With two major snowstorms, including near-blizzard conditions, in the past two weeks, you may be wondering how your landscape plants are faring in the snow and cold.
Contrary to popular perception, most woody or perennial plants that are killed in the winter die not from excessive cold or snow, but from desiccation or from extreme temperature fluctuations. The worst enemies of plants in the winter are drying winds, un-insulated soil, and freeze-thaw cycles. Winter-killed plants are usually freeze-dried plants.
In fact, the snow may be your plants’ best friend right now. Snow is an excellent insulator, and the snow covering the ground is protecting the soil, plant roots and crowns, and buries plants from exposure to much colder air temperatures and desiccating winds. There’s a reason it’s called a “blanket” of snow. In more northern climates, where snow cover is a given for most of the winter, snow is also called “poor man’s mulch” because it provides the same insulating qualities as a thick layer of mulch.
Of course, a heavy snow load on the branches of trees can be a problem, causing damage from breakage. Particularly on evergreens, if you are able to get to them, you can use a broom to gently sweep upward, dislodging any heavy snow that has accumulated on branches. This week the wind may have done the job for you.
Multi-stemmed upright evergreens such as arborvitae and junipers can be protected from limb breakage by loosely wrapping or tying the branches together – but that should be done before winter sets in and removed once spring arrives. If branches are coated with ice, leave them alone until warmer weather melts the encasing ice.
Hardy perennials, trees and shrubs actually start preparing for winter in mid-summer. Once day length begins to shorten (and nights get longer), plants begin to slow their growth and transfer food reserves from the leaves to the roots.
As temperatures cool in autumn, plants begin to withdraw water content from leaf and stem cells. This increases the concentration of sugars and other chemicals within the plant cells, forming a kind of “antifreeze” that helps to prevent the cells from freezing and rupturing. Within the leaf, this “antifreeze” lowers the freezing temperature to 28° F or lower. That’s why a “killing” frost is considered to be 28° F or lower and not 32° F.
This also explains why hardy annual vegetables, such as kale and Brussels sprouts, taste sweeter after freezing temperatures arrive; there are more sugars stored in the leaf cells. The bright foliage colors that we enjoy in fall are also due in part to the increased sugar levels within the leaves.
As winter approaches and temperatures continue to drop, deciduous plants drop their leaves, thus solving the problem of frozen leaf cells by getting rid of them before winter, and enter full winter dormancy. Winter acclimation is a gradual process, though, so even the hardiest of plants can suffer injury or death if temperatures drop suddenly or fluctuate wildly.
The foliage of some evergreens, such as arborvitae, cryptomeria, juniper and boxwood, often takes on a burgundy-bronze cast in winter. This is not winter damage, however; it is due to higher levels within the leaf cells of sugars called anthocyanins, red pigments that contribute to frost protection.
Unlike deciduous plants, evergreens retain their foliage in the winter, and so are particularly susceptible to winter desiccation. Even in winter, evergreen leaves continue to transpire, or lose, some moisture, and if the soil is frozen, the roots cannot replace that water.
The result is winter burn, in which leaf tips or edges, and sometimes entire leaves, die, turning brown and dry. This damage is exacerbated by cold winds and winter sun, which increase the rate of water loss from the leaf. Often, you don’t see this damage until spring, when growth resumes and dead areas become evident.
To endure winter conditions, conifers have needle- or scale-like leaves that are tough, with a waxy cuticle; the stomata, or pores, through which water is lost, are usually on the underside of the leaf and thus less exposed to wind and sun. Broadleaf evergreens, such as rhododendrons, often roll their leaves inward in winter; this is their mechanism to conserve moisture during winter’s cold temperatures.
You might read or hear recommendations to apply anti-desiccants, also called anti-transpirants, to evergreen foliage during winter to protect it from desiccation. The suggested guideline is to make at least two applications per season, one in December and one in February.
However, research does not support this recommendation. In the home landscape, anti-desiccants do not last long enough to provide protection, and in some situations interfere with normal plant processes. It is better to select hardy plants or to create a windbreak for vulnerable evergreens using burlap wrap or frames filled with leaves or straw.
Annette MaCoy is the consumer horticulture extension educator and master gardener coordinator for Penn State Cooperative Extension in Cumberland County. If you have garden questions, contact Annette at 240-6500 or by e-mail at firstname.lastname@example.org.