Some boxwoods, especially the Korean boxwood (B. sinica var. insularis) experience natural browning as the months turn colder and slides into winter. Though considered an evergreen plant, this particular boxwood features brown foliage throughout the winter, though the shrub does not drop its leaves.
In fact, winter becomes the most common season for spotting brown foliage on otherwise green boxwood shrubs. Exposure to cold winter winds, dry conditions and frost contributes to boxwood leaves turning brown, reddish-brown, orange or yellow. This process, known as bronzing, does not cause permanent damage to boxwood shrubs. Planting boxwoods in a shaded site, protected from winter winds prevents winter bronzing. Also, consider proving a wind-break for boxwoods, especially those planted on hilltops or areas exposed to winter winds.
During the winter months, boxwoods generally lose water through their leaves. Help avoid brown foliage due to drought conditions by providing boxwoods with extra water throughout the dry winter. Prior to the first winter freeze, allow a soaker hose to drip slowly over the soil, soaking to a depth of about 8 inches. Cover soil around the shrubs with mulch to keep in moisture. Check soil regularly; provide additional waterings if the soil becomes dry to the touch.
English Boxwood Decline
English boxwoods (B. Sempervirens Suffruticosa) sometimes experiences a slow decline which includes foliage browning symptoms. Most commonly caused by a combination of fungal infections and poor cultural practices, English boxwood decline takes several years to develop. Leaves and stems turn from a dull green to a brown, eventually falling off and leaving behind bare, gray colored twigs. Providing adequate water, removing debris around plants and protecting winter injury helps guard against English boxwood decline.