A great question!If exposed to full sun and frequent frost and wind, the foliage of some boxwoods may become orange or bronze in the winter. Late season pruning may exacerbate this problem. Creating a wind break or wrapping with burlap will provide winter protection from the sun and wind. Also, consider using Bonide Wilt Stop to help prevent winter kill, wind burn, and summer scald. Winter bronzing can also be caused by stress due to low pH, poor fertility, or drought. Fertilize with Holly-tone in late fall and spring to increase fertility. Maintain moisture of soil throughout the winter especially when the soil isn't frozen. The good news is, though unattractive, bronzing will not kill the boxwood and they should green up again once temperatures warm up in the spring.
Read our blog post about this for more information.