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BY DON MEEHAN
SERIES ON WINTER INJURY OF LANDSCAPE PLANTS - Part 1
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 information has been provided by Sharon J. Collman, WSU County Extension Agent, and Ray Maleike, WSU Extension Horticulturist.
PART 1: HOW WINTER INJURY OCCURS
Winter injury may be caused by a complex combination of circumstances rather than a single factor, and the extent of injury will also vary.
Some of the factors are:
Weather Factors
* Deviation from normal minimum winter temperatures
* Dramatic fluctuations in temperature.
* Duration of a severely cold period
* Time of year that the severe cold period occurs
* Bright sunny days with frozen soil
* Depth to which soil freezes
* Accompanying drying winds
* Low humidity
Site Factors
* Distance from a large body of water
* Low spots where cold air settles
* Solid fences, hedges, or barriers that trap cold air
* Soil moisture availability before a freeze
* Soil conditions, soil type, or mulches
Plant Factors
* Genetic hardiness of native vs. exotic species
* Genetic hardiness of a species genetically adapted to a geographic area (ecotype)
* Difference in hardiness of different plant tissue
* Degree of plant establishment
* Condition of the plant: e.g. dormant, partly dormant, stress from drought, fertilizer burn, insects, etc.
* Location of the plant in terms of site
* Protective reactions of plants: leaf drop, leaf rolling
Winter injury of landscape plants occurs in different ways. By understanding how plants react to winter temperature, predictions can be made about what type and extent of damage may occur. However, in the Pacific Northwest with its varying climates and microclimates, general predictions are difficult to make. Plants near large bodies of water, such as Lake Washington and Puget Sound, may react differently than plants in more inland areas and in the Cascade foothills.
Plants native to a geographic region have evolved in response to the weather patterns of that region. Plants within a species are genetically variable in hardiness and react differently to unusual or extreme weather conditions. Weaker or less hardy plants may succumb, leaving the hardier plants to produce hardier offspring.
Many landscape plants are introduced or considered "exotic". Therefore, they may be adapted to different environments than native plants. Exotic or introduced plants often respond differently to local climate cues than native plants. The development of hardiness in woody landscape plants follows four distinct stages. Cold temperature damage may occur at any one of the stages, depending on the severity of the cold.
STAGE I. With the onset of short days in the later summer and early fall, a hardiness inducing plant hormone is synthesized in the leaves of the plant. This hormone is moved throughout the plant. This hardiness factor makes all of the plant parts slightly hardier. The same plant that does not tolerate frost without damage while actively growing, now tolerates below freezing temperatures. The degree of frost tolerance will vary with the species.
STAGE II: Ultimate midwinter hardiness is genetically controlled and is extremely variable in different species of plants.
STAGE III: Deacclimation or dehardening is usually a gradual, but sometimes fairly rapid, loss of hardiness due to increasing temperatures in the spring of the year. Plants will deharden and become less hardy as temperatures get warmer.
STAGE IV: Once the plants start to grow and are in soft growth, they may not tolerate frost. Plants such as Higen cherry (Prunus subhirtella and its cultivated varieties), Phontinia, Rhododendron 'Unique' or 'Christmas Cheer', or any other plants which start growing very early in the spring, may be susceptible to late spring frosts.
With these variables, cold temperature damage can occur:
1. When fall temperatures are fairly warm and plants fail to acclimate and a rapid, severe drop in temperature occurs. (As in November 1985.)
2. When midwinter temperatures are well below the normal genetic midwinter hardiness of some plant species. (As in December 1983.)
3. When late winter and early spring temperatures are fairly warm, causing rapid deacclimation followed by a rapid and severe drop in temperature. (As in January and February 1989.)
4. When plant parts are warmed by winter sun during below freezing weather and a sudden temperature drop follows at sunset.
5. When a late spring frost kills soft, new growth.
Don Meehan is on the faculty at Washington Sate 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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