Trim boxwood shrubs at any time of year except in the late fall and early winter when new growth may be damaged by frost. Annual thinning, best done when the plant is dormant in late winter, keeps the bush healthy by allowing air, light and water to reach the center.
To thin boxwood, cut 6 to 12 inches off the branches using hand shears. Do not remove more than 10 percent of the outer branches. The bush should be about the same size and shape after thinning as before. Trimming the outer growth of boxwoods gives them shape, but shearing causes dense growth on the outside of the shrub and can leave the inner branches bare. To keep the plant healthy, thin after shearing. Severely prune an overgrown boxwood when the plant is dormant. For best results, cut it back gradually, removing one-third of the large branches each year for three years.