Evergreens are just that, green year-round. These include boxwoods, holly, arborvitae, pine, spruce, fir, etc.. This also means that they transpire all year, including during the winter. Basically, they take water in and sweat it out. No big deal unless the winter is severe enough so that the evergreens are unable to take in water but are still transpiring. Another term for this is winter desiccation. Winter desiccation occurs when the amount of water lost by a plant through its’ leaves and stems exceeds the amount available to the roots. Warm, sunny winter days increase the amount of water loss. So does wind moving across leaf surface which pulls moisture out of the leaves. If the soil is frozen or is low in moisture due to dry fall conditions, roots are unable to pick up enough water to meet the plant’s need.
The last few years, we have had many extremes, evergreen trees and shrubs have had to withstand many difficult conditions, and the problems have compounded. Summer drought conditions, above and below average winter air temperatures with solidly frozen soil. These extremes have led to much higher-than-normal rates of winter injury and undue stress on evergreens. These may include excessive needle drop, brown needles in leaved evergreens, and yellowing and dieback in broadleaf evergreens.
Many of us do not realize that Fall and early winter watering, until the soil freezes, certainly helps prevent winter desiccation, but use of an anti-transpirant product can also help protect plants. Anti-transpirants are basically a liquid wax that hardens after 4-6 hours. It slows the rate of water loss but does not interfere with normal plant respiration or photosynthesis. The coating gradually weathers and degrades so a second application, if weather allows, is recommended later in the season. Applications can be done as weather permits, temps above freezing are appropriate for application. Protect your evergreens and avoid winter desiccation.
Great advice to help combat winter desiccation. I see many boxwoods with this yellowing need discoloration. I will recommend the anti-transparent product to my customers, thanks for the info!
Yes, boxwoods can be very susceptible to desiccation due to cold harsh winds, as well as salt damage (as they are commonly planted along walkways). There are also other pathogens and pests out there that can impact boxwoods and cause discoloration. Anti-transpirant is a good protection either way from cold harsh winds, although we typically utilize it more often on evergreens such as spruces and pines.