The boxwood, a popular broadleaved shrub from the Buxus genus, is a fixture in many landscapes due to its dense, easily shaped form. Its primary appeal is its classification as an evergreen, suggesting it should maintain a lush, deep green color throughout the year. While boxwood is botanically designed to stay green in winter, its appearance often changes. This inconsistent color retention during the cold season is typically the result of environmental stresses.
Boxwood’s Evergreen Nature
Boxwood species, such as Buxus sempervirens and Buxus microphylla, are defined as broadleaf evergreens, meaning they keep their leaves through the winter rather than shedding them annually. The foliage is typically small, oval, and possesses a naturally glossy, dark green color. This year-round color provides structure and contrast in a winter landscape.
The leaves have a waxy cuticle, which is a natural adaptation designed to minimize water loss through transpiration. This feature allows the plant to sustain itself during the colder months. Boxwood’s hardiness and ability to be pruned into intricate shapes have made it a favorite for hedges and formal garden topiaries.
Defining Winter Browning
When a boxwood fails to maintain its vibrant color, the resulting discoloration is commonly known as winter browning or bronzing. This is a visual symptom of stress, not a typical leaf-shedding process like that seen in deciduous trees. The foliage takes on a reddish, rusty, or yellowish-brown hue, often appearing most intensely on the outer layers of the shrub.
This shift in color is caused by the plant producing pigments, such as anthocyanins, in response to environmental duress. The leaves themselves remain attached to the stem. This temporary color change is a clear sign the plant is struggling to manage moisture levels, but the original green color frequently returns with the arrival of new growth in the spring.
Environmental Factors Causing Injury
The discoloration is primarily the result of desiccation, a form of winter burn that occurs when the plant loses moisture faster than it can be replaced. Boxwood leaves continue to lose water through transpiration, particularly on sunny or windy winter days. The mechanism of injury is simple: the roots cannot absorb replacement water because the surrounding soil is frozen solid.
Strong winter winds significantly accelerate this water loss from the exposed leaf surface. The combination of intense, direct winter sun and frozen ground is especially damaging, as the sun warms the leaves, increasing transpiration while the roots remain locked out of the water supply. This effect is often most pronounced on the south and southwest-facing sides of the shrub.
Maintenance Tips for Green Winter Foliage
Preventing winter browning relies on minimizing moisture loss and insulating the roots from extreme cold. One of the most effective preventive measures is ensuring the boxwood enters the winter fully hydrated. This requires deep, consistent watering in the late fall until the ground freezes, providing the plant with a maximum reservoir of moisture to draw from during the cold, dry months.
Applying a two to three-inch layer of organic mulch around the base of the plant helps to maintain soil moisture and provides a layer of insulation for the shallow root system. It is important to avoid mounding the mulch directly against the plant’s trunk to prevent rot.
For shrubs in highly exposed locations, a physical barrier, such as an erected burlap screen, can be staked a foot or two from the shrub. This barrier helps to deflect harsh winter sun and drying winds.
Another technique is the application of an anti-desiccant spray, sometimes called an anti-transpirant, to the foliage. These products create a thin, waxy coating on the leaves that slows the rate of water loss through transpiration. This protective spray is typically applied in late fall or early winter when temperatures are above freezing, and a second application may be beneficial in mid-winter.