Like the non-reblooming macrophyllas, H. serrata blooms on old wood so I expect in zone 5 they could suffer from the same problem of buds freezing off unless protected over the winter. But if you're willing to go to the extra effort of protecting it over the winter I think it ought to bloom for you.
Ecrane hit it on the money. I am in zone 5a and here is the result I got from a hydrangea serrata shishiva that I overwintered in my unheated garage, with a light quilt over it and 8 ounces of water once a month, in zone 5a.
It has such a delicate beauty. Do buy it. You won't regret it.
I suspect the nursery owner you talked to was making a general statement about H. serrata (most of which only bloom on old wood), he may not have experience with this particular cultivar. If this cultivar does live up to its advertising then you wouldn't need to protect it, although just like with the reblooming macrophyllas if you do protect it you'll get more blooms because then you'll get both the old & new wood blooms.
ecrane3 wrote:I suspect the nursery owner you talked to was making a general statement about H. serrata (most of which only bloom on old wood), he may not have experience with this particular cultivar.
I have always kept in in my garage, under a quilt, and watered it with 8 ounces of water. Didn't want to take a chance. It appears to bloom May through early September, from my old pictures.
You know the one I'm after? Glen Church. Unusually hardy. I've only seen it in books. It was named for the guy who found it. He is from New Zealand, and it is in his two books on hydrangeas that I found in the library.