Winter injury on trees

Posted 3/28/01

Winter injury on trees

By Jerry Vitalis
Chisago County Master Gardener

Now that winter is on its way out, it is time to look at the damage it left behind on the trees and shrubs. I read an article by Janna Becherman, Extension Plant Pathologist. Parts of this article are from that reading.

Symptoms of winter damage are already appearing on conifer trees. Red-brown foliage has started to appear on Colorado blue spruce and Red Pines around the area. This injury is usually due to freezing, following a spell of warmer weather. Often, winter injury begins where the snow line ends. This results in the upper half of the tree having reddish-brown needles, while the lower half (which is protected by the snow) remains green. The snow cover also prevents winter injury of young conifers by providing shelter from drying winds and glare from the sun. The browning of evergreens, such as arborvitae and yew, may not show up until May or June.
Regardless of the type of evergreen you are dealing with, wait to prune affected branches until bud break occurs. Although many needles may be lost, the buds are usually protected and will leaf out come spring. By waiting for bud break, you can better determine the damaged area. Prune out any infected portions of the plant, removing only dead branches.

Winter injury on hardwoods is more difficult to diagnose. The root and stem injury doesn't show up until warm weather is here. The symptoms could include early leafing out or no leafing at all. This is followed by shoot death (the shoots wilt and die) when warm weather comes. Fortunately, these types of injuries would not be expected this year, due to the heavy snow cover.

Another type of winter injury that primarily affects hardwoods is sunscald. Sunscald and bark cracks occur primarily on the south and southwest sides of smooth-barked trees and shrubs. Maples and other thin barked trees seem to be particularly vulnerable, followed by apple and other fruit trees, linden and mountain ash. This injury is caused by the south side of the trees warming up while shaded portions remain at freezing temperatures. The heating results in the tree losing its dormancy, which is followed by freezing when the sun sets. Sunscald, coupled with drought, can result in a vertical frost crack and eventually, death of the cambium. The frost cracks also provides for an open door for insects, diseases and/or decay.

There is not much you can do this spring about sunscald or frost cracks. However, you can remind yourself to wrap your young trees next fall. If you did wrap your trees last fall; be sure to remove the wrap as soon as it warms up this spring.

If you are going to plant new trees or shrubs this spring, do some research first. Be sure itís for our zone and stay away from varieties that are prone to having continuous winter injury problems. Also, by knowing how and where to plant may eliminate the problem.