Don't know where in zone 5 you are, but I got snow in April just when the hydrangeas were waking up. That's a problem for Nikkos, and likely why there's only a bloom or 2 on mine. They bloom on old wood. The bloom buds froze.
Not sure about pruning for size. I figure they're as tall as they're supposed to be. What makes you think they'd stay small if you pruned them?
Karen

Hello, Nancy. It is too late to prune hydrangeas without affecting bloomage for next spring. The time period when you should stop begins somewhere around August-September. People in the southern states should not prune anymore starting in August while people in the northern states should not prune anymore starting in September.

This is an observation that has been passed around throughout the years. It is not written in stone and varies due to weather, etc. The safest thing is to stop pruning starting in early August. One year you could be safe to prune in early September while on another year you should not prune past the end of August.

However, if you have to prune -for example- due to safety reasons, ignore the above and prune away whenever you have to.

Luis

Thanks Karen and Luis.
I'll wait. I thought that they grew unusually big because of the lack of bloom....sent all the energy to leaf growth?
It seemed over the top.
We, too, had a late burst unusually cold weather which must've killed the blooms. Let's hope it's a better year. I missed the blooms!

What do you mean by "too large"? Nikko Blue gets to be a good sized shrub, 4-6' tall and as wide, often quite a bit larger in my area. If you need to prune to control size, then I'd suggest you relocate the plant to an area where it can grow undisturbed. Because of the growth and bloom habits of a bigleaf hydrangea, it's a risky business trying to maintain these at a specific size smaller than what they want to be. The best time to specifically reduce size is when the plant is in the middle of its bloom season - hard to do then (far less so if it's not blooming at all :-)), but really the only guaranteed time to be able to remove significant growth without affecting future blooms. The drawback is that pruning stimulates new growth and the plant will quickly return to its former size.

I've found that the following site has the best information on hydrangeas virtually anywhere on the 'Net. You'll notice that they clearly indicate that pruning should be kept to a minimum if at all possible on H. macrophyllas, but if that is impossible July is the latest preferred time.

Here is a link that might be useful: pruning directions

My established Nikkos are about 3 feet tall. They're happy and they bloom when they're not snowed-on in April!
That's the difference a colder zone makes.
It's best to also have reliable bloomers like Endless Summer. That way, there are no bloom-free years.
K.