Winter weather more severe than usual in some areas is causing considerable damage to trees and shrubs. Evergreens are suffering more than deciduous varieties and those planted last fall are being hurt most of all.
Three different kinds of winter injury can occur. One is extreme cold. Each plant variety has its own degree of cold-hardiness which depends on its genetic makeup.
One variety may be able to survive minus 20 degrees when fully hardened (cold-acclimated) while another may be damaged if the temperature goes much below freezing. The best way to prevent damage from extreme cold is to use plants that will not be damaged by the low temperature of the area.
Heavy mulching (three inches) with tree bark, wood chips, sawdust or something similar, and a windbreak to protect the plant from chilling winds will often bring it through in fair condition.
Another kind of winter injury is due to desiccation. If the soil is frozen deep for a period of time and all of the roots are in frozen soil, the roots are unable to supply moisture to replace that lost by respiration of the top of the plant. The greater the duration of the freezing, the greater the injury to the plant.
The three-inch mulch can reduce the depth of freezing. Sheltering the plant from wind and sun can reduce moisture loss by the foilage. Bright winter sunshine and brisk winds serve to accelerate transpiration (loss of moisture) and if it continues for long, death of the plant is almost certain. Evergreens suffer most but deciduous trees and shrubs also are vulnerable.
The other kind of winter injury is caused by failure of the plant to harden in time for winter weather, or by the plant breaking dormancy in mid-to-late winter when mild weather occurs and is followed by severe cold.
Bark splitting on young, newly planted shade trees can be prevented by wrapping the trunks with treated paper or burlap, according to Dr. Frances R. Gouin, University of Maryland specialist. Or, paint the stems and lower branches with the cheapest white latex paint you can buy, he says. The white paint reflects the light and minimizes rapid temperature fluctuations known to be responsible for much of the bark splitting that occurs in the East.
Anti-desiccants, such as Vapor-Gard and Wilt-Pruf, may be of some value in protecting evergreens from excessive drying, however, they offer little or no protection against freeze injury.
Trees and shrubs growing in containers outdoors are more suceptible to winter injury and the increased probability of desiccation caused by prolonged periods when root zones are frozen.
The containers and consequently the root sytems are above ground, surrounded by the cold, circulating air rather than the relatively warm, insulating environment of the soil. Roots of these containerized plants can be exposed to temperatures of 5 degrees F. at the same time the roots of field-grown plants three inches below the surface are only about 21 degrees F.
The roots are significantly less hardy than the aerial portions of the plant. For example, Laland pyracantha stems and leaves can survive -15 when fully acclimated but mature roots are killed at +2 and young roots cannot survive +22.
Mature roots of dogwood will be killed by +10 and young roots by +22; Hino Crimson azalea mature roots by +10 and young roots by +19; American holly mature roots by +8 and young roots by +23.
New growth stimulated in early fall by late-summer fertilization may not have had time to harden off sufficiently to survive sudden drops to below-freezing temperatures and may be killed by ice crystals rupturing the cell wall. Fall fertilization, after the plants are dormant, but before the soil temperatures drop below 45 degrees F., is preferable.
In the spring an application of fertilizer accompanied by adequate watering will sometimes induce new growth to compensate for winter injuries. But be careful not to over-fertilize since roots may also be suffering from winter injury.