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Gardening In Western Washington
Presented by WSU Cooperative Extension



Low Temperature Plant Damage

Seasons

Every so often in western Washington a winter comes along which may contain one or more periods of rather severe minimum low temperatures. Low temperature extremes may also occur during the fall (early freeze) or during spring (late freeze). These latter two are usually evidenced by a sudden drop in temperature. Winter lows are often, but not always, characterized by extended periods of very low temperatures

Garden Location

The depths to which temperatures plunge in gardens depends a great deal on the location of the land within and around them. For example, gardens which are located close to Puget Sound, Lake Washington or coastal areas do not experience as low temperatures as those established some distance from them. There is a great moderating effect supplied by the water. Low temperatures, whether in fall, spring, or winter, will therefore usually be more severe in gardens placed more distant from these waters.

In addition to the above, gardens which are higher than surrounding area will also usually not experience as cold temperatures because cold air is much heavier than warm, and it will flow downward to lower lying areas. Solid fences, walls and thickly planted hedges will impede the flow of cold air away from an area; consequently, gardens surrounded by these may suffer more cold damage than those which allow for free drainage of cold air.

Plant Conditions

Many of the woody plant materials grown in western Washington are quite able to survive most winters without any damage. They are generally able to withstand low temperatures if they are sufficiently dormant, and if very low minimum temperatures do not last too long .

There are, however, other circumstances which must be considered. For example, plants growing in poorly drained soils or in exposed, windswept areas are likely to suffer more from low temperatures than others. Plants which are excessively fertilized, especially with nitrogen fertilizers and which are irrigated too frequently during August and September, are likely to make considerable growth during this period. This growth will not have sufficient time to harden off, and will consequently not be able to withstand even normal winter temperatures. Along with this, older, more established plants are usually more cold resistant than newly planted ones.

Symptoms and Signs

Low temperature injury is usually manifested by several signs. First of all, extreme cold will cause the leaves, twigs, stems and even roots of plants, which do not have the basic genetic hardiness to low temperatures, to freeze. Essentially, this type of cold causes a disruption and breaking down of the cell contents and tissues. This damage can often also occur to normally hardy plants in which the tissues have not hardened off for one reason or another. Symptoms of this kind of injury may be evidenced by severe wilting, complete blackening or browning of the leaves and stems, and general collapse of above-ground plant parts. Occasionally, only the flower buds of certain plants, like some rhododendrons, whose leaves, stems, and other parts may be quite resistant, are killed. They will turn brown. Freezing temperatures in spring after the buds have begun to swell, will often result in abnormally twisted and curled new sets of leaves.

Very low temperatures are often accompanied by bright, sunny days in western Washington. If these conditions continue for a period of time the ground will freeze solid, thus preventing the roots from bringing up sufficient moisture to replace that lost in the process of transpiration. Signs of this damage are often characterized by the browning or "burning" of leaf margins, entire leaves and stems. If this condition continues long enough, it can often result in the dieback of portions of plants or even in their death.

Quite often, under conditions of bright, sunny days and very clear, cold nights, the stems and trunks of shrubs and trees will split longitudinally. These splits can be rather narrow, or quite wide, and will usually be located on the south or west sides of the plants. Frost cracks can even occur on plants considered to be quite hardy; such as Japanese Maples, flowering cherries, and fruit trees.

Shrubs and trees which are growing in containers above the level of the ground are very susceptible to extreme root damage. The roots of plants are far less hardy than the tops and even normally hardy species can be killed outright if the containers in which they are grown are subjected on all sides to very low temperatures. This kind of damage is often not noticed until spring, when little or no new growth takes place.

Some gardeners in western Washington are accustomed to leaving the roots (corms and tubers) of gladiolus and dahlias in the ground during the winter. These usually survive most winters, but, if the frost, and accompanying very low temperatures, penetrates the ground to their level, they will likely be killed. Heavy mulching with organic materials can help keep frost from reaching their level. Mulches can also afford protection to other less hardy perennial plants which may be damaged by cold or heaved out of the ground by frost.

False Alarms

The leaves of rhododendrons and other broadleaf evergreens will often droop and roll during periods of severe cold weather. This condition is normal and does not indicate damage. It is the plants way of exposing as little leaf surface as possible to the cold. If, however, after temperatures have moderated, the leaves do not return to their normal condition, then damage may have occurred. Also the leaves of some rhododendrons and evergreen viburnums enter a state of "winter wilt" at the onset of cooler weather. They hang in this fashion until spring when they return to a more normal horizontal position. The foliage of certain conifers, such as some arborvitaes, cryptomerias and junipers, will change from their normal green color to one of brown or purple, especially during periods of abnormal cold. This, too, is a normal condition and should not be a cause for alarm - unless, of course, the plants do not experience a return to their usual color in the spring .

What to Do

Once a period of extreme low temperatures has occurred, it's best for the gardener to wait until spring before attempting any remedial measures. This is especially true of pruning. Even though all the leaves on a broadleaf evergreen shrub or tree may have been killed, it is still quite possible that the low temperature may not have damaged the wood or at least that it has not extended too deeply into the plant's woody parts. It's always best to wait until spring to determine which wood is dead and needs to be removed.

Light fertilizing and making sure the plant does not suffer further stress during the following summer will help it to get back on the road to recovery. Mulching trees and shrubs with bark, sawdust or shavings, will not only help to conserve moisture during the summer, but will also serve to prevent winter frost from penetrating so deeply into the soil.

In the sad event that the above-ground portion of a shrub has been killed, it's still quite possible that the crown and roots are still in good shape. In spring the crown should begin to put forth new stems. It is therefore wise to wait until late spring or even early summer before digging a damaged plant out of the ground.

Frost cracks, if not too wide, will usually heal themselves. Large cracks in trees may benefit from treatment with fixed copper to help prevent infection by diseases.

Experimenting with Tender Plants

And finally, gardeners who wish to persist and experiment with plants that might be considered marginally hardy in western Washington should take some precautions. First, it would be wise to determine that your garden is located in a warmer mini-climate, such as near a large body of water, in a city whose temperatures are generally higher than surrounding areas, or perhaps on a higher location in relation to the surrounding area. Secondly, always plant these species in protected areas. All the following qualify as protected areas: the north side of a structure, against a warm wall, in a right angle between two walls of a house, underneath high trees. And finally, it would be wise to take some precautions each fall, at least until the plants are old enough and established enough to better endure the extreme winter conditions that sometimes occur. The use of burlap windbreaks, anti-transpirants and heavy mulches all may be of some help, at least in protecting against winter wind and sun.

Plants Liable to Damage

The following list is by no means complete. It is only offered to serve as an indicator. In theory, all woody plants, even native species, can be damaged by extreme low temperatures that occur in spring after they have broken dormancy or in fall before they have sufficiently hardened off their tissues and gone into winter dormancy. Although this list is composed mainly of broadleaf evergreen shrubs and trees, there are other deciduous and coniferous plants which may also suffer damage because of a lack of genetic hardiness to extreme low temperatures. For further information contact your local WSU Extension office.
Written by George J Pinyuh. Retired Area Extension Agent. (PC 91, 1981) Reviewed 1996.


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