For blooming, like many plants hydrangeas like to have about 1/3 pruned out each year. If you prune down to the ground each year you will not get the blooms you want! Here's a picture of the bush I've been talking about that I took from cuttings from my rental house. We are just coming into the bloom season, as I said, I pruned it pretty hard this year -- next year it will be just solid blue from the flowers. It's our acid soil here that makes the blue so strong, good for growing rhodys and azaleas too.