Boxwood shrubs (Buxus spp.) help create a welcoming space in your yard where your kids and pets can play. The shrubs can increase privacy, create living fences and add year-round color and texture. These evergreens grow in a range of shapes, sizes and colors, thanks to the many cultivars developed by nurseries and botanists over the years. Some boxwood varieties are more winter-hardy than others; plant hardiness can be determined by using the U.S. Department of Agriculture's plant hardiness zone map, which categorizes regions of the U.S. by their average annual low temperatures.
The common boxwood (Buxus sempervirens) is hardy in USDA zones 5 to 8, which means it withstands average annual lows to minus 20 degrees Fahrenheit. The species grows to 20 feet tall with a 15-foot spread and has small, dark green leaves, making it a good choice if you want to create a hedge or screen for your yard. It prefers moist, well-draining soil and grows in sun to partial shade. The common boxwood is native to northern Africa, western Asia and southern Europe and has been grown in cultivation for many years, so you can choose from several cultivars that come in different colors, shapes and sizes. Cultivars hardy to USDA zone 5 include "Suffruticosa," a compact, low-growing shrub that reaches heights of 3 feet and "Arborescens," an upright shrub that grows to 10 feet tall.
The Korean boxwood (Buxus sinica var. insularis) is a bit more cold hardy. This Asian native grows in USDA zones 4 to 9, and is hardy to minus 30 degrees. The spreading shrub grows to 32 inches tall and 4 feet wide, but can reach heights of 5 feet over time if it's not pruned. It has yellow-green foliage and blooms with fragrant, if not showy, spring flowers. Korean boxwoods grow well in moist, loamy or sandy soil in full sun to partial shade. Varieties hardy to USDA zones 4 include "Wintergreen," a 4-foot-tall shrub with a 5-foot spread. "Wintergreen" is less likely to discolor in winter cold than other varieties, so your family can enjoy the rich color year-round.
The Japanese boxwood (Buxus microphylla var. japonica) grows in USDA zones 6 to 9, which reach lows of minus 10 degrees. Japanese boxwoods reach heights up to 8 feet and spread to 15 feet wide. They bloom in spring with white-green flowers and have shiny, round leaves. When grown in full sun, this shrub's foliage may turn red or copper over the winter. Japanese boxwoods are great for shady sites, because they prefer partial or dappled shade and moist, loamy or sandy soil. Varieties hardy to USDA zone 6 include tiny "Morris Midget," which grows to 12 inches tall and 18 inches wide. This boxwood needs protection from winter winds.
Hybrid boxwoods include "Green Gem" (Buxus "Green Gem"), which grows to 4 feet tall and wide. "Green Gem" grows well in sun or partial shade and moist, well-draining soil; it rarely requires pruning. It's hardy in USDA zones 7 to 9, or to average annual lows of zero degrees. The more cold-hardy "Green Mountain" boxwood (Buxus "Green Mountain") grows in USDA zones 6 to 8. This boxwood reaches heights of 5 feet with a 3-foot spread and has dark, glossy leaves. It tolerates air pollution, making it a good choice for urban yards.