Beware the Siberian Express

The unusually warm temperatures we experienced in January 2006 were a pleasant contrast to the cold snap in December. In mid-Missouri, it was the second warmest January on record. Daffodils are coming up, robins are singing, witch hazels are blooming and it is downright pleasant to walk about comfortably with only a light jacket. However, the balmy conditions are causing some concern among gardening enthusiasts regarding the potential for winter injury to landscape plants that may result from a sudden temperature drop.

Woody plants in the temperate zone survive freezing temperatures by going dormant and attaining the cold hardiness associated with the dormant condition. Shortening days and cooling temperatures in fall start dormancy development. Under ideal conditions, gradually dropping temperatures cause trees and shrubs to undergo physiological processes (called acclimation) leading to cold tolerance. Temperatures slightly above freezing are generally most effective for promoting dormancy and hardiness. In most years, landscape plants in our region reach maximum cold hardiness by early December. Plants hardy in USDA zone 5 should then be able to survive temperatures down to -15 or -20 degrees Fahrenheit. In contrast, non-hardy or nonacclimated plants will suffer freezing injury resulting from ice crystal formation within their cells if exposed to the same temperatures. This can lead to death of buds, shoot dieback, bark damage or, in extreme cases, trunk cracking or girdling.

Once dormant, temperate zone woody plants require a certain number of hours of chilling (below 40 degrees Fahrenheit) before buds will begin to grow, even at warm temperatures. This generally prevents them from actively growing during a warm spell at a time of the year when there is a high probability that the new growth will be killed by freezing. However, warm periods in winter commonly cause woody plants to become less cold tolerant (de-acclimate) even though growth is not apparent. Although plants can re-acclimate when exposed to more average winter temperatures, while de-acclimated, they may be somewhat vulnerable to freezing injury during a sudden cold snap in winter or early spring. Fortunately, warm temperatures in mid winter cause less de-acclimation than they do in late winter after chilling requirements have been satisfied. Therefore, plants are less vulnerable to freezing injury after a warm spell in January than they would be a month later.

A look at the weather records reveals that the probability of cold weather in February and March is fairly high. In 1905, for example, the minimum temperature on February 13 was -25 and on March 12, 1948, the minimum was -4. Perhaps even more frightening is the fact that, in 1933 (the warmest January on record in Columbia), the minimum temperature dropped from +24 to -10 during the second week of February. If a similar drop occurred in February or March of 2006, it is likely that there would be damage to some species of woody plants in nurseries and landscapes. Given the extremely cold current conditions in Russia and Alaska (-45), a sudden change in the jet stream could very well create such a temperature pattern in Missouri as the Siberian Express brings us an icy blast.

Unfortunately, there is not much that can be done to protect our de-acclimated woody friends from damage during a sudden cold snap. With luck, temperatures will gradually settle back toward normal, allowing landscape plants to re-acclimate before the jet stream takes a sudden southern plunge. With even more luck, normal temperatures will persist through March and we will have a spectacular spring. Regardless of what happens, it is likely that most landscape plants, especially native species, will avoid serious damage. At any rate, the winter of 2006 has given gardeners an interesting topic of conversation.

Chris Starbuck
MU Division of Plant Sciences
573-882-9630