Effect of Winter Storms on Spring Foliage
May 3, 2013
It's "springtime" on the high prairie, and many neighborhood trees are suffering instead of flourishing. Here in Denver, we don't have an orderly progression of seasons. Instead, we have summer days and winter days, either of which can occur during almost any time of the year. Sometimes there is more of one kind than the other. In January there are more cold days, but a few hot ones—and in July there are more hot days, with occasional cold snaps. We all know how early it snows in the fall and the late snows in the spring can break very large branches, or tear off green leaves, under the weight of ice or wet snow.
But it is the wide fluctuation of daily temperatures when the trees are dormant that is a more serious problem. In particular, the spring blooming shrubs and trees are vulnerable to damage if the buds are already swollen and starting to open: many flower petals can't handle a sudden freeze, and some of our introduced species can't even handle a minor freeze, once their buds have opened. This year many flowers and leaves were flushing out when three winter storms, spaced one week apart, devastated their display. Magnolia, quince, forsythia, and some of our fruit trees were in full bloom when their flowers were frozen. Most will recover to have normal foliage for the warm season.
But the more insidious long-term damage comes from the very warm days, and even weeks, that can occur in the dormant season. The sap starts to rise in the trunk and stems, and if the soil is dry or frozen, the roots can't supply enough water to the crowns, which then are stressed and may die back. On young woody plants, these warm sunny days in winter can also mean sun-scalding of the bark, and this may kill sections of cambium, causing long-term damage and decline of the tree, as the damaged bark peels, decays, or both.
Our Grounds staff is preventing this kind of damage every year by wrapping the stems of young and vulnerable trees in the fall, before the winter days take over. Once the trees begin to mature, their bark is usually thick enough to withstand the crazy changes in temperature, which can be as much as 60 degrees in the course of a single day. So happy spring, and good gardening!
Director of the Chester M Alter Arboretum
Kurtz Professor of Botany, University of Denver