Historical Catastrophic Fall Freeze Events

Adjusting to cooler temperatures in the fall can be a real challenge for Missourians of the species Homo sapiens. However, since they can’t put on a sweater or move next to the fireplace by themselves, plants can have even more trouble with the transition. This is particularly true since our weather sometimes changes from Alabama- like to Alberta-like within a day or two at this time of year. Although most woody landscape plants can easily withstand sub-zero (F) conditions if the temperature drops slowly over a period of weeks, the same plants may be injured by temperatures only slightly below freezing if the change is very abrupt. Unfortunately, we can find a number of examples of drastic temperature drops in our weather records with accompanying descriptions of plant devastation to illustrate this point.

November 11 is a memorable day for two reasons: First, it commemorates the signing of the Armistice that ended WWI on November 11, 1918. Additionally, Armistice Day (now referred to as Veterans Day) is the date when catastrophic freeze events hit the Midwest both in 1911 (11/11/11, before the first Armistice Day) and in 1940. During the 1911 event, 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.

Most readers will have a few painful memories of a more recent freeze event, usually referred to as the "Halloween" freeze of 1991. While this episode was not as serious as the Armistice day freezes, it did significant damage to a wide range of landscape and nursery plants. As with the 1940 freeze, the severity of the damage from the Halloween freeze was increased by the weather conditions over the preceding several months. The temperatures during the summer of 1991 were above normal, and there was a significant drought during August. Then, as fall rains began, and warm temperatures persisted, plants began to grow again and thus failed to develop dormancy and cold tolerance as quickly as they normally would have. Leaves were still green when Halloween came. The temperature began to drop from about 55 F on October 31 to around freezing on the morning of November 1 and then, to about -5 F over the following week. The spring of 1992 brought hundreds of calls to the Extension Center in which concerned homeowners described severe dieback, trunk cracks and peeling bark on plants in their landscapes. Nursery growers reported severe injury to the bark of many trees in the field, especially those in the genus Prunus. Container grown plants that were not protected at the time of the freeze had significant freezing damage to their roots, often leading to collapse when growth began the following spring.

The take-home message from this article is that freezing injury to landscape plants in Missouri is not due to the fact that it gets very cold here; rather, it is because temperatures sometimes go from very warm to very cold very quickly. Given time to acclimate, woody plants can tolerate amazing extremes in temperature. However, since they live in a location where Gulf influence can give way to an arctic air mass at any time, they are often damaged before they can develop dormancy and cold tolerance. We have had two 500-year floods in the past 10 years. Hopefully the three episodes described above constitute our 500-year freezes.

Thanks to Pat Guinan, MU Department of Atmospheric Sciences, for information on historical freeze events.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630