Cultivating Knowledge on the Flathead Reservation

Plants growing actively in the fall can be damaged by unseasonably early cold periods when temperatures drop into the low 20s or high teens. This type of damage is called fall freeze damage and occurs before the plant has become dormant as evidenced by development of fall leaf color and normal leaf drop. The extent of damage ranges from some or all leaves remaining dead on the tree until spring with no twig damage, to death of entire twigs along with the leaves present at time of the freeze. The extent of damage depends upon the duration and depth of the cold spell and upon what stage of growth the tree was in when the cold happened. Proper fall care of trees usually reduces such damage, except in unseasonably cold seasons. Withholding water, fertilizer, and heavy pruning in the early fall encourages trees to prepare for winter. However, extreme drought can interfere with a tree's acclimating for winter. To fill the tree's water reserves for winter, water only after the leaves have begun to turn color and drop. Continue watering deeply once each week until the soil freezes. Most mature tree and shrub species concentrate their feeder roots in the upper 10 to 12 inches of soil. Therefore, strive to keep this soil volume moist during fall watering. Tissue at the base of branches, in the branch crotch, and at the base of the tree is last to acclimate to cold. If very low temperatures occur in late fall or early winter these areas may be damaged.

This is also the time to talk about trees and winter desiccation because once you see it, it's too late to do anything about it. Most winter damage is caused by drying of the plant tissues. This is made worse by extreme cold, which freezes soil water, and by dry conditions, such as low humidity and drying winds, which increase evapotranspiration (loss of water from the soil both by evaporation and by transpiration from the plants growing from the soil). All of these conditions exist in Montana so winter damage is very common. The ability of the plants to withstand long continued hard freezing (physiological drought) and long continued winter drought (atmospheric) depends on the supply of available moisture in the plant tissues going into winter. Broadleaved and needled evergreens can be severely affected by winter desiccation because the leaves can transpire all winter long. Look for death of twigs and leaves on the windward side or on the side facing the afternoon sun. Symptoms may be more severe in recently transplanted trees that have not yet re-established a complete root system. The best line of defense against winter desiccation is to water all trees after the leaves have fallen but before the soil freezes. In addition, winter watering during extended thaw periods when the soil is not frozen is also good practice. Wind and sun barriers are helpful for small and newly transplanted trees. Do not cover the plants entirely with the barrier, and never place plastic garbage bags over the plants. Air inside these bags can warm to dangerous levels on bright winter days. You can also spray plants in late fall with a film type (latex or wax) antidesiccant to reduce moisture loss from twigs and needles.

Bark on the south and southwest sides of tree trunks and in branch crotches may be killed by sunscald, a particularly prevalent problem in Montana in late winter and early spring. Bark is warmed and the cells dehardened by afternoon sun. Rapid temperature drop after sunset then kills the cells and bark. Damage is most common on smooth-barked trees such as mountain ash, apple and maple, on trees with dark bark, such as cherry, and on young, thin-barked trees. Shrubs and evergreen trees are rarely affected. There are several ways to reduce sunscald. Tree wrap, such as that made from heavy craft paper, can be applied to the trunk in October to reflect the sun and reduce abrupt temperature fluctuations. This also helps keep rodents from feeding on the bark. Remove the wrap in April. White latex paint also reflects the sun and prevents rapid temperature changes. Don't use oil-based paint. Wrap or paint the trunks from the soil line to the lowest branch. The purpose of tree wraps is not to keep the trunk warm but instead to keep it cool. Evergreen shrubs interplanted with trees help shade the tree bark during winter and reduce sunscald.

This article was taken from the MSU Montguide #MT199518AG. For a complete copy of this montguide go to http://msuextension.org/publications/YardandGarden/
MT199518AG.pdf. For more information regarding winterization of your home and garden please contact the Flathead Reservation Extension Office at 406-675-2700, ext 1247 or send an email to flatheadreservation@montana.edu.
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