Master tips from a Master Gardener (1/5/17)

Bill Koellner · Wednesday, January 4, 2017
Winter can provide multiple injuries to our plants. The winter environment can affect trees and shrubs.

Included are: low temperature injury and frost injury; desiccation injury; winter sunscald; frost cracks; frost heaving; and snow and ice breakage.

Low temperature and frost injuries

Most often plants frequently injured by low winter temperatures are those which are planted in plant zones that are not the appropriate plant hardiness zone.

The plants that are from the correct hardiness zones were bred to live and flourish in areas sufficient to withstand prevailing winter temperatures.

However, temperatures can drop rapidly or fluctuate frequently, which is not good for plants. If hardy plants are not managed properly by both fertilization and pruning as well as watered; they may also suffer.

The following are areas that can be injured: flower buds, vegetative buds, branches, stems, crowns, bark, roots, or even whole plants may be injured.

Plants that grow in containers are particularly vulnerable to low winter temperatures since their roots are not protected by being below ground but the roots are exposed to the temperatures of the container.

Late spring and early autumn frosts can injure active tissues that are subject to the air temperatures which reduces pot temperatures. Remember, plant tissue freezes hard at 28 degrees, and will not recover.

Temperatures below 33 degrees will affect plants, so protection needs to take place. A result of late spring frosts can be the death of dormant but, particularly, expanding flower buds on plants.

Cold temperature injury that occurs during winter may not be evident until injured tissues fail to grow the following spring.

Containerized plants should be placed in protected areas, primarily on the south side of structures, sunk into the ground, and heavily mulched to avoid low temperature injury to roots.

Avoid heavy applications of nitrogen fertilizers in late summer. As fall progresses, always, mulch heavily around the base of root-tender plants, like roses, so their crowns and roots are protected from freezing temperatures.

Wait to mulch when the roses freeze and the leaves die, and the plants are ready to enter the dormant stage. Roses with northern exposure need to be mulched more heavily.

However, gardeners are aware of the type of roses selected for your yard, noting specifically their hardiness zone.

Injured and dead tissues should be pruned and discarded or destroyed to discourage invasion of the plants by disease organisms.

Replace plants which are completely killed with species adapted for the appropriate plant hardiness zone.

Desiccation injury

(winter burn)

Desiccation Injury called "winter drying" or "winter burn", is usually observed in late winter or early spring on evergreen plants.

Broadleaved evergreens such as rhododendron exhibit browning or even total leaf scorch of their leaf margins depending on the extent of injury. Narrow leaved evergreens, such as white pine, exhibit slight browning of needle tips when injury is slight.

Extensive injury may result in browning and premature loss of entire needles. The injury occurs during sunny and/or windy winter weather when plants lose water from their leaves through transpiration faster than it can be replaced by roots which are in frozen soil.

Winter sunscald

Winter sunscald is injury that occurs when the sun warms tree bark during the day and then the bark rapidly cools after sunset.

These abrupt fluctuations are most common on south or southwest sides of trunks and branches, and they may kill the inner bark in those areas. Young and/or thin-barked trees are most susceptible to winter sunscald.

Wrapping trunks of susceptible trees with protective "tree wrap" is the most effective way to minimize this type of winter injury.

Frost cracks (bark splitting)

Frost cracks are splits in bark and wood of a tree that result from rapid drops in temperature. They may be associated with internal defects resulting from previous injury to the trunk years prior to splitting.

Defective wood does not contract as readily as the outer layers of healthy wood do when winter temperatures plunge rapidly. In winter, the crack may become wider or narrower during colder or warmer periods.

Such frost cracks often close over during the summer only to open again in subsequent winters. Avoid wounding the bark of trees when they are young. Be particularly careful not to bump trees when mowing.

Large frost ribs can be braced to prevent reopening during the winter, thus enhancing callusing and healing. Frost cracks in trees are ideal sites for the entrance of wood decay organisms.

Frost heaving

Frost heaving of new transplants and small shrubs during the winter will expose plant roots to severe above-ground winter conditions which include cold temperatures and drying wind and sun.

Freezing and drying injury to roots, if extensive enough, can result in the death of the heaved plants.

Proper mulching around the base and entirely over the root zone of plants will help prevent the soil from frequent freezing and thawing conditions which are most responsible for heaving.

Replant heaved plants quickly if possible and mulch around them. Wait until spring to determine the extent of injury and need for replacement.

Snow and ice breakage

Heavy snow or ice on weak limbs with foliage can result in breakage.

Even strong healthy limbs of deciduous trees and shrubs can be broken if the weight of ice or snow is extremely heavy.

If the ground is saturated prior to a heavy snow or ice storm, and enough weight is placed on the upper portion of a tree, it can lift the root system right out of the ground.

Prune trees and shrubs to reduce the amount of snow and ice they will collect and/or to eliminate those branches which will be inherently weak.

Plant trees and shrubs away from places where snowmelt from roofs will drip on them. Dripping water may freeze on the plants and accumulate sufficiently to break branches.
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