There are a number of confusing varities. The local favorite is called "Nikko Blue". They like acid soil and ONLY FLOWER ON THE SECOND YEAR GROWTH! So if you cut back hard the wrong "dead looking stem" you will get no flowers next season. Go to real good nursery and talk to them, they will show you how to do it.
Why it is confusing is that they now have new varities that flower on the NEW growth...and they look similar. I thought I was losing my mind. I have 40 or so of these plants and they are mixed types.
Lastly, last winter and spring was *really* bad for these plants. We actaully moved down a zone last winter. We also had a late spring frost that killed most of the flowers that were starting to form. Many plants (esp exposed ones) did not flower this year according my local expert. He told me hang in there, if they are still alive they will come back.
I guess my message is don't just cut anything that looks dead. get some advice. Frankly I would not prune it UNLESS it is too big or you want to take the flowers...let'em get big.
http://www.hydrangeashydrangeas.com/pruning.html
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