Although most of us remember the date November 11 because it is Veterans Day, horticulturists may also remember it as a calendar date when two different freeze events caused massive damage to plants in the Midwest. Many of us begin to worry at this time of year, wondering if we have done all that we can to encourage our landscape plants to develop dormancy, with its associated cold tolerance. If not, our woody friends may be vulnerable to a rapid November temperature plunge.
On November 11, 1911, an arctic cold front moved through Missouri so quickly that the temperature in Columbia dropped from a record high (at the time) of 82° F to a record low of 13° that same day. Most of the state went from the low 80’s at 2 pm to below freezing at 5 pm, with many locations reporting a 30-degree drop within 20 minutes. Associated with the temperature drop were rain, sleet and snow, and there are reports of hunters freezing to death before they could get to shelter. While I have not found good records of the effects of this freeze event on woody plants in Missouri, there is no doubt that they were devastating.
The Armistice Day blizzard of 1940 is often mentioned during discussions of disastrous weather events in the Midwest. The fall of that year was mild and rainy with very little cool weather to promote the development of dormancy and cold hardiness in woody plants. On November 11, the temperature dropped from near 60 degrees F over most of Missouri to about 10 degrees within about a 3-hour period and then continued to drop to below zero by the morning of the 13th in some parts of the state. This combination of the theretofore warm fall and the precipitous temperature drop caused massive damage to woody plants all over the Midwest, destroying most of the fruit trees in Missouri in a single night. Dr. Dan Millikan, MU Professor Emeritus of Plant Pathology (now deceased), who was a student at the time, told of standing in an orchard and hearing the trunks of the trees crack as they froze.
While there is not much that Missourians can do to influence the weather, there are certain horticultural practices that we should avoid that could make our landscape plants more vulnerable to an early, rapid freeze event. Avoid applying soluble nitrogen fertilizer to woody plants in late summer and early fall. This could easily prolong vegetative growth and interfere with development of cold tolerance. Avoid fall pruning in which more than 5% of a plants branches are removed. This can have a localized invigorating effect which could delay dormancy. Even if no bud growth is observed in response to the pruning, cambial activity may be stimulated, leading to freezing injury near the pruning cut. Major pruning should be done during the dormant season. If you plan to add mulch to trees and shrubs to replace that which has thinned by microbial decomposition, consider waiting until early December. This will allow the soil around the plants to cool, promoting dormancy development. Make mulch bagels, not volcanoes. Mulch can be several inches thick a foot or two away from the trunk but never more than one inch thick next to the trunk.
Counter intuitive as it may seem, evergreens may be vulnerable to desiccation injury this winter. When the soil stays wet for long periods, some deep roots may suffocate, resulting in a shallower than normal root system. Then, if we have long periods of low temperature with windy conditions, evergreens may dry out more quickly than in a more normal year. Make certain to mulch your evergreens well after the surface of the soil starts to freeze. Also, consider dragging out the hose to water the evergreens after wintertime periods of low temperature with no precipitation. The cold winter winds are continually removing water form leaves of evergreens and they need to be able to rehydrate whenever the soil and stems are not frozen.
REVISED: August 1, 2012