English boxwood, Buxus sempervirens 'Suffruticosa', is a popular evergreen shrub. It is easily grown in many parts of the country and its needs are not difficult to meet. It has only one major annual need for maintenance–thinning. English boxwood has a very dense growth habit, and it lends itself beautifully for use in hedges, parterres, and accent plantings. The thick foliage of English boxwood is, however, a major factor in disease development that can kill branches. Fungal diseases such as Volutella canker are favored by the damp, airless conditions inside the canopy of dense foliage. Infected branches first turn olive green and may gradually turn orange and then straw-colored before they die. Annual thinning brings light and air into the interior of the plant and encourages the growth of new foliage within the canopy that can take over for branches damaged by ice or snow. The best time to thin boxwood is early winter, and the branches that are removed make great holiday decorations. Follow these instructions to learn how you can thin your boxwood to keep it healthy. |
![]() <<< Boxwood keeps its foliage for three years. This year's growth almost completely covers the leaves that grew in the previous two seasons. It is clearly in need of thinning. ![]() >>> If you carefully bend the branches back, you can see that the foliage is growing only on the outermost reaches of the branches. This severely reduces air circulation and favors the development of diseases. <<< To thin boxwood, use a bypass pruner that is sharp. Reach inside the shrub about six inches and prune off the stem. Continue thinning until the small interior twigs just start to become visible. You should remove about ten percent of the outer branches. >>>
Some gardeners are familiar with the technique known as 'plucking', which is a type of thinning done by breaking off branches rather than pruning them off with a pruning shear. The branch on the bottom has been plucked and has created an irregular wound on the shrub that might heal poorly. By contrast, the top branch has been pruned off, leaving a small, clean cut that will heal well. <<<
After proper thinning, small voids appear in the canopy that permit air and sunlight to enter the interior portions of the shrub. Older leaves are now visible. They were covered by new growth before thinning. >>>
After two or three years of annual thinning, new growth will start to appear in the interior portions of the shrub. This results in a healthy plant with strong stems and greatly decreases the incidence of disease problems.
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When thinning is complete, the size and shape of the boxwood is unchanged. You may be surprised by the volume of branches removed in the thinning process. The pile of clippings in the foreground is from just ten minutes of thinning.
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Last Updated January 6, 2004 11:35 AM
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