As humans, we hate the bitter cold and heavy snowfalls, but have you ever though about the stress our tree’s undergo? Winter damage is determined by a number of factors, including the plant species, the location in which the plant is grown, and the exact timing of the weather extremes during a plant’s dormant. Low temperatures are usually not the primary cause of winter injury, but rather the extreme fluctuations in weather.
Plants have to become acclimated to temperatures that are below freezing. They become acclimated by experience a slow decrease in temperature and a few other factors. Plants that are dormant, but not acclimated can be easily stressed by a sudden, hard freeze. This drastic change in temperature, tends to affect woody plants. Extended periods of mild winter weather can de-acclimate plants and pose a large stress on them, if their is a sudden temperature drop.
It is very important to know what species of plants and trees you are planting. Some species tolerate cold temperatures, while others need warm to mild winters. Reading labels and understanding the needs of each plant, will help the average homeowner pick out hardy plants for the region. Species such as rhododendron, holly, and some magnolias may survive several mild winters in the Midwest, before winter causes injury. Flower buds are often the most susceptible to low temperatures. If plants with marginal hardiness are used, they should be planted in protected sites, to help decrease stress. For the most part, cold temperatures are not as damaging as extreme fluctuations in weather.
Frost cracks also known as radial shakes, appear as shallow to deep longitudinal cracks in the trunk of trees. They are most evident when temperatures drop below 15°F. Frost cracks are typically seen on the south or southwest sides of trees, because this area experiences the greatest temperature changes between day and night. A sudden drop by temperature causes the outer layer of wood to contract earlier than the inner layer, which endsin an exceedingly long vertical crack at weak points within the trunk. Frost cracks most likely will re-appear after they have once appeared on the tree. Trees most susceptible to frost cracks include oaks, red maple, crabapple, walnut, linden, and willow.
An elongated canker found on the trunk of thin-barked trees, such as beech, maple, willow, white pine, and linden, is often referred to as "sunscald". Sunscald is typically found on the south or southwest side of trees, due to a larger exposure to direct sun. In winter, the temperatures on the sun-side of the trunk may exceed air temperatures by as much as 20°F. This can cause the tree to de-acclimation, causing injury. Trees that experience sun scald will have a read of darkening bark that turns reddish brown, and becomes rough. With time, the callus tissue eventually cracks and falls away. Sometimes only the outermost cambium layer is damaged and a sunken area appears on the trunk. Affected trees often have sparse foliage, stem dieback, and stunted growth.
WINTERBURN ON EVERGREENS
Not all evergreens stay a lush green color after the winer months. Some evergreen experience winter burn, which causes a browning or scorched leaf tip come late winter and early spring. Browning usually occurs from the needle tips downward. Symptoms of winter burn are present on many narrow-leafed evergreens, such as junipers, pine, and yew, and broad-leaved evergreens, such as boxwood and rhododendron. This phenomena happen when their is a loss of water through leaf transpiration. The harsh effects of gusty winds and the winter sun tends to dry out the needles. The majority of the water in the trunk an stems is frozen, so it unable to replenish the water lost. Applying an anti-transpirant in December and February, will help reduce transpiration and minimizes damage to the foliage.
We all love to see new growth in the spring, but mother nature isn’t always ready for warmer temperatures. A spring frost can cause damage to woody stems, blossoms, and new shoots. The new tissue will become water soaked and will eventually wither and die. Spring freezes can sometimes resemble blight, so it is important to have an arborist diagnose the tree.
Root tissues has a very hard time becoming acclimated to temperatures below freezing and can be severely injured by soil temperature below 15°F. This is especially true for shallow rooted plants. Although it is imperative to not over mulch, the presence of mulch, leaf litter, or snow helps to insulate the soil. This insulation will help prevent soil temperatures from falling below freezing and allow for a better change of survival during the winter. Plants with frozen roots may wilt and have stunted growth, when spring hits.
SNOW AND ICE BREAKAGE
Heavy snow and ice storms cause damage by bending and breaking branches, due to excess weight. Common evergreens, such as yews, arborvitae, and junipers, are prone to snow damage. It is best to tie their branches together, prior to winter so they don’t bend and split. The branches of many hardwoods, such as Siberian elm, maples, and birch, may be seriously damaged in ice storms. It is not suggested to try to remove ice from these trees because it can cause more harm. It is best to allow the ice to melt naturally.
Salts used for deicing pavements can cause damage to the roots of trees and shrubs. Visual symptoms of salt damage may appear in spring and early summer and include browning of evergreens, leaf scorch, branch die back, and dead areas in turf. The excess salt will leach through well-drained soils, but can’t always leach in poorly drained soils, causing significant damage. It is best to choose salt-tolerant species near streets and walkways, where salt stress is more common.
MINIMIZING WINTER INJURY:
Select hardy species and cultivars.
Avoid late-summer fertilization or pruning, which might stimulate new growth.
Water trees and shrubs, especially evergreens, during dry periods until the ground freezes.
Use mulch to conserve soil moisture and insulate the roots from cold temperatures.
Apply anti-desiccant to evergreens starting in late fall, following label instructions.