WNLA e-Newsletter - January 26, 2009
WESTERN University Professor Shares Implications and Remediation for Ice and Freeze Damage to Ornamentals in the Midwest
EDITOR’S NOTE: In addition to his presentation “How to Save Soil, Freight and Labor Costs by Planting Bare Root Trees All Year Long” during WESTERN University (sponsored by KAT Nurseries) at the 2009 WESTERN Annual Meeting and Trade Show, Christopher Starbuck, Ph.D., Associate Professor in the Division of Plant Sciences at the University of Missouri, contributed this article:
Landscape and forest plants in the Midwest sustained extensive damage from several storms of historic proportion over the past several years. The purpose of this article is to provide some background and historical perspective, to describe the types of damage observed and plant responses to it, to predict some long term effects and to suggest some approaches to remediation. It should be noted that some of the weather events causing this damage were unprecedented in scope and severity. It is, therefore, somewhat difficult to predict how damaged plants will respond over time, since we have little experience upon which to base projections.
Background
Freeze Damage: Temperate zone woody plants survive freezing temperatures by development of a dormant condition in response to shortening days and cool temperatures in the fall. In the Midwestern U.S., many species do not attain full dormancy, with its associated freezing tolerance, until mid-November. Cold hardy plants are able to survive freezing in various ways. These include freezing of water in intercellular spaces, where ice crystals do not rupture cell membranes, and supercooling (cellular water remaining unfrozen at sub-freezing temperatures). Once dormant, woody plants require a certain number of hours when temperatures are below about 40 degrees F before cambium and buds become active in response to warm temperatures. This tends to prevent plants from growing actively during short, unseasonably warm, periods in mid-winter. If plants are exposed to sub-freezing temperatures before they can attain dormancy in the fall, or after they begin growing in spring, they can be severely damaged at temperatures which would cause no harm during the dormant period. During the wide-spread freeze event which occurred in April of 2007, many woody plants were devastated because they were in active growth when the temperature suddenly dropped well below freezing for six consecutive nights. Most plants had received sufficient chilling by mid-March to break bud dormancy. Temperatures during the two weeks between March 21 and April 3 were commonly 20 degrees above normal for that time of year. This was the third-warmest March 21-April 3 in 118 years of weather records in Missouri. Then, when a frigid air mass descended on the Midwest, the temperature dropped very quickly to the mid 20s, causing death of expanding buds, flowers, leaves and bark on a wide range of woody plants. This was the coldest April 4-9 period in 118 years. This sudden drop from 20 degrees above normal to 20 degrees below normal during this period in spring was unprecedented.
Ice storms: Missouri is listed by the National Bureau of Standards among states with a high probability of experiencing storm events leading to ice accumulation up to ½” in thickness. However, according to weather statistics, ice accumulation of more than 1” is likely to occur only once every 50 years. The ice storm that occurred in Southwest Missouri on Jan. 12-14, 2007, deposited nearly two inches of ice in some parts of Greene, Polk, Dallas and Laclede counties in Missouri. A storm in mid-December 2007 deposited up to one inch of ice in Central and Northwest Missouri. In addition to extensive power outages, these storms caused massive splitting, bending and breakage of trees over wide areas of the state.
Types of Damage: Many types of damage were observed after the various storms of 2007. With the tremendous weight of ice that accumulated during the January and December ice storms, many trees had broken trunks and main branches, leaving large jagged holes on the bark of the remaining stems and branches. Many trees exhibited bending, splitting and uprooting of non-broken trunks. Some freezing injury was evident to the new growth of plants within days of the April freeze. However, many plants showed evidence of severe damage to the cambium and bark leading to dieback, bark splitting and weak growth of buds and shoots that appeared, initially, to have survived the freeze. Some trees and shrubs were killed back to or near the ground, with suckers arising from the soil line or lower trunk. In many cases, stem dieback led to a proliferation of vigorous, competing shoots at the uppermost point where the bark survived the freeze. In some cases, branches that had been pruned back to apparently live wood showed additional dieback from the pruning cut throughout the growing season.
Possible Long Term Effects
The storms of 2007 are likely to have some long lasting effects on trees and shrubs. Branch loss due to dieback and breakage greatly reduced total leaf area on many plants. This stressed plants by reducing the carbohydrates available to roots. It also led to sunscald of exposed bark and, in some cases, to attack by stem borers and other insects. Wounds and dead branches will increase the incidence of decay, leading to more branch loss in the future. Opportunistic disease organisms, such as Phytophthora fungi and crown gall bacterium, will take advantage of lesions to infect susceptible plant tissues. In some cases, damaged trees will be unstable due to imbalanced form and to the weak attachments of epicormic branches that grow from latent buds released by dieback.
