Answer: The "prune right after blooming" rule of thumb generally applies to spring blooming shrubs such as forsythia -- so you are quite right to wonder about these two.
The best strategy for potentilla is to prune it back quite hard in late winter, then tidy it again several times during the season if needed. Assuming it has enough water and nutrients, it will continue to bloom for most of the summer.
Rose of Sharon blooms in late summer on the current year's growth, so it is best pruned once in late winter or very early spring. At that time, you may prune it back as hard as needed to bring the size under control.
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