Extension Educator, Horticulture
Ice cubes, ice skates and ice tea are a few of my favorite things. Ice on trees is another story. Illinois certainly can have its share of ice storms. This past Valentine's Day our area braced itself for another ice storm, with visions of major tree damage and power outages. While we were fortunate to miss such a storm, it serves as a good reminder of the importance of making good tree planting decisions.
While no tree is 100 percent resistant to ice storm damage, there are trees and tree characteristics that will enable certain trees to come through ice storms with only minor damage.
Bill Vander Weit, city arborist with the City of Champaign outlines a few characteristics of trees prone to ice damage. One characteristic that makes a tree particularly vulnerable to ice damage is a narrow angle of attachment between the branch and trunk. Branches with a tight angled, V-shaped crotch have a weak connection and are prone to failure under ice loading and high winds. For best branch strength, the ideal branching angle in many broadleaf species approximates 10 o'clock and 2 o' clock. Branches that are growing at tighter angles than these should be removed or reduced in trees when they are young.
While any tree can develop branches with a tight angle of attachment, there are certain species that are known for this characteristic. Bradford Pear is the poster child for poor branch attachment, and it is rare for these trees to reach maturity without experiencing the loss of major branches. Other trees known for poor structure are green ash, littleleaf linden and redbud.
Another characteristic that makes a tree prone to storm damage is a broad spreading crown that arises from the presence of several large competing trunks. The broad crowns increase the surface area of lateral branches, and with increased surface area, more ice can accumulate. The problem is compounded because trees with this form typically have narrow branch attachments. Among trees that will form broad open crowns, increasing their susceptibility to ice storms, are Siberian elm, American elm, hackberry, green ash and honeylocust.
Trees that have fine branching, such as river birch and Siberian elm are also prone to ice storm damage. The fine branching will result in a greater volume of ice for a given surface area, compared to species that are coarsely branched, such as Kentucky coffeetree and ginkgo.
A University of Illinois publication Trees and Ice storms: The Development of Ice Storm-Resistant Urban Tree Population lists several trees that show good resistance to ice damage. These include American sweetgum, baldcypress, eastern hemlock, ironwood, Norway maple, white oak and swamp white oak. However ice storm susceptibility should not be the sole criterion for selecting trees for urban planting, but the numbers of susceptible species should be limited.
Other important factors to consider are proper tree placement and pruning trees on a regular cycle. Trees should not be planted where they will interfere with above ground utilities. When planting large trees next to electrical lines, trees should be located a minimum of 30 feet from these lines. If planting closer, choose trees that will be no greater than 20 feet at maturity.
Pruning trees on a regular cycle will reduce a tree's susceptibility to damage by removing deadwood and structurally weak branches. Cyclic pruning will also allow regular visits to assess hazardous trees before they fail. Also pruning and training early in a tree's life will encourage a good framework.
Remember, in Champaign or Urbana you cannot plant a tree on public right-of-way without a permit. Both cities regulate planting to insure that good trees are planted without interfering with above or belowground utilities. For more information, visit: