Boxwoods (Buxus spp.), with their rounded forms and glossy leaves, thrive in U.S. Department of Agriculture plant hardiness zones 6 through 8. Japanese or littleleaf boxwoods (Buxus microphylla) and their cousins, the American boxwoods (Buxus sempervirens), have a slow rate of growth that is unaffected by trimming. But fast growth isn't the only reason to prune these shrubs.
Both American and Japanese boxwoods grow slowly -- usually less than 12 inches per year. This rate of growth is not influenced by trimming or pruning, but is the natural pattern of these plants. The mature size of a boxwood varies, depending on the variety. American boxwoods usually grow 10 to 15 feet tall, while Japanese boxwoods remain under 4 feet tall.
Benefits of Trimming
Although trimming won't speed a boxwood's rate of growth, it can keep the shrub healthy -- indirectly influencing growth. As a boxwood grows, the inner portions of the shrub are often shaded by outer branches. If not corrected, this tendency can cause the inner branches to die back or succumb to fungal diseases. Pruning can also improve the shape of the shrub and maintain its size.
Three specific types of pruning are usually needed to maintain a boxwood's health. Thinning, which is usually done in the spring, is done by cutting back branches throughout the shrub. Thinning opens the interior of the plant to light. Shearing can be done any time of year. Shearing is removing the tips of the plants to maintain a hedge or topiary shape. Heavy pruning should be reserved only for neglected boxwoods that have grown out of control. Heavy pruning involves removing or cutting back up to one-third of the plant's branches each spring for three years. More severe pruning can harm or kill a boxwood, so this process must be done gradually.
Although trimming won't make boxwoods grow faster, proper cultural care can help promote vigorous growth. Boxwoods thrive in partial sun and need consistently moist, but not soggy, soil. In heavy, wet soils, growth slows and boxwood are vulnerable to numerous diseases. A 2- to 3-inch layer of wood chip mulch can keep the plants' roots cool and conserve moisture. Boxwoods sometimes suffer from a nitrogen deficiency, which is usually evidenced by yellowing lower leaves. An application of 1/2 cup 5-10-5 fertilizer in spring can encourage new growth and a fuller plant.
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