Q. How do I prune a mophead hydrangea?
There is really no need to do anything as the plant does not seem to be over grown and the dead flower heads can be quite attractive. If you wish you can dead-head the pant by removing the flowers. Cut just below the flower and above the next set of leaves. As the plant gets larger you could prune it occasionally to shape it and keep it healthy. First, remove any dead or sickly branches. Then cut down about one third of the branches down to just above the root, or remove old dead branches. Do this every year for three years. Prune the remaining branches slightly to shape the plant. Any pruning you do should be done before August as this species flowers on old wood, i. e. buds are produced the summer or fall before the next spring. Pruning too late in the summer will result in no flowers next year.
Courtesy of NYBG Plant Information