Karl Foord, UMN Extension Educator
Preparation for winter
Plants acclimate or develop hardiness to freezing temperatures in response to changes in light duration and temperature. Acclimation is a two-stage process. The first stage is initiated by decreasing day length and results in partial hardiness. The second stage is initiated by subfreezing temperatures and results in full hardiness and acclimation. The hardening response in a plant may vary from year to year because of variation in temperatures over that year. The degree of cold hardiness of a plant is determined by the genetic capacity of a plant to acclimate to freezing temperatures and transform its tissues from a non-hardy to a winter hardy state. Plants are often given a cold hardiness rating based on the lowest midwinter temperature that plant tissues can endure relative to the USDA winter hardiness zone temperature bands. For example, Honeycrisp has a winter hardiness rating of USDA hardiness zone 3 (-40 to -30 F).
However, extreme low temperature is only one of a number of factors affecting winter survival. Winter damage frequently occurs during late fall or early spring due to extreme changes in temperature when the plant is not at its maximum hardiness. Injury is a function of the acclimation status of the plant at the time of the radical temperature change, and is often difficult to determine, and may not show for several years.
No two winters are the same
Every winter may have an episode that could cause some damage to some trees. For example the winter of 2008-9 was considered to be a more stressful year than the winter of 2009-10. This is borne out by the temperature graphs of the two years. Note that the winter of 2008-9 had a great deal of significant temperature fluctuation while 2009-10 was much smoother (Figures 1 & 2). Dr. Jim Luby noted three potentially damaging events that stand out in the winter of 2008-9 as follows: 1. The 43 degree temperature drop on December 15, 2008, 2. The 37 degree temperature drop on March 11, 2009, and 3. The five nights of low temperatures below - 20 from January 13 - 17, 2009. Many areas experienced a freeze event on Mother's Day (May 9, 2010) with varying amounts of damage. Some Minnesota apple orchards lost almost all their apple flowers and reports from Iowa noted lots of damage with bark splitting off the trees.
Carryover environmental effects
A dry season or a particularly heavy crop can reduce the vigor of the tree and make it more susceptible to winter injury. As Kathy Zuzek would tell you a rose variety that is defoliated by black spot in July is more susceptible to winter injury then one that has not been defoliated.
Other influencers of hardiness
Lack of snow cover during the coldest period can lead to root damage.
Pruning apples before they have accumulated their full hardiness can set trees up for winterkill, especially so with cultivars that are less winter hardy. In general, pruning cuts are dehardening in the early winter and the larger the cut the more dehardening occurs. Prune fruit trees in late spring before the buds become active.
Orchard topography is important because it affects cold air drainage. Lower areas where cold air accumulates can cause frost to settle and damage trees.
High intensity sunlight on a sunny winter day heats up the south and southwest side of thin-barked young apple tree trunks causing the cells to come out of dormancy and become active. After sunset temperatures can drop precipitously to levels well below freezing which kills active cells and conductive tissue. This often appears as a longitudinal crack running up and down the trunk.
Commercial tree wraps made of crepe paper, plastic spiral wraps or longitudinally cut drain pipe will intercept the sun and insulate the bark preventing sunscald. Wrap the trees from base to the lowest branch in the fall after leaf drop. I have used all methods. I have left the plastic wraps on for at least three years until the bark thickens and is less prone to sunscald. I removed and rewrapped the crepe paper each year.