Boxwoods (Buxus species) are favored broadleaf evergreen shrubs prized for their dense foliage and ability to provide year-round structure. These plants are cold-hardy, retaining their leaves even through freezing temperatures. As cold months progress, many gardeners observe a distinct change in the shrub’s vibrant green color, often shifting to shades of yellow, red, or bronze. Understanding whether this discoloration signals a healthy physiological response or damaging stress is important for effective plant care.
Understanding Boxwood Bronzing
The shift in color observed on boxwood foliage during winter is known as bronzing, which is generally a temporary and protective physiological reaction. This change is not a sign of death, but rather the plant producing specific pigments in response to environmental stressors like intense sunlight and cold temperatures. The plant reduces its chlorophyll content, the green pigment responsible for photosynthesis, making other pigments visible.
The resulting reddish-brown or orange-yellow color comes from anthocyanin pigments, which act as a natural sunscreen for the leaf tissue. This process helps protect the leaf cells from photo-oxidation caused by intense light and cold stress. The degree of bronzing can vary based on the boxwood cultivar and its placement, with plants in sunnier, more exposed locations showing a greater color change. Once warmer temperatures return in the spring, the boxwood typically resumes full chlorophyll production, and the foliage reverts to its characteristic green color.
Identifying the Causes of Winter Burn
While bronzing is a natural reaction, severe yellowing or browning indicates a condition called winter burn, which is a form of desiccation. Winter burn is a physical injury where the evergreen foliage loses moisture faster than the plant can replace it. This damage occurs when three environmental factors combine to create water stress.
The first factor is frozen ground, which effectively locks up the water supply, preventing the roots from absorbing moisture. Evergreens continue to transpire—release water vapor through their leaves—even in winter, particularly during sunny periods. The second factor, intense winter sunlight, triggers this transpiration process as the leaves warm up and attempt to photosynthesize.
The third component is wind, which accelerates the loss of moisture from the leaf surfaces. This drying effect is especially pronounced on the sides of the shrub facing south and west, or on sides exposed to prevailing winter winds. When water loss exceeds water uptake over a prolonged period, the leaf tissue collapses and dies, resulting in straw-colored or bleached-brown foliage. Unlike bronzing, winter burn represents actual dead tissue that will not green up in the spring.
Preventing and Treating Winter Discoloration
Preventing winter discoloration requires mitigating water loss and water unavailability before the onset of cold weather. A deep watering regimen in late fall, just before the ground freezes solid, is one of the most effective preventative measures. This practice ensures the boxwood’s shallow root system has a saturated reservoir of moisture to draw upon throughout the winter, reducing the risk of desiccation.
Applying a 2- to 4-inch layer of mulch around the base of the shrub helps to regulate soil temperature and conserve moisture. Mulch delays the freezing of the soil, extending the period during which roots can absorb water, and insulates the root zone. Another preventative action involves using physical barriers to shelter the shrubs from harsh environmental elements.
Temporary burlap screens or windbreaks placed on the south and west sides of the boxwood can shield the foliage from intense afternoon sun and drying winds. This protection minimizes the rate of transpiration, preventing the desiccation that leads to winter burn. If discoloration occurs, patience is required for recovery.
Treating Winter Burn
For boxwoods affected by winter burn, avoid pruning the damaged, brown foliage until new growth begins in the spring. The dead stems may still offer some protection to the underlying living wood. Once new growth is evident, prune the dead or severely damaged branches back to the nearest point of live, green wood to encourage new, healthy branching. Adequate spring watering and a light, balanced fertilizer application can support the plant’s recovery and help it quickly replace the injured foliage.