Ice Damage to Trees

I doubt there's a gardener living who can contemplate an ice storm without deeply mixed emotions. On the one hand, it's beautiful, possibly the most beautiful thing the winter landscape offers. Trees that have been encased in ice, that shimmer and twinkle in the least light and shine with their own cold brilliance are winter trees at their best - line drawings electrified.

Iced viburnum

On the other hand, that’s “best” in a strictly visual sense. Unless you count being covered with ice while still in leaf, there’s no greater stress for a tree’s crown than having to bear great weight on frozen, unbendable branches.

Unless you count ice plus wind.

Our friend Eric over at Yale isn’t mentioning any woes that may have befallen Marsh Gardens, but he does have a lot of good advice in the “how to save your trees” department.

Ice Storm Damage to Trees

By Eric Larson

One of nature’s severest tests for man, beast and plant is an ice storm. We can of course stay indoors, salt the walks and roads, and put on our ice-gripping slip-ons to fight the effects. For birds and animals, shelter is also the order of the day. But for plants there is no refuge, save in their structural ability to withstand the weight.

Ice build-up can increase the branch weight of a tree by up to 30 times the normal.  When ice builds up to a half-inch, small branches, dead limbs and weak structural members are at risk. One-half to one-inch accumulations will result in the loss of larger branches, sound wood and other severe damage to the tree.  Strong winds add to the stress on the structure of branch and limb.  An additional cause of injury to trees due to ice is from vehicles skidding into them.

We in New England live in an area of the country that is susceptible to ice storms, along with a large band that extends across the central and northern midsection.  As recent Weather Channel reports show, ice storms can happen almost anywhere, including the South. I will leave it to those august prognosticators to describe the climatological conditions that result in an ice storm. We can discuss what it does to trees and shrubs.

The factors that influence a tree’s susceptibility to ice damage include: inherently weak wood, dead and decaying branches, broken branches, a broad or imbalanced crown, large horizontal structural wood, fine branching habit and ‘included’ bark.

This last is a feature that you most often see on Bradford Pear.  When a crotch angle is too tight, the bark between the two structural members becomes ingrown, as in a fingernail. This weakens the structure of the branch junction, making the Bradford Pear one of the worst trees for ice damage.

Examples of species with a broad crown and fine branching habits include the Siberian Elm, Honey Locust, Green Ash, Hackberry and American Elm.

Factors affecting a tree’s resistance to ice damage include its general shape, with pyramidal being the best. This prototypical evergreen architecture is also found in some young deciduous trees, like the Sweet Gum.  Later in its life span, the Sweet Gum crown will broaden and become more susceptible to ice damage.  It should be said that there are some varieties within a species generally known for broad crowns that exhibit tighter more pyramidal structure, so if you are set on, say, an English Oak, look for the ‘fastigiate’ or narrow pyramidal form.

Another factor in resistance to ice damage is the propensity of a species for coarse branching habit, or fewer thicker branches.  These tend to have less horizontal surface area, which means less ice load.  Under story trees and trees that mature at smaller heights also have more resistance to ice damage.

The other factor in my experience involved with major tree failure during ice events is the root depth of different species. For instance, Red Oaks have shallower root systems, which makes them easier to transplant, but it also makes them more susceptible to blow-down than their cousin the White Oak, which has a deep tap root.

So what can we do to lessen the devastating affect of ice damage on trees?

First –  in areas where ice storms are likely, plant resistant species and varieties. (See table below).

Second – plant the tree or shrub in the correct place. Those with preference for good drainage should not be planted in wet lowland areas for instance.  Pay attention to the tolerance or preference of trees and shrubs for sun or shade.  Trees that are grown too close together or in deeper shade than preferred tend to have imbalanced crowns and long horizontal branches.

Third – Train and prune young trees for good structural integrity: remove crossing branches and branches with poor angles of connection to the trunk, and maintain good spacing of branches as the tree grows. Remove dead wood periodically, and have an arborist check for disease and insect problems.  Trees near the home, sidewalk, road or other sensitive area should be monitored more closely.

An additional note about prevention of ice damage concerns the common practice of allowing English Ivy to grow up the trunks of large deciduous trees. There is no data to support the fact that this harms trees, except for one important point: the increased surface area of this evergreen plant adds to the ice load for the entire tree to an extent that may spell dire consequences. So in areas with frequent ice storms it may be wise to periodically remove the ivy from all but the main trunk of the tree.

After ice damage has occurred, it is probably best to contact an arborist for help in assessing the damage and deciding what action to take. In some cases, correct pruning will suffice in minimizing the long-term effects of the injury. In a few cases, the injuries are devastating enough to warrant removal of the tree. It is always best to seek the advice and estimates from at least two tree services.  Also make sure they are licensed, insured and bonded before starting work.

(If your place looks anything like our place, heaps of snow are giving the ice layers a real run for their money. Tips on snow removal are here. LL)

• Susceptible ……………..Moderately Resistant …………..Resistant
• American Elm                      Bur Oak                                          American Sweetgum
• White Oak                             Eastern White Pine
• American Linden                                                                                   Arborvitae
• Black Cherry                         Northern Red Oak                                Bald Cypress
• Black Locust                         Red Maple                                               Black Walnut
• Bradford Pear                       Sugar Maple                                            Blue Beech
• Common Hackberry           Sycamore                                                  Catalpa
• Green Ash                             Eastern Hemlock                                    Ginkgo
• Tulip Tree                              White Ash                                                 Ironwood
• Honey Locust                                                                                             Kentucky Coffee Tree
• Pin Oak                                                                                                        Littleleaf Linden
• Siberian Elm
• Silver Maple
• Norway Maple
• Silver Linden

Adapted from Hauer, R.J., Wl Wang, and J.O. Dawson. 1993.Ice Storm Damage to Urban Trees, Journal of Arboriculture

19:187-193.

Photos by Bill Bakaitis