Winter protection is necessary under the following conditions: First - When a plant is removed from its natural habitat to one more severe; Second - When plants are not sufficiently hardy to withstand the new climatic conditions or exposures; Third - When it is desirable to retain ground moisture during the winter for planting on exposed places, mounds and banks. Fourth - When plants (especially evergreens) are transplanted into a stiff clay soil under climatic conditions where they will be subjected to considerable freezing and thawing and it becomes necessary to protect them against heaving; Fifth - When plants, especially rhododendrons and other evergreens, must be protected against wind and sun which cause so much damage on account of excessive evaporation of moisture from the leaves at a time when no moisture is being taken into the plant through the root system. Continued, steady cold and a permanent covering of snow are generally sufficient to tide a plant through the severest part of winter, but an open winter, followed by severe cold, or the alternate freezing and thawing in spring, will work havoc. Mulching, therefore, is equally important, not only to control sudden changes in temperature in extreme weather, but also to maintain a cool, even temperature in early spring.
One of the common impressions in connection with mulching for purposes of winter protection is that plants are really being protected against extreme cold. Nothing could be farther from the truth. It is not possible to protect plants against freezing conditions, especially during the cold winters of our north when frost enters the soil to a depth of two to three feet. Under such conditions no normal depth of mulch consisting of rotted leaves or manure could keep frost out of the soil surrounding the roots of the plants.
The real reason then is a different reason from that of being a desire to ward off a freezing condition. It is the desire not of warding off the cold but of preventing abnormal evaporation of moisture from the leaves and especially from the lower part of such plants as the roses which are particularly sensitive to this type of evaporation. It is for this reason that plants which do not have well-ripened wood like the golden bell and certain varieties of the deutzia are subject to injury because of this evaporation. Other plants like the bush honeysuckle and lilacs which succeed in developing thoroughly ripened wood before winter approaches do not suffer so much. There are two important sources of injury (outside of the gnawing by animals) which can cause plants to be injured during the winter months and as a protection against which mulching is necessary. The first is abnormal evaporation from the roots and stems of plants that are in exposed situations, and the second a liability of being heaved from the ground where a clay loam soil is subjected to violent changes of freezing and thawing. It is for this same reason that standard roses and climbing roses are frequently lost during the winter months because they are protected by a thin covering of straw, and evaporation is allowed to continue, when, in reality, they should be taken down and buried in soil which remains moist throughout the winter months and thus protects the stems against evaporation.