In winter, low temperatures as well as too intense light, lack of water (when the soil has frozen the moisture in it cannot be absorbed by the roots), and wind may cause damage. Some harm is directly due to the effects of below-freezing temperatures on the tissues of tender plants; other damage is indirect for example, the tearing and drying of roots that follow heaving of the soil as a result of alternate freezing and thawing.
It is well known that plants in poorly drained soils are more susceptible to winterkilling than specimens of the same kinds growing in well drained places. Plants located in hollows or “frost pockets” are much more likely to be damaged than those planted where there is free air drainage. Selection of favorable planting sites is important as a protection against damage by the cold.
Importance of Fall Watering
Trees and shrubs, particularly evergreen kinds, are less likely to suffer winter damage if the soil in which they grow is kept moist throughout the autumn. If this is done, their tissues go into the winter well supplied with moisture and they are better able to withstand the dehydrating effects of sun and wind when the ground is so cold that it is difficult or impossible for them to absorb adequate supplies of needed moisture.
If fall rains are inadequate, evergreen shrubs should be soaked thoroughly each fall before the soil freezes. A mulch of leaves or coarse compost applied immediately after the final watering is beneficial in cold climates.
Where severe winters prevail much damage may be done to trees and shrubs, especially to evergreen kinds, by dehydration caused by their above ground parts being exposed to the drying effects of sun and wind at times when their roots are in frozen soil or in ground so cold that absorption of moisture is prevented or retarded.
An effective way of reducing the damage of this type is to spray the foliage at the beginning of severe winter weather and once or twice more at about monthly intervals with an antitranspirant (antidessicant) liquid. Such sprays based on rubber latex or plastics are sold by dealers in garden supplies under various brand names.
Antitranspirants are of especial value when used on evergreens growing in exposed locations and for use on specimens that have recently been transplanted.
Mulching for Winter Protection
Very fine protection against cold is provided if an insulating layer of more or less loose material is spread over the surface of ground occupied by the roots of many plants. Such a layer either may prevent the underground portions of the plants from freezing or may simply prevent the ground from freezing as deeply as it otherwise would. In the latter case, at least some of the roots are likely to be in unfrozen soil and, if the plant is evergreen, it may still absorb at least some moisture to compensate for that loss from the aboveground foliage.
Winter mulching is really resorting to Nature’s way of protecting plants. When leaves fall they are blown among shrubs and other perennial plants. Before they decay and return plant food to the soil, they serve as a protection for roots and other below ground parts by keeping a more even temperature in the soil over which they lie. Many gardeners, instead of taking a lesson from Nature, are inclined to neatness and artificiality and so rake up these leaves in fall. This is not always good practice. If it is done, in many areas it is desirable to replace them with a winter mulch.
Small shrubs, especially those that have been recently planted or are not reliably hardy, can be protected by placing a layer of dry leaves, several inches thick, around them. The leaves should not be packed down so that air is excluded but should be left loose. Chicken wire may be used to contain them and prevent them from being blown away.
Larger shrubs and evergreens benefit greatly from having the soil covered with a 3-6 in. layer of half-rotted compost, loose, strawy manure, half-rotted leaves, or any other appropriate mulch material that remains fairly loose and does not pack down and exclude air.
In part, this term is interchangeable with winter mulching, but not wholly so. Winter covering includes the various types of protection that cover the tops, the above ground portions, of plants. Winter covering is normally removed with the coming of spring; winter mulches are often left in position to decay gradually and form nourishment for the plants, or are forked into the ground to decay beneath the surface.
Among plants that are winter covered in cold regions are trees, shrubs and evergreens likely to be injured by cold, perennials and bulbs likely to be damaged by low temperatures and deep freezing of the soil, and young or newly transplanted specimens that may be harmed by heaving of the soil due to alternate freezing and thawing.
Young plants of many perennials and biennials are not thoroughly winter hardy in all regions. Where they are somewhat tender, or where there is danger of the small plants being heaved out of the ground by alternate freezing and thawing, they are benefitted by a light winter covering. Plants such as Foxgloves and Canterbury Bells that retain their foliage during winter will not survive, however, if a heavy layer of winter covering material is spread over them. It should be just thick enough to shade the soil, and so check too frequent alternate freezing and thawing. Branches from Pines, used
Christmas trees or other evergreens are very satisfactory as a winter covering, as also is salt marsh hay.
Delphiniums are sometimes protected by having sand or coal ashes heaped around and over their crowns. This is especially worth while in gardens that suffer from excessive moisture.
Irises and Peonies may not need winter protection if they were planted early. However, young plants set out rather late in the season may suffer injury from heaving by frost unless it is prevented by a light covering. This is especially desirable if the soil is clay.