(Editor's Note: This article was originally published on December 8, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)

Beautiful, but deadly. The intricate ice formations on this Cedar were the result of a leaking gutter. Fortunately, the ice did not damage the tree which was over 80 years old and quite sturdy. Not all specimens are so lucky.

How Plants Survive the Winter

Our trees, shrubs, and perennials enter a period of dormancy as days shorten and temperatures cool in the fall. During this period, woody plants must have a certain number of hours of temperatures below 40˚F in order for them to come back in the spring. Many species in the Midwest do not attain full dormancy until late fall/early winter (mid-November or December). Unexpected cold snaps in early fall or late spring will severely damage plants that either have not entered full dormancy, or have begun spring growth. A perfect example is the devastating effect of the unprecedented April 2007 freeze, when temperatures dropped into the 20's for six consecutive nights. Experts predict that the true consequences of this event will not be known for a few years.
An interesting side note to the damage done by the 2007 freeze: apple trees have a cycle of alternate bearing where a season of low fruit production is followed by excessive fruit set.[1] Orchard owners and home gardeners lamented the loss of buds in 2007, but 2008 saw apple trees groaning under the weight of the crop (at least here in Ohio).

What is an Ice Storm?

In some parts of the country, these storms are referred to as "glaze storms" (or "silver thaws" in the Pacific Northwest). When surface temperatures remain at or below the freezing point (32F/0C), super-cooled rain freezes on contact, sometimes accumulating up to 1 inch. (Weather statistics indicate that accumulations of this magnitude only occur once in 50 years.)[1] Parts of Missouri saw up to 2 inches of ice during the 2007 storm. This heavy layer can increase the weight of a tree branch up to 30 times, causing breakage. Add a strong wind, and the chances of losing an ice-covered branch increase exponentially.

Aside from breakage and/or uprooting, what impact does an ice storm have on trees and shrubs? Branch loss reduces the total leaf area, which reduces the carbohydrate available to the roots. Exposed bark is subject to sunscald and infestation by stem borers and other insects. Breakage wounds are open to decay, fungi, and crown gall bacterium.

Which Plants are at Risk?

The United States Forest Service states that trees with an excurrent branching habit (conical form) and species with less branch surface area are the least likely to sustain damage from ice accumulation. horizontal

Horizontal branching seemed to resist breakage better than upright branching.

Fast-growing trees have brittle wood and develop V-shaped crotches which easily split.



Examples of Conical Form Trees
  • Fir
  • Pine
  • Spruce
  • Cypress
  • American sweetgum (young)
Examples of Trees with Less Branch Surface
  • Black walnut
  • American sweetgum (mature)
  • Ginkgo
  • Kentucky coffeetree
  • White oak
  • Northern red oak
Examples of Trees Which are Easily Damaged
  • Chinese elm
  • Siberian elm
  • Poplar
  • Silver maple
  • Birch
  • Willow
  • Japanese Maple
Prevention or Repair?

A Stitch in Time...
Nothing can be done in an established landscape to prevent damage to mature trees. But if you are replacing specimens or designing a new landscape, planting storm-tolerant varieties is one way to prevent the ravages of ice and snow. Check with local experts to learn which ones will grace your landscape and survive the winter weather. Plan ahead when planting tall trees, positioning them at least 45 feet from overhead utility wires. Prune them regularly to establish a well-distributed crown branching habit. Occasional light pruning of older trees might help them resist ice damage. Weak branches and interior branches can be removed before winter sets in, but avoid late summer pruning which will stimulate new growth.

Choose hardy native plants or those recommended by local arborists. Site selection is paramount. Easily-injured broadleaf evergreens such camellia, azalea, rhododendron, holly, and daphne should be located on the north, northeast, or eastern side of a building or fence to protect them from wind and sun. Low places in the landscape create frost pockets and can accumulate excess moisture around the roots. Don't place shrubs under the eaves of the house--snow and ice can slide off onto the plants, and overflowing gutters can drench plants in heavy ice during freezing weather.

