Frozen 2: The tree edition

Posted by: Jane Rozum, Douglas County Horticulture Agent

It’s a bad omen for plants when the weather changes from the relatively mild temperatures we received in October and early November into the flash freeze the Front Range of Colorado experienced the week of November 10, 2014. Plants that weren’t snow-covered and subjected to the bone-chilling winds and below zero temperatures were the worst-hit.

We’ve heard from Tony Koski on the effects of the November ‘Polar Vortex’ on grasses and weeds; Alison O’Connor reported on the plight of roses subjected to the extreme cold. What about those plants that live more than a few feet above ground? What about the trees?

In a typical year, landscape plants can handle the below zero temperatures we routinely receive every winter.  However, the biggest problem with this year's November chill was timing. Plants acclimate well to slowly- declining fall temperatures as we approach winter. When plants experience a rapid drop in temperature following mild weather which doesn't allow for proper hardening,  significant plant tissue damage may result.

We are already seeing the effects of this cold snap on our evergreen trees and shrubs. Austrian, Bosnian, and Norway pines and other non-native evergreen trees are exhibiting brown tip foliage, especially on the south, east and west sides of the tree. The north sides of trees may exhibit less damage because that side of the tree likely acclimated to a greater extent than the sides more exposed to full sun.  Brown, brittle buds, if present, may indicate severe cold injury and death of the growing point.  

Deciduous trees were affected by the cold-snap as well. Some trees never dropped their leaves because the cold-snap interfered with formation of the abscission layer that allows the leaves to fall from the trees. Retention of leaves by deciduous plants in the fall is called "marcescence"; leaves die and wither, but do not fall from the tree. For some tree species like oaks, this is a normal, genetically built-in occurrence. Many of our maples and ornamental pears exhibited marcescence because of the November freeze. This leaf retention won’t hurt the tree, but with the leaves still present, heavy wet snow may cause excessive snow loading and lead to broken branches. The full effect of the November freeze on deciduous trees, however, may not be exhibited until this spring and beyond.

Calllery Pear with  marcescent leaves



What can one do about potential damage caused by this rapid onset cold weather?  CSU Extension recommends winter watering trees every so often when we get relatively warm days (40-50°F) and there isn’t any snow cover. A mulch ring around trees, especially around newly planted trees, is always a good practice. No fertilizers should be provided during the winter season.   At this point, we’ll have to wait and see what happens this spring with our woody plants.