• Deviation from normal minimum winter temperatures
  
  
• Dramatic fluctuations in winter temperatures
  
  
• Length of a severe cold period
  
  
• Time of year that the severe cold period occurs
  
  
• Bright sunny days with frozen soil
  
  
• Depth to which soil freezes
  
  
• Drying winds
  
  
• Low humidity
  
  
• Snow cover
  

• Distance from a large body of water
  
  
• Solid fences, hedges or barriers that might trap cold air
  
  
• Soil moisture availability before a freeze
  
  
• Soil conditions, soil type or mulches
  
  
• Plants grown in soil, raised beds or containers
  
  
• Windbreaks

• Hardiness of a native vs. exotic species
  
  
• Hardiness of a species genetically adapted to a geographic area (ecotype)
  
  
• Differences in the degree of hardiness of different plant tissues
  
  
• How well the plant is established
  
  
• Condition of the plant: dormant, partly dormant, stress from drought, fertilizer burn, insects, etc.
  
  
• Location of the plant relative to site factors
  
  
• Protective reactions of the plant: leaf drop, leaf rolling

Winter injury to landscape plants occurs in different ways. By understanding how plants react to winter temperatures, predictions can sometimes be made concerning the type and extent of damage that may occur.

However, in the Pacific Northwest's varying climates and microclimates, general predictions are difficult to make. For example, the climate is more moderate around a large body of water; lower winter temperatures and harsher climates occur with increases in elevation.

Plants native to a geographic region have evolved in response to the climate and weather patterns of that region. Hardy plants survive and produce offspring which pass on the genetic hardiness. Through natural selection, these plants will have a better survival rate. Native plants usually survive winter cold.

Many landscape plants are introduced or are considered exotic. They may be adapted to completely different environments than native plants. Exotic or introduced plants often respond differently to local climatic cues than native plants.

Hardiness to cold develops in organized patterns. Cold temperature damage may occur at any time depending on the severity of the cold and the stage of hardiness of the plant. A series of physiological changes that contribute to hardiness take place. These changes are influenced by day length (longer nights) and cooler temperatures. In response to cooler temperatures, plants acclimate or harden to cold. The rate of acclimation to cold varies by species and the degree of cooling temperatures. If fall temperatures remain fairly warm, plants may fail to acclimate to cold.

Ultimate midwinter hardiness is genetically controlled and is extremely variable among plant species and even among plants in the same species. Douglas-firs (Pseudotsuga menziesii) that evolved in the Rocky Mountains are hardier than Douglas-firs that evolved in the Cascade Mountains. Likewise, flowering dogwoods (Cornus florida) from New York are hardier than dogwoods from Florida or Georgia, even though they are the same genus and species.

Deacclimation or dehardening is the loss of hardiness. Deacclimation is the plant's response to warming temperatures in late winter and early spring. Typically, deacclimation is gradual but can be rapid during an extended warm period.

Once plants start to grow, the soft growth usually cannot tolerate frost. Plants that bloom or start to grow early in the spring, such as flowering cherries, magnolias, photinia and some very early-blooming rhododendrons, are susceptible to damage from late spring freezes.

Roots in an aboveground container may freeze, killing the roots. Because in some plants the stem tissue may be much hardier than roots, the top of the plant is not damaged by the freeze. The plant may 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. Dead roots are usually brown to black and may be soft. Live roots may have white growing tips and will be white to greenish under the bark.

This type of damage may be reduced by putting containerized plants in a protected area, such as a cool garage or greenhouse. In western Washington, the containers can be buried in sawdust or ground bark.

This occurs during periods of severe cold or extended cold weather, combined with bright sunshine. The leaves of some broadleaved evergreen plants can heat up to 50° to 60° F during these sunlit days. This causes a type of rapid deacclimation. When the sun sets, the deac-climated leaf tissue freezes rapidly. Ice crystals form in the individual cells within the leaf, rupturing and killing them.

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 leaves that are perpendicular to the sun's rays. Plants vary in their susceptibility to sun scald.

Wind and sun, alone or in combination, can damage evergreens because they cause the plants to transpire or lose water through their leaves. The water is not replaced because the roots cannot pick up water in cold or frozen soil. The leaves turn brown, 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 happens on the south and west sides of the plant.

These maladies may be prevented by protecting the plant from the wind or shading the plant.

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 cambial tissues 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. Wrap the trunks of recently transplanted trees and those which may have been stressed during the growing season with a light-colored wrapping from the soil line to the first set of branches. Leave this material on for the first winter and through the first growing season.

Bark splitting at the crown of the plant where roots and stem meet is a common injury. 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. This will result in the death of roots and eventually the entire plant.

In the spring, the twigs and leaves above may appear alive and green, but the plant is actually dead. Leaves may start to grow in the spring and then wither and die for no apparent reason. Sometimes, instead of the bark splitting, it adheres to the wood instead of cracking, and as it dries it forms a sunken area or a canker. Bark splitting and frost cankers may be found at the crown and on the trunk. Stem wounds may increase the chance of bark splitting.

Branches may break due to heavy snow or ice. Prune the broken portion of branches left on the tree or shrub back to another branch or the main trunk. On large branches make this cut just outside the branch collar.

Wait to prune other branches that may have been killed by the cold. It is easier to determine which stems are actually dead after growth starts in the spring. The leaves may be dead, but that does not necessarily mean that the stems and vegetative buds are dead.

The results of winter injury sometimes take months or years to appear. Sometimes the leaves live on their reserves until they are depleted. This occurs slowly in cool weather or rapidly when the weather suddenly becomes hot. Prune as described above. Graft unions may be sensitive to damage from cold winter temperatures. Only a portion of the graft may be injured. It may function for years until another kind of stress causes it to fail.

Winter-damaged tissue may allow the entry of disease organisms and insects.

Root systems, especially shallow ones, may be injured by cold. The damaged root systems fail and the top of the plant may thin and start to die.

Before pruning a sad-looking plant to almost nothing or pulling it out altogether, check to see if it is still alive.

Scrape the bark away with a fingernail or make a shallow slant cut just under the bark with a pocket knife. Live branches are bright green or white just beneath the bark. Dead branches are brown and may be soggy.

Check the tree or shrub in several places: at the twigs, down the branches, and at the crown or soil line. If the outer twigs have died, move towards the trunk until you hit live tissue; older wood may be more hardy than younger wood.

Sometimes faded green branches may begin to regrow and do not die. Remove damaged tissue after you give the plant a chance to recover and it starts to grow again.