Answer: In my experience this may happen for no apparent cause (possibly wind or storm damage?) but usually happens to older branches. This is one reason where an annual renewal pruning can be desirable, in which a portion of the oldest stems are cut away as close to the ground as possible, thus encouraging strong vigorous new growth from the base. (This procedure can also help to limit the overall size and help keep a tidier outline.) Annual pruning would also include removing any winterkill damage, and after rough winters, this may in fact require removing most of the plant. Again, the result is fresh new stem growth. Some gardeners will cut the entire plant back very short each spring and this also helps spur vigorous growth. Since the shrub blooms on new growth of the season, this will not decrease flowering. In fact, a hard annual spring pruning followed by a light pruning in June can promote flowering by increasing the number of branch tips where the flower buds form.
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