A simple dynamic is responsible for this winterkill damage. Leaves are always transpiring, even in winter, and as they transpire they give off water. When it's very cold roots lose some, even most, of their ability to absorb water. The result is leaves end up with a water deficit, and they wilt and turn brown. Wind exacerbates the problem.
Snow, which we’ve had plenty of this year, offers considerable although inconsistent protection against winterkill. The snow helps in three ways.
The ambient temperature of the earth in southern New England and New York is somewhere just above 50 degrees F. In winters where there is no snow on the ground and air temps are below freezing, the ground freezes and this inhibits water uptake by roots and contributes to water deficits in leaves. If there is considerable snowfall, like this year, the snow insulates the soil from the cold air and, because of the warmer ambient soil temperatures below, soil around roots will thaw out and more water will be available to tree and shrub roots. In turn, water deficits in leaves will diminish or cease, and leaves will be more likely to remain lush and green.
Second, if the snow is high enough to enshroud leaves, leaves will be in a high moisture environment as they are surrounded by frozen water, at least some of which is volatilizing and becoming water vapor. It’s like a little snow cave. Any mountaineer who has spent a night in a snow cave will tell you they are relatively warm humid places.
The last way snow helps leaves is by protecting leaves from wind. Wind in summer or winter has a tremendous drying effect. It accelerates transpiration and dessication. When snow melts after a very snowy winter like the one we’re having, it’s common to see a demarcation line in the spring right where the snowline was on a rhododendron, holly or other broadleaf evergreen. Below the snow line, where leaves were protected from drying winter winds, leaves will be green and lustrous. Above the snow line, where the leaves were still exposed to drying winter winds, leaves will be wilted and brown.
As great as snow is for trees and shrubs in, it isn’t reliable protection against winterkill. It doesn’t get high enough to protect the upper parts of many susceptible broadleaf evergreens, and in many years it just doesn’t snow much. A worst-case scenario for winterkill is to have a very cold windy winter with little or no snow.
As Mother Nature is unpredictable, it’s wise to take steps each year before the onset of winter. There are two things that can be done to protect evergreen trees and shrubs in anticipation of damaging drying winter weather.
Trees and shrubs can be wrapped in burlap. This is unsightly to many people, and can also be very expensive. If interested in this means of protection, we can refer you to reliable landscape maintenance contractors who provide this service each year.
A more cost effective and also nearly invisible alternative is to apply a winter anti-transpirant to broadleaf evergreen trees and shrubs. These materials slow down transpiration rates thereby greatly reducing water deficits in leaves, winter drying, and winterkill.
For more information on how your trees and shrubs can be protected, and also what can be done to help damaged plants recover from winterkill damage, feel free to contact us and speak with a Green Cross ISA Certified Arborist.