Winter tree safety


Written by Debbie Nelson, Master Gardener, U of M Extension Service, Hennepin County



Normally this time of year I like to think of my garden as all snug and cozy under a layer of snow, but this year it appears that we may have a snow deficit. For those of us who have to shovel it or drive through it the lack of snow has probably been welcome. But for those who like to play in it or for gardeners it may not be such a good thing. Cold damage to our plants usually occurs when there is little or no snow cover for insulation and /or when temperatures repeatedly fluctuate.

Snow is a natural insulator. Without it the ground freezes deeper, and perennials deciduous, trees and shrubs can experience root damage from the cold. This root damage can be manifest by a late leafing out or no leafing at all. Because the symptoms do not appear until long after the damage is done, we seldom associate them with winter damage and the lack of snow cover. Also when the winter temperatures fluctuate repeatedly without snow cover, the ground can go through thaw /freeze cycles in the upper layers of soil. These cycles can cause the crowns of perennials to heave out of the soil exposing them to colder temperatures that might kill the crown. This is the primary reason for mulching your perennials in the fall. The good news is that it is not too late to place a layer of mulch over your plants. If you have a bag of leaves that didn’t make it into the yard waste pick up, hay or straw, or even some evergreen boughs, you can layer them over your perennial beds to protect them from the freezing (and thawing) temperatures we may well get in January or February.

Unlike perennials and deciduous trees and shrubs, evergreens do not always remain dormant all winter. On bright sunny days, photosynthesis and respiration take place in evergreens. Photosynthesis is the process by which plants produce food (carbohydrates) and respiration is the process by which the food is converted to cell development. Respiration requires water and if the ground is frozen too deeply the water in the plant used for respiration cannot be replaced. Even though we had adequate moisture in the fall, if the ground is frozen little of it will be available for uptake during the winter months resulting in desiccation (drying out) of needles. The result can be tip die-back or red brown needles all around the tree. Sometimes this does not show up until May or even June so it is not always associated with winter injury, but confused with disease.

Red brown needles on conifers can also be the result of tissue damage. On bright sunny days the needles are warmed up. The rapidly falling temperature after sundown causes the moisture in the needles to suddenly freeze resulting in tissue death. Usually this type of foliage damage occurs on the south or west side of the tree or shrub where the late afternoon sun reaches them. You can minimize this type of winter damage by not planting your conifers in susceptible locations, but since most of us are dealing with mature trees that were planted long ago our best option is to provide them with some kind of protection on the south and west sides. This can be done by putting some type of screening on those sides of the tree or shrub, possibly a barrier of burlap or simply pine boughs propped against the tree. Should your conifers suffer from browning of needles wait to trim the affected branches until after bud break in the spring. Although many needles may be lost, usually the buds will leaf out in spring and you want to make sure you do not trim off any viable buds.

Bright sunny days in winter can also cause winter injury in deciduous trees. The bark of the trees is heated up which stimulates new cell growth in the tissue layer just beneath the bark. When the sun sets or is covered by clouds the temperature drops suddenly killing the activated tissue. Symptoms of “sun scald “are long cracked areas of bark usually on the south or southwest side of the trunk. While sun scald cracks will not necessarily kill the tree, cracks will allow insects and disease pathogens to enter the tree and do additional damage resulting in a decline in the tree’s health. Trees most susceptible to sunscald are those that are young, newly planted or thin barked, such as apple or maple. To protect these trees, wrap the bark either with a commercial tree wrap, some light colored material or plastic tubing that will reflect the sun and prevent the bark from heating up. However be sure to remove the wrap early in the spring. Moisture that collects beneath the wrap may provide an environment for disease when the weather warms up.

Lack of snow in the winter is also the equivalent of a drought. So if our winter continues with below average snowfall, to minimize damage be sure you start watering your trees and shrubs as soon as the ground thaws in the spring.