Protecting small upright evergreens with multiple leaders (e.g., arborvitae, juniper) or clump trees such as birch can be accomplished by wrapping the leaders together about two-thirds above the weak crotches, using something soft (strips of cloth, nylon stockings, etc.) Be sure to remove the strips in spring. A professional arborist can prepare larger, wide-spreading trees by cabling the branches together.

Finally, photograph your trees and landscape every few years. In the event that you have major damage during a winter storm, you'll have evidence to provide to your insurance company.

Damage Control

snowWhen the worst of the weather has passed, assess your trees and shrubs. Do not try to remove ice or snow by beating the branches! Plants have some flexibility and the accumulation of ice or snow happened gradually. By knocking the weight off these branches, it causes them to "snap" back, which stretches and damages the circulatory system of the plant. If a branch is severely bent, you can try propping it up to prevent breakage, but do it gently. The best course of action is to wait until the temperatures rise to above freezing, at which time snow will melt on its own. If ice accumulation is significant (half-inch or more), Iowa State University Extension suggests spraying the branches with COLD water to accelerate melting. Never use hot water.

Remove broken branches as soon as possible to prevent further breakage or tearing of the bark. Trees don't heal--the bark grows over the damaged area. Hence, proper cleanup of a broken stem is important. Smooth the ragged edges of the dead bark using a sharp knife, removing the bark back to its attachment point. Tree experts have determined that wound dressings neither prevent decay nor keep out insects. Dr. Alex Shigo found, during a 13-year research project, that leaving the tree alone was more beneficial than applying wound dressings, and that many products such as shellac, paint, or roofing tar actually damaged the tree.

The weight of snow and ice can sometimes uproot small to medium-sized trees. When the temperatures warm, assess the situation to see if the tree can be straightened and saved. If one-half to one-third of the original root system is still in the soil, with the remaining exposed roots compact and undibreaksturbed[2], it might be possible to set the tree upright and brace with guy wires.

For catastrophic damage, the question is: "Should I save this tree or have it removed?" Major tree repair is expensive. If the tree is an important part of the landscape and a major portion of it is intact (has more than 50% of its branches), then the homeowner might opt to save it. Removing a mature beloved tree is an emotional experience, but if the condition of the tree is such that it will fall victim to either future weather- or disease/insect-related events, one must consider that into the decision.

Professional or DIY?

Large limbs can weigh over 2,000 pounds and most mature trees should be dealt with by professional tree services. Beware the ibrokenndividual knocking on your door with an offer to repair or remove your damaged trees. Workers with professional tree services are:

• Established in the community or nearby, not "moonlighters"
• Listed in the phonebook or community services directory
• Fully insured for property damage, personal liability, worker compensation
• Certified tree care professionals

Get several estimates and be sure to ask for references, then check them out. Be sure you understand who is responsible for removal and cleanup. Find a tree service professional in your area.

The homeowner who wishes to prune damaged trees should spread the repair project over 2 or 3 seasons. An excellent article on this subject resides on the International Society of Arboriculture website.

When the storm is over and the repairs and removals are finished, it is still important to continue evaluating and nurturing the recovery of damaged plants. Supplement rainfall to 1-inch per week, mulch to conserve moisture, and avoid excess soluble nitrogen fertilizer (less than 2 pounds actual nitrogen per 1,000 square feet[1]). Storm-damaged plants can be victim to insects and diseases, so check for symptoms and signs, then act quickly to minimize further damage.

To see the devastating effects of the 2007 ice storm in Tulsa, Oklahoma, read Aunt_A's excellent article, "CityGreen: Regreening a City After a Deadly Storm".

[1] University of Missouri Extension. http://extension.missouri.edu
[2] University of Illinois Extension. http://www.urbanext.uiuc.edu

Other resources
University of New Hampshire Extension
Virginia Cooperative Extension
Ohio State University Extension
Virginia Cooperative Extension
Iowa State University Extension
University of Minnesota Extension
Unites States Forest Service