When talking with homeowners, this is the question that comes up the most, hands down. Their lovely, old plantings of hydrangea are gorgeous, but they are blocking the windows, doorways, driveways, or ocean view. They are flopping on to the deck, or on the plants in front of them.
Technically, the answer to the question is yes- mophead hydrangeas, even those varieties that bloom on old wood, like “Nikko Blue”- can be cut back if it is done before August, when they set buds for next year’s bloom. I hate to do this, but it can be done. Believe me, it isn’t pretty. And it is only a short-term solution. The mopheads will keep growing back, and they will get just as big as they were before.
My best advice in these situations is to move the hydrangeas. They are going to be what they are going to be. There is no way to keep them small and still attractive year after year. Find a nice home for them with plenty of room to grow, morning sun, some afternoon shade, and plenty of moisture. They will require much less maintenance, (which is always the case when you work with nature, not against it) and reward you with years of beautiful blooms.
If you want hydrangea close to the house, there are plenty of well-behaved varieties that will stay smaller. The Cityline series of Hydrangea are compact, topping out at 3-4 feet. They are available in a variety of colors. My favorite is Cityline Rio, below.