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After nearly a week of temperatures in the teens and twenties during late December, some gardeners may find damaged plants in their landscape. Hard freezes were rare through most of this decade. Some winters, certainly in the Northwest's maritime areas, gave us warm temperatures that allowed gardens to keep a deceptive green face. Snapdragons and even potted geraniums lived through winter. Warm winters provided impetus for experimenting with lots of marginally hardy plants, even tropicals.
So what can we expect of gardens hit by this past month's freeze? With so many variables in our maritime Pacific Northwest landscapes, the individual performance of plants will vary from garden to garden. Moderation of cold temperatures can be expected at lower elevations and near large bodies of water. More frigid temperatures always prevail on hills, and certainly toward the Cascade foothills and above. Exposures also vary from the shelter of woods to a fully exposed bare hillside.
Plants don't experience the wind chill factor that affects warm-blooded creatures, but drying winds can cause water loss from plants. Some may be damaged by both freezing and dessication. Cloud cover helps to moderate the effect of freezing, because plants don't dry out quite as fast as they do in a long period of sunny days with frozen soil. Snow also moderates plant damage, acting as an insulating layer.
In addition to the variables presented by the weather and the site of the plant, plants react according to their own genetic hardiness and their place of origin. Plants such as Scotch pine and Mugo pine originated in climates generally colder than western Washington. A plant such as New Zealand flax (Phormium tenax) often used as a garden ornamental for its stiff, vertical foliage, will die with sub-freezing temperatures. Many gardeners have been able to grow this plant and many like it through the last five winters but may find in the spring that they are totally dead.
Was the plant growing actively when the cold temperatures hit? If the plants haven't gone dormant, they will experience more freeze damage. Just before the temperature dropped, many plants in my garden were in bloom: hardy fuchsias, fragrant winter viburnum, sasanqua camellia, and sweet box (Sarcococca.) I expect freeze damage on all the flowers and shoots of hardy fuchsias frozen entirely.
Plants that have poorly established roots or are experiencing stress are more susceptible to freeze damage. Lack of soil moisture can also make freezes more damaging, but most plants in western Washington had well-watered roots from late fall rainfall. Surprisingly, roots are more susceptible to freezing than stems or buds. Container plants are more susceptible to damage because the soil freezes from all sides and more tender plants are likely to die.
What can a gardener do now? Do not panic at the miserable appearance of plants just after a hard freeze. Plants that appear to be dead may regrow from the roots in the spring. Do not start pruning out dead wood now. Wait until new growth begins when it's easier to see the extent of damage. If you wonder about the life remaining in a branch, gently scrape away a bit of bark. Live wood will show a greenish tinge where the cambium is still growing. Dead wood is brownish or blackish, and may even be slimy under the bark.
Check evergreens and other plants under eaves to be sure they aren't drying out. Water them properly when temperatures moderate a bit. You can also mulch plants with 2 to 3 inches of an organic mulch to help protect plant roots. However, if you mulch over frozen ground, it will tend to stay frozen. This may not be a problem, since we want plants to stay dormant now and through winter.
If your garden has newly planted trees with south or western facing trunks, wrap them with white or light-colored bark wrap to prevent bark splitting on freezing, sunny days.
Do not fertilize any plants until the extent of damage shows up in spring. Fertilizer won't revive a plant with dead roots.
Freezing damages garden plants, but may not be as destructive as wind and ice storms. Ice storms in recent winters killed or damaged thousands of trees. The extent of this past damage may be smaller than you suspect in walking around the garden this weekend. Think of it philosophically. Gardeners enjoy watching spring's revival. After a hard freeze, each renewal of plant growth can be especially welcomed and prized.
For more information, see Washington State University bulletin EB 1645 "Winter Injury of Landscape Plants in the Pacific Northwest," or you may order the bulletin from your local WSU Extension office.
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