When we spend time outdoors in deep cold, we often get red noses and chapped cheeks. In a young tree, sudden, deep cold can cause more severe damage: cracks in the bark.
Such cracks, called frost cracks, are most likely to occur on young trees, whose bark is still thin and relatively flimsy. The cracks are always vertical, and they can be shallow or deep. There can be more than one crack in a single tree.
Cracking is most likely to be seen on winter days after temperatures have fallen to 15 degrees or colder at night.
“The sudden change is what causes frost cracks,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “If the temperature declines slowly and the tree has a chance to get acclimated, its bark is likely to hold up better.”
A swift drop in temperature causes the outer layer of bark to freeze and contract faster than the inner layer, putting them both under tension. At weak points, the strain may crack the bark.
The cracks often appear on the south or southwest sides of a tree’s trunk. “That’s where the temperature fluctuations are greatest,” Yiesla said. During the day, the sun from the south and southwest will warm the bark more on that side of the tree, so when the cold night falls, that side faces a greater temperature drop.
Certain species of trees are especially susceptible, including walnuts, crabapples, lindens, willows, horse-chestnuts, oaks, London plane trees, sycamores, and red and Norway maples. “At the Plant Clinic, we get a lot of calls about frost cracks in maples,” she said.
A frost crack is often permanent. “When the growing seasons begins, the tree will start to seal over the crack,” Yiesla said, “but it will be a weak place where the bark is likely to crack again in winter.”
The crack itself usually does not do serious harm to the tree, but it creates an opening where insects, diseases or decay organisms can enter the wood.
Wrapping the trunk of a young tree to protect it in winter is not a practice the Arboretum recommends. “Tree wraps tend to trap moisture against the bark, which leads to disease and decay,” she said.
Instead, the best way to help a tree withstand frost cracking is to make sure it is generally healthy. Spread an insulating layer of mulch over its roots, and water it well all through the year, especially in fall.
“Trees that are generally well cared for will be better able to ride out all kinds of stresses, including cold,” Yiesla said.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice or [email protected]). Beth Botts is a staff writer at the Arboretum.