet commonly drop their needles. BUD DROP Bud drop is much like leaf drop. Perfectly healthy looking buds may drop from the plant. This is common with azaleas and camellias. LEAF DROOP AND LEAF ROLL Drooping of leaves is most dramatic on fatsia where the large leaves bend at the stem end of the petiole and drop flat against the stem looking like death itself. If the plant is not killed by extreme cold, even partially damaged leaves will recover and the plant will return to normal when the weather improves. Entire leaves may roll inward on some rhododendrons. This is a protective reaction to cold which reduces the amount of leaf surface exposed to cold or drying winds. Leaves return to normal as temperatures warm. LIMB AND BRANCH BREAKAGE Branches may break due to heavy snow. The broken portion of branches left on the tree or shrub should be pruned back to another branch or the main trunk. On large branches this cut should be made to just outside the branch collar. With respect to other pruning, such as branches which may have been killed by the cold, it might be best to wait and see how extensive the damage is. It is easier to determine what stems are dead after growth starts in the spring. The leaves may be dead, but that does not mean that the stems and vegetative buds are also dead. Part 3 of the series will cover symptoms of winter injury. Don Meehan is on the faculty at Washington State University working in Island County as a County Extension Agent. He is located at the Coupeville WSU Extension Education Center. The phone number is 679-7327. Cooperative Extension programs are available to all without discrimination.




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