The ravages of a Maine winter play havoc with the garden’s trees and shrubs. Winter sun, wind and cold can bleach and desiccate evergreen foliage, damage bark and injure or kill branches, flower buds and roots. Hungry mice burrow beneath the snow to feed on bark and twigs while deer and rabbits nosh on flower buds and foliage.
What can the gardener do to mitigate this damage?
Protecting against sunscald
On cold, sunny days, the bark exposed to direct sunlight (usually the south and southwestern sides of the tree) heats up to the point where living cells beneath the bark become active. These cells, called cambial cells, are responsible for producing new water and food conduction tissues within the trunk. When the sun becomes blocked by a cloud or building, the bark temperature drops precipitously, killing the cambial cells. The resulting damage is called sunscald.
Sunscald is characterized by sunken, dried or cracked areas of dead bark. Young trees and newly planted trees are highly susceptible, as are thin-barked trees such as cherries, crab apples, maples, birches and mountain ash. Also, pruning evergreen trees or shrubs in late summer or fall to remove lower branches may expose previously shaded trunk tissue to direct winter sun, resulting in potential sunscald injury.
Protect sensitive trees by wrapping the trunk with light-colored material that will reflect sunlight, keeping the bark temperature more constant. Commercial tree wrap, a polyurethane spiral wrap that expands as the tree grows, or any light-colored material will work. Wrap the tree in early November and remove the wrap in April.
Newly planted trees should be wrapped each winter for at least the first two years, thin-barked species for five years or more.
There is no remedy for sunscald after it has occurred, other than to carefully cut away the damaged bark with a sharp pruning knife and hope for the tree’s natural wound-healing capacity to work. Do make damaged trees a priority for wrapping in subsequent winters.
Protecting evergreen foliage from browning, bleaching
Whenever the winter sun warms conifer needles, transpiration occurs. Water is lost from the needles while the roots are frozen, and this results in desiccation of the needles and destruction of chlorophyll, followed by needle browning or bleaching. Browning or bleaching of broad-leaved evergreens, such as rhododendrons, occurs in the same manner.
Among the conifers, the most susceptible types are yews, arborvitae (Mainers call it “cedar”) and hemlock. All conifers, however, can be affected.
Solutions to this problem begin with proper placement of conifers and broad-leaved evergreens in the landscape. They are best planted on the east side of buildings, certainly not on the south or southwest sides or in windy, sunny sites.
To protect low-growing conifers from winter wind and sun, prop pine boughs against or over the plants once the ground has frozen. The boughs will act as a windbreak and catch insulating snow.
For larger conifers and sensitive rhododendrons, burlap wind barriers can be constructed on the south, southwest and windward sides of plantings. These barriers, if tall enough, also may protect against salt-spray damage to plants near driveways and roads.
Stakes for the barriers should be installed in early November, before the ground freezes. Later in the month, attach the burlap sheets to the stakes with staples or sturdy twine. Make the enclosure as tall as feasible to block wind from hitting the uppermost branches. Leave the top of the enclosure open.
Water-stressed trees and shrubs are ill-prepared for winter winds and cold. Throughout the growing season, your trees should receive an inch of water a week from rain or irrigation. Beginning in late autumn until freeze-up, they should receive an inch of water per month by rain or irrigation. Waiting until October to begin watering as needed will not maximize stress resistance.
Some gardeners spray evergreens with anti-desiccants or anti-transpirants to reduce winter damage. Save your money. Most studies show these materials to be ineffective.
Rabbits and mice and deer, oh my!
Most of our garden mice spend the winter in the woodpile below the porch sunflower feeders. In a really hard winter, however, we have experienced mice damage on the lower trunks of newly planted shrubs and trees, enough to start placing cylinders of quarter-inch hardware cloth around the bases of sensitive plants. To be effective, these wire cylinders must extend two to three inches below ground.
Cylinders made of the same wire will deter the garden’s rabbits from feeding on specific shrubs or trees, but they should extend at least 18-24 inches above the ground to deter nibbling of tender lower branches. In all cases, these wire barricades may be left in place all year, but be sure to enlarge them as the trunks grows larger.
As for the noshing deer, we built a fence to keep them away from the blueberries and raspberries. Beyond that solution, you’re on your own. In my mind, deer at the edges of the winter garden are part of the joy of gardening in Maine.