Winter brings several threats to trees in terms of ice, snow and wind — and even the perennially warm climates must occasionally deal with potentially damaging cold temperatures. Let’s examine each problem, starting with the worst of winter weather, an ice storm:
Ice, ice and more ice
Probably no winter weather element does more damage to trees than freezing rain, the main component of ice storms. Freezing rain is liquid rain that freezes on contact with exposed surfaces, covering them with a layer of ice or glaze. The weight of the ice can cause bending and breaking of branches and even trunks. As little as 0.25 inches of freezing rain is considered the threshold for producing damage. Keep in mind that a gallon of water weighs over 8 pounds and ice weighs only 8 percent less than liquid water. Extreme cases of 2 or 3 inches of accumulated ice have occurred and resulted in catastrophic tree damage.
How fast the ice will accumulate depends on the rainfall rate and the temperature. Heavier rain and lower temperatures lead to more ice. Freezing rain can occur with surface temperatures close to 0 degrees Fahrenheit. The length of time the tree must bear the weight of the ice is also important. If warmer temperatures arrive shortly after the precipitation ends, melting will ensue and lessen the ice load. But if the storm ushered in a prolonged period of below freezing temperatures, the tree damage will continue well after the freezing rain has ended.
When the rain freezes
Freezing rain occurs when a shallow layer of very cold air near the surface lies below warmer air aloft where clouds and rain are being generated. Shallow layers of cold air with the potential for freezing rain can penetrate well to the South in winter months from Texas in the West and into northern Georgia and Alabama to the East. From the Rocky Mountains westward, freezing rain is rare except for some valley locations in the Pacific Northwest. Overall, the interior Northeast has the greatest likelihood of freezing rain. Freezing rain is most common to the north of a warm front, which is typically east of a surface low. Occasionally behind, or west of some cold fronts, the cold air layer is shallow and freezing rain may occur, especially in the Midwest and Plains states.
Let it snow
Occasionally, snow can cause similar problems as ice. When temperatures are close to freezing, snow can be wet and heavy. The adage that 10 inches of snow equals 1 inch of water infers that snow weighs one-tenth of liquid water. But for wet snow, that ratio can be down to 5 to 1. And wet snow clings to everything unlike dry, powdery snow. It can cover branches, etc. with a heavy coating of snow. The excess weight can cause limb and even trunk breakage. Normally, you would need at least 4 to 6 inches of heavy snow to cause problems.
Wet snow, with temperatures close to freezing, is more common the further south you go and in fall and spring. It’s typically associated with a major low or storm system and an Arctic high providing the cold air.
Blowing in the wind
Strong winds also bring a threat to trees, from broken limbs to snapped trunks to complete uprooting, especially if the ground is wet. High winds exasperate problems caused by freezing rain or heavy, wet snow.
Although damaging winds can occur in the warm season with thunderstorms and the occasional tropical cyclone, they are more prevalent in the winter when weather systems are more intense. Major low pressure areas can produce damaging winds over wide areas. Strong cold fronts can often bring gusty northwest winds behind them that can also result in tree damage. Powerful ocean storms are common in the Pacific Northwest during a typical winter. On the East Coast, nor’easters can produce strong winds from the Mid-Atlantic up into the Canadian Maritimes. Although not as common as ocean storms, powerful low pressure areas can batter the Plains up into the Great Lakes.
When the temps drop
Cold temperatures themselves can do damage under some circumstances. The greatest threat is for trees in the Deep South, when abnormally cold conditions penetrate well into the lower latitudes. Keep in mind that native and introduced species can’t tolerate temperatures below freezing; citrus trees and palms are good examples.
A rapid drop in temperature often does the most damage, including frost cracks in the trunk bark. (This can be expected after a strong cold frontal passage; also, when the sunlit portion of the tree becomes shaded.) Damage can range from loss of leaves to death of the tree.
New growth is most susceptible to cold damage, as warm weather in the late fall or early spring often leads to this type of disturbance. Arctic outbreaks occur when there’s a big dip in the jet stream. Oftentimes, a massive Arctic high pressure area follows this dip far further south than usual.