by James Freed, WSU Extension and Sandy Salisbury, WA DOT – WA Community Forestry Council
Summer is a good time to think back to the ice storms of last winter, try to learn some lessons from the damage they caused, and plan for the future. Ice storms are caused by super-cooled rain that falls on surfaces, such as tree branches that are at or below freezing. Accumulations of ice can increase the weight on branches by 30 times or more. Ice coatings can be from a trace to an inch or more. Ice accumulations between ¼ and ½ inch can break small limbs and branches. 
Some trees are more resistant to ice damage than others and can be planted to reduce tree and property damage from ice storms. Healthy trees with a lateral branching structure and a broad crown are more susceptible than trees with narrow crowns. Trees with fewer, thicker branches have less surface area on which to accumulate ice. Small forest understory trees are relatively resistant to ice damage. Trees with dead or diseased limbs are especially vulnerable. So tree selection and maintenance are both important to minimize ice damage (Hauer, et al, 1994).
Many of the trees used in our Northwest landscapes are not native to the Pacific Northwest. If they are from growing areas that do not experience ice storms their natural susceptibility to ice damage is very high.
We can plan for future ice storm impacts by carefully selecting tree species that are more resistant to ice damage.
Trees with a conical structure and strong branch attachments are relatively resistant to ice damage. However, when native trees are used in landscapes they often lose their resistance to ice damage because they are now growing in more open conditions that favor large crowns and weak limb attachment and are often pruned for view and looks and not for strong crowns.
Our native trees in their forest environments may be damaged by ice but offer little threat to property and people. The major exception is the edge-effect trees. These are forest trees growing along roads, streams and fields. Their unbalanced crowns make them very susceptible to uprooting and crown breakage.
Ice storm susceptibility should not be the only criterion for selecting trees for landscape plantings, but the numbers of susceptible trees should be limited to areas that will not pose a threat to utilities, buildings and people. If we want to use species that are susceptible to ice damage, we can locate them so that falling branches are not a hazard to structures, transportation routes, or power lines.
Now is a good time to examine and treat trees that have experienced broken limbs in past ice storms. An arborist certified through the International Society of Arborists (ISA) can help evaluate your tree(s) and prescribe the best course of action to treat a tree. You can find a certified arborist near you by looking in the phone book under “tree services” (be sure to look for the ISA Certified Arborist shield), or go to http://www.treesaregood.org/, the consumer webpage for the ISA .
Ice Storm Susceptibility of Tree Species Commonly Planted in Urban Areas
Editor note: If you would like a copy of “Trees and Ice Storms. The development of ice storm-resistant urban tree populations,” contact us at firstname.lastname@example.org We’ll send you a copy
 Richard J. Hauer, Mary C. Hruska, and Jeffrey O. Dawson. 1994. Trees and ice storms: The development of ice storm-resistant urban tree populations. Special Publication 94-1, Department of Forestry, University of Illinois at Urbana-Champaign. Urbana, IL 61801, 12 pp.