Are the Woodies Winterized?

Every November, I get a little nervous, wondering whether my woody landscape friends are far enough into dormancy to avoid serious damage if a precipitous temperature drop occurs. As noted in the November, 2002 issue of MEG, there are several memorable, historic examples of what can happen to trees and shrubs when the temperature drops by 60 or 70 degrees F in less than 24 hours (see http://ppp.missouri.edu/newsletters/meg/archives/v8n11/meg3.htm). Temperate zone woody plants are incredibly tolerant of low temperature if the change occurs gradually, allowing them to attain dormancy and the cold tolerance associated with it. During an ideal fall, the shortening days of September initiate the slow process of dormancy development. Gradually decreasing temperatures with a few light frosts in October continue the process, until, by mid-December, most woody plants have attained significant cold hardiness. Unfortunately, however, we rarely have an ideal fall, and landscape plants are often vulnerable to the effects of a sudden plunge of the mercury in November. Freezing of non-hardy plant tissue causes formation of ice crystals inside the cells, leading to instant death from rupture of cellular membranes. In a hardy plant, tissues can survive sub-freezing temperatures because ice forms outside the membranes, in the cell walls or water in the cells can remain unfrozen due to “supercooling.”

In many cases a tree growing in Minnesota is less vulnerable to freezing injury than the same tree growing in Missouri. While we may be experiencing warm southerly breezes in a Missouri October, the temperature is steadily dropping in Minnesota, promoting dormancy development. Thus, when the first wintry blast comes in from the north, the Minnesota tree is better prepared to handle it. Another factor that often interferes with dormancy development in Missouri is summer drought combined with high temperatures. This year, for example, we had a total of 0.56 inches of precipitation in central Missouri from July 10 through August 20. During that period, we also experienced a week of triple digit temperatures. Many trees and shrubs suffered serious drought stress, dropping leaves and coloring early. Then, during September and October, we had regular rainfall and relatively cool temperatures. Some trees and shrubs came out of their drought-imposed shallow dormancy and began to grow again. I observed several, particularly confused ornamental pear trees in full bloom in mid-October. Although my petunias were still going strong in the last week of October, temperatures had been cool enough to promote dormancy of most woody plants and, with a few frosts, they should be ready for serious winter by December 1. Recall, however, that the summer and fall of 1991 followed a similar pattern, leading to the devastating “Halloween Freeze.” Following the summer drought of 1991, the fall was warm and rainy, and leaves of many trees froze bright green on the trees on Halloween night. In contrast, the fall of 2003 was cooler and slightly drier. By Halloween time this year, most trees were dropping their leaves and so were much less vulnerable to damage from a sudden temperature drop. While we are not out of the woods yet, my level of nervousness is dropping steadily.

There are horticultural advantages and disadvantages to living anywhere in the country. In Oregon, Camellias grow like weeds, but you can’t grow a good tomato. In Missouri, tomatoes grow like weeds, but you need a greenhouse to grow Camellias. One advantage that we have over Oregon is that explaining the responses of landscape plants to our erratic weather keeps us mentally active.

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