Dead-heading
Dead blooms on mophead hydrangeas can, in mild areas, be removed just after flowering, but it is best to leave them on the plant over winter to provide some frost protection for the tender growth buds below.
Remove the dead flowerheads in early spring, cutting back to the first strong, healthy pair of buds lower down the stem.
Lacecaps are hardier, and the faded flowerheads can be cut back after flowering to the second pair of leaves below the head in order to prevent seed developing, which saps energy from the plant.