Hydrangeas: it helps to know if the plant blooms on old or new wood. Generally speaking, you should prune back by half. Almost anytime when you are tired of seeing dead blossoms. If the plant blooms on new wood you can cut anytime.
Peonies should be pruned late fall. The new blossoms come from the tubers. I usually pruned hard on late winter.
Hibiscus, prune lightly in late summer but no pruning in late fall or winter. Wait until new growth begins to show in the spring. The branches can be cut back to leave at least 2 or 3 nodes.
I have never grown a Cornus capitata( evergreen dogwood) so you might google that.