Garden Carefully To Prevent, Repair Winter Freeze Damage

Q: What should I do with shrubs that may be freeze damaged?

A: Temperatures certainly dropped suddenly, into the low 20's in some areas during the icy blast of last week.

This cold occurred when many winter and spring-blooming plants were opening buds as a result of generally mild weather previous to the freeze. And winter isn't over, so the alternate mild and cold spells can continue to happen.

First, don't worry unnecessarily about the somewhat scruffy appearance exhibited by some shrub buds and leaves. The winter-blooming shrubs, such as sarcococca, witch hazels and mahonia, will stop their bloom during very cold weather, and may even have some buds killed back. They then resume opening buds gradually throughout January and into February. This on-and-off again blooming is often seen on the early cherry trees, Prunus subhirtella `Autumnalis.' Most of the winter-bloomers will revive with warmer weather.

Spring bulbs and early perennials came through the cold just fine. Hellebore buds are still tucked down at ground level and most spring bulbs are just emerging. Snowdrops, the early white flower of winter (Galanthus nivalis), popped out of the snow in my Seattle garden blooming beautifully. A 2-3 inch mulch over bulbs and perennials (using compost, fine leaves or even aged sawdust), can help with some superficial protection from light freezes.

Severe winter injury, with plants losing buds or stems, or frozen to the ground, may occur where temperatures drop even more severely than the recent freeze. How plants survive cold depends on a number of factors that will differ from garden to garden. If the temperatures drop into the teens or below, with bright sunshine and bare frozen ground that may last for days with low humidity, plants will suffer. If snow falls, some insulation results.

Microclimates within gardens also affect winter freeze damage. If gardens are near water, temperatures moderate. Mulches, soil moisture available before the freeze and "cold spots" near solid structures that trap cold air can all affect what happens. Plants in containers or raised beds, where the roots may be exposed to cold air without soil protection, can be particularly vulnerable to freezing. The same plant, such as a small Japanese maple, could be killed if in a container, but survive if in open ground.

Plants themselves differ in genetic hardiness factors. In addition, older plants that have been established in gardens for a few years have root systems less vulnerable to freezing than those that are newly planted.

Symptoms of freeze damage will vary. Buds are often killed, and perhaps killed sporadically, so that a camellia or rhododendron may have bud kill on the most exposed part of the plant but not within the protected areas. Leaves will roll tightly during cold, but open again in warmth. This isn't generally a dangerous symptom.

Sun scald can cause scorched spots to appear after long periods of extended cold weather combined with bright days and low humidity. The leaf thaws during the sunny days, and freezes again at night, causing ice crystals within the leaf tissue that kill the area of the leaf. Sun scald can also cause bark splitting on the southwest side of shrub and tree trunks.

Several techniques will help in caring for the winter-affected garden. Prune out any broken or split branches from ice or snow breakage. Other pruning should proceed very cautiously. Some shrubs may appear dead but will revive with warmer weather. It's not advisable to prune until growth shows where the branches are alive, and where they are not. Before pulling a plant out, check under the bark to determine if the cambium is still alive. Scrape gently at the bark with a fingernail or sharp knife. Live branches will show bright green or white just beneath the bark. Dead branches are brown, and perhaps soggy.

Check in several places on the plant; branches may be dead at the top but the crown may be alive.

Tips for garden care during winter include:

-- Don't prune winter damage too early; wait until growth begins.

-- Prune properly, back to live wood. Don't leave stubs.

-- Check all plants under eaves to be sure they aren't drying out during winter hard freezes.

-- Mulch with 2-3 inches of loose, organic material. Keep mulch away from plant trunks and crowns of perennial flowers.

-- Select, as much as possible, plants that are locally winter-hardy.

-- Wrap trunks of newly planted or transplanted young trees with white or light-colored bark wrap to prevent bark splitting. Leave it on for the first growing season.

-- Do not fertilize too late in the season, to avoid shocking the plants into soft growth that will be susceptible to winter-kill. Do not begin fertilization too early before growth begins.

For more details, call your local WSU Cooperative Extension office during business hours Monday-Friday to request publication EB1645 "Winter Injury of Landscape Plants in the Pacific Northwest." There will be costs for the bulletin and for mailing, which each office will explain. In Snohomish County, call 425-338-2400; in King County, 206-296-3900. In Pierce County, call 253-798-7170 9 a.m.-3 p.m. Monday-Friday.

Gardening runs Sunday in Home/Real Estate. It is prepared by Mary Robson, area horticulture agent, Washington State University/King County Cooperative Extension.