Even boxwoods get the winter blues, bronzing blues, that is.
Boxwoods (Buxus spp.) are broadleaf evergreen plants that boldly retain their foliage in the winter months, unlike deciduous plants that shed their leaves to avoid stress and damage from winter weather. However, fortune does not always favor the bold, as photosynthesis still occurs in winter for evergreens, but it is not without risk to their aesthetic appeal. So if the foliage on your boxwood shrubs has started to take on a sickly, bronze/orange-rust tinge this winter, fear not; increased sun exposure and drought stress are usually to blame.
In the winter, frozen or dry soils can cause a drought stress response in boxwoods as their root systems are unable to replenish moisture lost through photosynthesis. The symptoms of drought stress appear as orange to bronze foliage. Boxwoods planted in sunny, unprotected locations experience more prevalent bronzing when harsh winter winds pull excess water out of the leaves. In contrast, those grown in more protected and shadier sites have very little to no bronzing to worry over. Foliage almost always reverts to its sharp green glory in the spring, especially on those with mild cases of bronzing. In severe cases, the foliage that is most impacted by wind or sun exposure may appear completely brown or even bleached. Desiccation (drying out) is the term used for this type of advanced symptom of drought stress, and it is a common sight on many species of evergreen plants in addition to boxwoods.
Water your boxwoods in fall and early winter before freezing temperatures arrive to help prevent this type of stress response. Install 2 to 3 inches of mulch around shrubs to retain soil moisture and help insulate their shallow root system. For more information, see HGIC 1604, Mulch. Minimize potential desiccation by locating your shrubs in protected areas in your landscape. Temporary physical barriers can be used for existing shrubs. For example, a burlap screen staked on the windward side about a foot and a half from the shrub will offer some protection for more established boxwoods.
Wait until spring to assess any dried out branches for overall damage. Prune dead stems back to live woody tissue with diagonal pruning cuts just above leaf nodes, and properly sterilize any pruning tools before and after use. See HGIC 1003, Principles & Practices for Pruning Trees for more information on pruning.
A few diseases can also cause similar leaf symptoms in boxwoods. An excellent way to distinguish between these and desiccation is to assess the pattern of symptoms over the whole plant. While diseases usually have symptoms that appear more randomly over the plant before spreading to other branches, desiccation from drought stress affects foliage uniformly. Also, leaves affected by drought stress remain firmly attached to the branches. If foliage has fallen off the plant after drying out, this is usually a good indicator of disease. More information on these problems may be obtained from HGIC 2052, Boxwood Diseases & Insect Pests.
Several boxwood cultivars are less susceptible to bronzing, but color change varies depending on cultural conditions. Hybrids between Buxus microphylla var. koreana and Buxus sempervirens are known to be less susceptible to bronzing. Examples include Buxus x ‘Green Gem’, ‘Green Mountain’, ‘Green Ice’, or ‘Glencoe’.
For more information on boxwoods, see HGIC 1061, Boxwood.