Answer: There are several reasons this can happen when clipping or trimming it into a formal shape. Probably the most common is due to using electric hedge shears. These tend to tear or grind or crush the stems and leaves rather than neatly snip them. A well sharpened, hand operated, hedge shear tends to cause less of this type of damage. If you are just trimming the occasional wayward tip, a sharp bypass hand pruner is the best tool as you can cut each stem individually - cut just above a leaf leaving a minimal stub and also cut without snipping foliage in half. It may also be that you are trimming too much at a time and cutting back to older wood. These shrubs should be trimmed lightly in winter or early spring, then tidied as needed in spring and up to mid summer after the early growth spurt. The frequent light trimming avoids leaving any bare spots. Avoid trimming in late summer or fall as this can stimulate late season growth that will not have time to harden before the cold weather sets in. Actually it is better for the plants to prune by selectively removing some of the older branches by reaching deep into the shrub and cutting with a bypass hand pruner or possibly a saw on an older neglected plant. This allows you to maintain a fairly regular overall outline but avoids creating a dense outer shell over time. This allows air and light into the interior of the plant and keeps it healthier. Here are instructions on how to do this along with a photo showing the overall result.
I hope this helps.
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