Prune the stems that flowered last season – don’t prune un-flowered stems, because these are going to produce the next lot of blooms. The best time to prune Hydrangeas is after they have flowered and the blooms have died. That’s not to say that normal pruning rules shouldn’t apply: cut back anything that’s very weak and spindly and, of course, any dead stems.
Large -leaved Hydrangeas: these Hydrangeas bloom on old wood. A severe trimming in spring may prevent blooming. Just trim out dead wood and trim to shape in late March.
Other types: trim after flowering.
The NEW Endless Summer Hydrangea will bloom on old, as well as new wood and almost never fails to bloom!