The ravages of a Maine winter play havoc with the garden’s trees and shrubs. Winter sun, wind, and cold temperatures can bleach and desiccate evergreen foliage, damage bark, and injure or kill branches, flowerbuds, and roots. Hungry mice burrow beneath the snow to feed on bark and twigs while deer and rabbits nosh on flowerbuds and foliage.
What can the gardener do to mitigate this damage?
Protecting against Sunscald
On cold sunny days, the bark temperature on the south side of a tree can be as much as 77ºF warmer than the bark on the northern side of the tree. When the south or southwestern sides of young trees are exposed to the warmth of direct sunlight, living cells beneath the bark respond to the increase in temperature by losing some of their cold hardiness and becoming active. These cells, called cambial cells, are responsible for producing new water and food conduction tissues within the trunk. During the subsequent night, or when the sun becomes blocked by a cloud or building during the day, the bark temperature drops precipitously, killing the cambial cells. The resulting damage is called sunscald.Sunscald injury, often not noticed until spring, is characterized by bark that is discolored and cracked. Young deciduous trees, newly planted trees, and thin-barked trees are highly susceptible to sunscald. A partial list of susceptible trees includes apples, ash, aspen, birch crabapple, cherry, cottonwood, honeylocust, linden, maple, mountain ash, oak, peach, tuliptree, and willow. Evergreen trees are generally considered less susceptible to sunscald, although 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 sunscald injury. Protect sensitive trees from sunscald by wrapping the trunk with light colored material that will reflect sunlight, keeping the bark temperature more constant. Tree wraps are made in a variety of materials, including plastic, burlap and paper. We use a brown corrugated paper sold in rolls, wrapping it around the trunk starting at the base; starting at the top results in open spaces between layers of the wrap, spaces where water can seep behind the wrap. The wrap should be applied in early November and removed in April; wrap left on the tree can harbor bark-boring insects. Newly planted trees should be wrapped each winter for at least the first two years, thin-barked species for five years or more.
Drought-stressed trees are more susceptible to sunscald, so be sure that your young trees are fully hydrated as they enter winter. A layer of mulch that extends from just outside the trunk to the tree’s drip line will conserve soil moisture.
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 Evergreens from Browning and Bleaching of Foliage
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.
Browning and bleaching of evergreen foliage can also result when the bright winter sun causes leaves temperatures to rise, initiating cellular activity. When the sun is suddenly blocked, foliage temperature drops to injurious levels.
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 evergreen 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 as well as 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, may also 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. And 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 per week, either 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 1/4-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- to 24-inches above the ground to deter nibbling of tender lower branches. In all cases, these wire barricades can 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 7-foot-tall 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.Rutting deer can also do substantial damage to garden trees as they rub their antlers against the bark. On a recent visit to Longwood Gardens in Kennet Square, Pennsylvania, Marjorie and I came across a group of American hornbeam trees (Carpinus caroliniana, also called “musclewood”), most two inches or less in trunk diameter at breast height. To deter deer from damaging these trees during rutting season, each trunk, from the ground up to the first branch, had been wrapped with a heavy-duty plastic mesh tube.