Expose your skin to freezing cold and wind, and it'll become dry and chapped. Trees and shrubs react similarly to winter's onslaughts, only they can't come inside for hot chocolate and moisturizer.
When leaves transpire, their surfaces release water into the atmosphere. In moist soil, the roots conduct more water to the leaves than the leaves release, so the plant stays hydrated. If water is pulled from a plant's leaves faster than the roots can replenish it, the leaves and stems dry out. That's desiccation.
Wind, strong sunlight, or low humidity occurring along with dry or frozen soil can desiccate a plant. If drying conditions persist without the plant's roots getting a sufficient supply of water, all or part of the plant can die.
Plants can desiccate any time of year, but in North America, they're most likely to do so in winter, when roots can't draw moisture from frozen soil. Symptoms are yellowing, browning, or curling needles or leaves; split bark; blasted (dead) flower buds; or a limp, tired look to the foliage. Most prone to desiccation are evergreens, especially broadleaf types such as rhododendrons, azaleas, boxwoods, hollies, pieris, mountain laurels, and leucothoe.
What to Do Now
Surround susceptible plants with a burlap screen (leave the top open to allow light and air in).
An Ounce of Prevention
Healthy plants are more likely to survive winter unscathed, so use these tips next year to keep your trees and shrubs in top condition.
Be sure that your plants are hardy in your growing zone and are getting the amounts of light and moisture they require.
Give new plants plenty of water, and water all your evergreens during a drought.
Test your soil in early fall and provide potassium (greensand) and calcium (gypsum) if indicated.
Plant evergreens, especially broadleaf evergreens, where winter winds can hit them full force. This usually means avoiding the south or west sides of your house.
Fertilize with nitrogen after mid-August. It spurs new growth that won't acclimate in time for winter and will probably die back.
Ignore pest or disease problems on plants that have a tendency to desiccate. Stressed plants are more susceptible to winter injury.