Tree and shrub winter injury

By the month of May we expect to see our trees and shrubs fully leafing out and beginning to grow.  But this year there have been many reports of various problems with trees and shrubs that have perplexed homeowners across the area.

The symptoms

Many have noticed problems with trees such as the tips of branches failing to leaf out or large parts of tree canopies not leafing out at all.  Other trees have been very slow to begin to leaf out and there have been cases of bark splitting and falling away from the trunk on certain trees.

The tips of this Sawtooth oak died back due to winter injury
Suspected winter injury to the top of this Austrian Pine
Boxwood winter injury

Problems have also been noticed on shrubs.  Roses and other semi-woody shrubs have been killed back much harder than normal and many evergreens are showing signs of brown tips or dead areas within the plants.

The cause

The cause of a majority of these problems is winter injury due to two cold events that occurred between November 2020 and February 2021.

In between November 10th through 13th 2020 we experienced a sharp and fast drop in temperatures that led to a hard freeze early in the fall.  These types of freeze events can often catch tree or shrubs at a vulnerable point where they are not fully hardened off for winter and have cells that are still very active.  These cells can be damaged by the sudden drop in temperature. 

The record cold snap in February of 2021 was even more extreme and although it came when plants were dormant, it was an extreme and fast shift in temperature that can have the same effect on any plant cells that were not fully dormant due to the relatively normal and even more mild temperatures we had experienced before the cold event occurred.

How damage occurs

The sharp drop in temperature can damage at least a portion the phloem and the cambium which is the living tissue of trees and shrubs that is just underneath the bark. The phloem carries food made in the leaves to all parts of the plants including the roots. The cambium produces new phloem. If the phloem and cambium are killed, the cambium cannot produce new, living phloem, and, therefore, the roots don’t receive the food needed to survive. Trees so affected may not die immediately because a healthy root system has stored energy reserves that it can use to keep the tree alive. When those reserves are depleted, the tree will decline very quickly. Usually this occurs during the summer following the year the damage occurred.

What to do

If a tree is showing signs of being slow to leaf out or not leafing out, it is best to have patience.  Woody plants have latent buds that can sprout and grow from healthy cambium tissue.  These secondary buds can begin growing but there will be a delay.  If no growth occurs by the beginning of June, then any dead areas should be pruned out and removed if possible.   If a tree has significant dieback in the canopy then it is time to make a decision about the tree’s health and future.  In some cases pruning to remove the dead areas may be an option, but if too much damage has occurred, removal and replacement may become necessary.