Many gardeners loathe pruning hydrangeas as pruning at the wrong time will eliminate flower buds, which means no flowers for that year. Knowing the type of hydrangea you have and if it blooms on old wood (the previous year's growth) or new wood (the currant year's growth) is crucial. Nikko blue hydrangeas bloom on old wood, so you should prune to shape them after they are done flowering, just as their flowers start to fade. Prune nikko blue hydrangeas in spring to remove dead or winter-damaged stems.
Prune nikko blue hydrangeas in early to mid-spring just as leaves begin to emerge to maintain health.
Cut damaged or dead stems, another name for trunk and branches, back to the ground.
Prune nikko blue hydrangeas as their flowers begin to fade (late summer) to establish shape. Nikko blue hydrangeas look best with a rounded or slightly triangular shape.
Remove fading flower heads, making cuts above strong buds (where leaves and stems emerge).
Cut stems back to the ground that are crossing or rubbing other stems.