Steps to prevent winter damage
All trees and shrubs are susceptible to damage in winter, but, generally, evergreen trees and shrubs are injured more frequently than deciduous plants. Of the evergreens, broadleaf evergreens, such as yew, boxwood, azalea, holly, arborvitae and Oregon holly grape, are most likely to be damaged. Needlelike evergreens, such as muhgo pine, concolor fir, juniper and Douglas fir, are affected, but not as often as the broadleaved species.
There are many reasons why damage occurs during winter, but for the purposes of this article, we’ll focus on three. First, in a most basic sense, winter winds drive moisture away from the leaves. Surrounding the leaf openings (stomates) are zones of water vapor in the process of leaving the leaf (transpiring). Normally, high, medium and low zones are recognized, with the high level of transpiration existing closest to the stomata.
Secondly, adding insult to the water loss is the occurrence (in severe winters) of the soil freezing deeply around the roots of the plants. When this takes place, the roots have reduced capacity to replace the moisture given off by the leaves.
Finally, periods of fluctuating temperatures in winter can “de-harden” evergreen plants to the point where they are susceptible to damage. In the transition zones of the U.S., daytime temperatures might be in the low 20s, but then a week or so of 60-degree weather occurs. Of course, even with 50 or 60 degrees during the day, in most cases, freezing conditions occur during the nighttime hours. Several periods of cold and warm temperature fluctuations can cause serious damage.
As mentioned above, wind is a leading factor in causing damage. However, in many cases, the plants on neighboring properties are not affected equally. Most likely this is due to differences in microclimate, the immediate surroundings of a particular plant or group of plants. It can be quite different from the front to backyard of a property, or even a spot 15 feet away.
Many factors affect microclimate, including sun exposure, overhead canopy, soil type (with either extreme water holding capacity or excessive drainage) and wind patterns. These influences can create conditions that kill roots in winter. Seemingly small windbreaks of either plants or hardscaping can make a big difference in this regard. When assessing or analyzing the specific conditions of your client’s properties, take notes from other nearby plantings.
Installing physical wind barriers, such as snow fencing, burlap and rows of other plants, helps prevent damage by slowing down wind speed. To maximize their effect, place them between the susceptible plants and the prevailing winds. Screens significantly reduce wind up to a distance of three times their height. A 5-foot fence or dense group of plants will protect broadleaf evergreens for up to 15 feet away.
- Wrap in burlap
Wrapping plants in burlap is another method of placing a physical barrier between the leaves of vulnerable plants and the wind. This is especially important for newly planted specimens, or when plant supplies are scarce.
Applications of leaf-coating products, such as Wilt Pruf, Folio-Cote, Envy and Dwax, can reduce damage from excessive moisture loss. To be effective, these products need to be applied early and often to “at risk” plant materials. On average, antidessicants are effective for four to six weeks. Use date markers that are easy to remember, such as the winter holidays of Thanksgiving, Christmas and Valentine’s Day to help time the applications. Because these products are waxy and gluelike, they can clog a sprayer if not thoroughly rinsed after using. A triple rinsing with hot soapy water will suffice. Be sure to read and follow all label directions explicitly.
- Thorough watering
Before the onset of dormancy, be sure to thoroughly water susceptible plants to make sure that they are fully hydrated and no air spaces are present in the soil. A severe winter can cause enough damage on it’s own, let alone a plant that only has half a tank of water starting out.
- Avoid certain evergreens
If a particular plant has a bad reputation for winter survival in your area, it’s probably well deserved. This is true of broadleaf and needlelike species alike. Compare notes with extension specialists and other grounds managers at landscape or turf conferences conducted by the state university in your area. Visit botanical gardens and arboreta in your area to check out how each plant looks in late winter, the worst time of year for most evergreens.
If you use one of these prevention methods, and have a mild winter, chances are good that minimal or no damage will occur. However, because there is no way to predict how severe a given winter will be, using two or three methods is a prudent course of action, particularly on your most valuable accounts.
Don’t beat yourself up if the methods you select don’t work. Prevention of desiccation and ice damage is not an exact science. Many factors are involved, including soils, winter wind patterns, snow load, exposure, species tolerance and microclimate. In almost all cases, some of these factors are beyond your control.
When plants are installed in rows, and one or two are damaged, the visual effect is dramatic. Conversely, if masses of plants are installed, and one or two are damaged, they can be removed without causing a hole.
If severe injury occurs to the evergreen shrubs and trees on the properties you manage, all is not lost. Chances are, if your customers’ plants were damaged, the plants of your competitors/ colleagues were as well.
The author is a horticulturist and certified arborist located in Omaha, Neb.