PROTECTING TREES AND SHRUBS FROM COLD
Dr. Leonard Perry, Extension Professor
University of Vermont
Cold damage can injure plants through sun scald, dieback, root injury, and frost heaving. A few cultural practices in fall can help prevent such damage from occurring.
Sun scald is caused by the bark heating up on cold winter days, then chilling rapidly when the sun goes down or is blocked such as from trees and clouds. The heating causes the growing tissue in the bark, near the surface, to become active. The sudden drop in temperature then kills this tissue. The result is sun scald, or sunken, elongated, darkened areas of the bark. This is usually on the south sides of trees where the sun has warmed the bark.
Sun scald is most common on young trees with thin bark. Older trees, with thicker bark, are less sensitive to such temperature extremes. Plants that have been pruned to expose previously shaded bark also can be injured more easily. Certain species with thin bark are more prone to sun scald, including cherry, crabapple, honey locust, linden, maple, and mountain ash.
To prevent sun scald, wrap the bark of susceptible trees in late fall with either tree wrap tape, plastic tree guards, or similar light colored material. These can be found in complete garden stores. Remove the wrap in the spring after the last frost, to avoid insects living under the material during the summer. Wrap newly planted trees or young trees for at least two winters. Wrap thin-bark trees for at least five winters.
Cold also can injure evergreens through discoloration, particularly in late winter. Anyone with rhododendrons in cold climates has probably seen this. It appears as browned leaf tips, large areas, or browning of needles of conifers. Such browning is often referred to as “dessication”, or a drying out of leaves. Sunny days in winter, or wind, can cause evergreen leaves to lose water or “transpire”. Since the ground is frozen, roots can’t take up replacement water, so the result is the leaves drying out and browning.
Similar to sun scald on bark, warm days can stimulate cells to be active in leaves, only to be killed by sudden temperature drops when the sun disappears. In fall before plants are fully hardened, or in spring as new growth is occuring, low temperatures can injure non-hardened tissues.
One obvious solution is to make sure evergreens go into the winter well-watered. If there is less than an inch or two of rain a week in mid fall, make sure you water such plants. Another solution is to wrap dense evergreens such as Alberta spruce in burlap (not plastic that can heat up and cook the plants), or erect a screen around evergreens such as rhododendrons. Such screens should be on windward sides of the plants, and on the south sides to lessen sun heating.
Snow is one of the best protectors, so low evergreens can be protected by making sure they are covered with snow. You can lightly shovel snow on them, or place holiday boughs and trees next to them to trap the snow. If you have evergreens that usually get damage, consider moving them if possible next season, or planting new ones in sites more protected from winter sun and wind. Sprays to protect the foliage from drying out, called “anti dessicants” have been recommended in the past. Most studies seem to show they are not effective though.
Dieback is simply plant parts, such as twigs and buds, being killed by cold. This is often seen on deciduous shrubs, or those that lose their leaves. Many plants such as forsythia have flower and leaf buds, the latter being much more hardy. This is the reason in some years with extreme cold you see the plants leaf out but with no flowers above the snow line where they were protected. Not much can be done to protect such plants, except to select more hardy varieties. Make sure next season in late summer you don’t overwater, prune, or fertilize. All may stimulate vigorous growth which wont harden properly.
If a tree or shrub is injured from dieback, don’t be too eager to prune off the apparent dead branches. Often these only may be dead on the surface, with living tissue underneath which will produce buds. So wait until mid-spring to prune such branches, especially on less hardy roses which often show at least some dieback.
Roots can be injured by cold soil temperatures, generally at about 10 degrees (F) or lower. The good news is that due to latent heat in the soil, snow, and other factors, root and soil temperatures seldom get this low. Dry soils can get colder than moist soils, so fall watering will help to prevent not only dessication of tops, but also cold injury of roots.
Making sure any cracks in the soil around newly planted trees or shrubs are filled will help. Covering the root zone with up to six inches of organic mulch, such as bark or pine needles, will keep roots warm much longer in the fall. Mulched soils also fluctuate less in temperatures during winter, which is better for root hardiness. Make sure and keep such mulches away from trunks and shrub stems, otherwise mice may live in this material and feed on the bark.
A final way cold may damage trees and shrubs is through repeated freezing and thawing of soil in spring, called “frost heaving.” Such expanding and contraction of soil around roots can damage them, and raise new plantings or “heave” them from the soil. Mulching, as well as planting early in the season so plants can be well-rooted by fall, minimizes frost heaving.
In addition to such tips on cold damage, an online publication from the University of Minnesota describes how to prevent other winter damage to trees and shrubs from snow and ice, road salt, and animals.
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