Remediation Approaches
The objectives of remediation practices should be to maintain reasonable vigor, to promote the compartmentalization of wounds and to re-establish a stable, well spaced branching structure. In addition to pruning, cultural practices such as irrigation, mulching and moderate fertilization can help plants maintain vigor. Trees and shrubs require a certain level of vigor to be able to compartmentalize (wall off) decay resulting from wounds to limit its spread.
Pruning
When a tree has less than 50% of its branches remaining, the best approach may be to remove it. Otherwise, use good pruning practices. Never leave stubs. Cut damaged branches back to laterals or to the trunk but preserve branch collars by making cuts that angle out from the trunk to the under side of the branch (Figure 1). Use the 3-cut method for larger branches to prevent bark stripping.
If possible, spread the pruning over 2 or 3 seasons. Excessive pruning forces unmanageable, rank growth that is prone to breakage, insects and diseases.
Irrigate damaged trees to supplement rainfall to 1 inch of precipitation per week. Use a rain gauge or calculate inches of irrigation applied (0.62 gallons per square foot = 1 inch). Make certain, however, that drainage is adequate to prevent waterlogging. Don’t forget to water during drought and then start watering in the fall. This may stimulate late growth that may be susceptible to fall freeze events. Try to maintain uniform soil moisture content throughout the summer and fall.
Mulching may be the single most important cultural method to help damaged plants recover. Use a wide, shallow mulch ring, no more than 4 inches deep and never more than an inch thick near the trunk. This conserves soil moisture and reduces competition between roots of turf and those of recovering ornamentals. Never allow broadleaf herbicides to wash into mulched areas.
Base fertilization on soil test results. Avoid excessive application of soluble nitrogen fertilizer. This may force rank growth that will be crowded, drought inefficient and susceptible to insect and disease problems. Use less than 2 pounds of actual nitrogen per 1000 square feet, split over multiple applications, using slow release material if possible. Avoid fertilizing after July to reduce the probability of fall freeze injury. Keep fertilizers containing broadleaf herbicides well away from recovering plants.
Pest Management is important during the recovery period. Storm damaged plants may be affected by insects and diseases that normally do not cause problems. Minimize stress through mulching and irrigation practices. Inspect plants regularly for pest symptoms and signs. Identify the problems accurately and use labeled pest control measures quickly to minimize damage.
Plan for a Storm Tolerant Landscape in the future. Unfortunately, we have learned a lot over the past few years about the factors that make Midwestern trees and shrubs vulnerable to storms. The specie and condition of a tree are major determinants of the probability of ice or freeze damage. Thousands of silver maples and elms with poor structures due to prior topping or neglected pruning when young were destroyed in the ice storms of 2007. Japanese maples proved extremely vulnerable to freezing injury during the April freeze catastrophe. When replacing trees and shrubs, it is important to consult recommendations by tree experts regarding which species have historically exhibited the best tolerance to ice loads. One good reference on this subject is: Trees and Ice Storms: The development of ice storm-resistant urban tree populations by Richard Hauer, Jeffrey Dawson and Les Werner (see references below).
When storm damaged trees are replaced, it is critical that new trees be sited correctly and pruned properly during the first few years after planting. Storm tolerant, large trees must be planted at least 45 feet from overhead utility wires. All trees should be pruned regularly when young to establish a crown with branches that are well distributed vertically and radially and are attached at an angle of at least 45 degrees from vertical. Additionally, occasional light pruning of older trees can reduce ice damage considerably when storms come. We should attempt to remember 2007 so we can do what is possible to avoid unnecessary tree damage and power outages.
It should be noted that many large trees, even in Southwest Missouri, survived the storms and are salvageable. Given that storms of this magnitude are relatively rare, it would be unfortunate if cities and homeowners completely stopped planting large shade trees. If so, Missourians will sorely miss the many economic, environmental and aesthetic benefits provided by urban tree canopies.
Selected References
- First Aid for Storm Damaged Trees: http://extension.missouri.edu/explorepdf/agguides/hort/g06867.pdf
- Pruning and Care of Shade Trees: http://extension.missouri.edu/explorepdf/agguides/hort/g06866.pdf
- Trees and Ice Storms: The development of ice storm-resistant urban tree populations: http://web.extension.uiuc.edu/forestry/publications/pdf/urban_community_forestry/trees_and_ice_storms_2006.pdf
