Endless Summer hydrangeas, Hydrangea macrophylla 'Bailmer,' bloom on new growth and old growth. This enables them to bloom continuously over the summer, rather than just once during the growing season. They must be pruned without damaging the new growth or interfering with the next bloom cycle.
Deadhead the hydrangeas in the summer by removing only the withered flower heads. Use sharp hand clippers or garden scissors to cut the base of the stalk just under the flower head. Do this for the first few years until a wooded base develops.
Do not cut off the last wave of flower heads in the fall; leave them to insulate the tender buds over the winter.
After the first new growth appears in the late spring, prune the old growth to just above the new growth line. Only do this if you want to keep the plant from getting too large. Otherwise, the plant does better when it isn't pruned. Don't expect as many flowers during the years it's pruned.