Answer: In general, spring bloomers are pruned immediately after blooming, while others are pruned in early spring. This way you don't cut off flower buds.
Forsythia, mock orange, lilac, spirea, snowball and pussy willow would be pruned in spring right after they bloom. You could also prune them earlier than that but you will remove the blooms.
Red twig dogwoods are usually pruned in very early spring both to control overall size and to force new twig growth which is brighter colored than old growth. For this reason it is better to cut entire stemst low to the ground.
Rose of Sharon should be pruned in mid winter or early spring, but not during summer because it blooms on the current year's growth. Potentilla does better if pruned to remove any winterkill and improve shapeliness in early spring as it also blooms on new growth -- this will help it look tidier. You can trim it back a bit after the first flush of bloom to encourage more blooms later in the summer, also. Butterfly bush should be pruned back in late spring to remove dead wood and encourage branching. Some gardeners will tip prune again to encourage more blooming tips, but this will delay flowering. It blooms in summer on the current year's growth.
Finally, hydrangea care varies drastically depending on the type you are growing. Burning bush has a natural shape which is quite tidy, but it does grow into a large bush. Since you aren't expecting flowers from it the pruning time is not critical.
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