Winter is a perilous time for garden plants, but damage is usually not caused by low temperatures alone. Leaves become scorched if plants cannot obtain enough moisture. Sudden temperature changes can cause the bark on trees and shrubs to split, and alternate freezing and thawing can break plant roots. Good planning is the key to protecting your garden from cold and frost damage. If you anticipate the need for mulches or other protective devices, get them ready in the fall.
Protection against cold damage
Some plants are damaged by extreme cold. Choose plants that are not normally damaged by cold in your area. Tender exotics can be grown almost anywhere with enough attention, but the extensive care needed may not be worth the extra trouble.
After the first hard freeze in the fall, untie plants that are trained up on supports or trellises, lay them on the ground, and cover with mulch. If a shrub (such as a rose) can’t be bent over to lie on the ground, dig up the roots on one side, tip the plant over, and cover it with soil or a heavy mulch.
Shovel snow onto short plants as an insulating cover. Build wire cages around taller plants and fill them with snow, straw, or dead leaves for insulation.
Spongy plastic fabric is an excellent insulating material. Cover plants with it, or use it inside a wire cage for extra insulation. It is susceptible to deterioration from ultraviolet radiation, however, so an additional white plastic covering is advisable for plants in full sun.
Protecting against winter scorch
What many people assume to be frost damage on trees and shrubs is often actually scorch caused by a lack of available water. This happens when the soil is frozen and cannot yield water. Wind on a sunny winter day is especially damaging to plants. Give plants a thorough watering in the fall, so that the soil is moist when it freezes. Mulching the ground around plants reduces soil freezing and makes it easier for roots to continue functioning during cold weather. Shade plants during the winter with snow fencing, lath, or other covers.
Winter bark scald
A sudden temperature change as the sun emerges from behind a cloud and warms the bark of a tree on a cold day can kill the bark. Sun scald is most likely to occur on the southwest side of the trunk on sunny days. Paint the trunk with whitewash or white interior latex paint, or shade it with burlap, tree wrap, or a board for protection.
Cycles of alternate freezing and thawing of soil may cause plants to be shoved up out of the ground, breaking and exposing some of the roots. This is known as heaving. Small, shallow-rooted plants are particularly susceptible to heaving. Mulch plants with straw or sawdust to keep the soil from thawing until spring. Apply the mulch after the ground is frozen. If a plant has heaved, keep it covered and frozen until spring approaches and the ground can be worked to permit replanting.
During very cold weather, wood freezes and expands, causing explosive cracks. These cracks usually occur where there is some rot or other weakness in the wood. As the weather warms, cracks usually close and heal over, but they may open in the same place the following winter. Paint the exposed edges of bark with a pruning sealer to keep them from drying out. If the bark is loose, nail it back to the trunk. If the same crack opens every winter, put long bolts through it to hold it together.
Shake or sweep snow and ice off limbs that are near the breaking point. Excessive buildup may deform plants even if they are not broken. Protect small plants with boards leaning teepee-fashion over them. Spreading shrubs, such as boxwood, can be tied into shape with twine in the fall so the weight of the snow doesn’t cause them to open and deform.
Road salt thrown onto foliage or absorbed through the roots causes a browning on the side of the plant that faces the road. New growth in the spring will be normal, but stunting and eventual death will occur if salt accumulates in the soil.
Protect streetside plants from salty water and slush with a wrap or screen of burlap or plastic. Leach excess salts from the soil in the spring with long irrigations unless spring rains are heavy.