Answer: You can prune them back hard. I routinely reduce them to 15-18" from the ground and remove some of the oldest (grayest) stems clear to the ground. Flowers are produced on new stems which grow from old wood. The more stems you have, the more flowers you'll have. But, too many flowers mean competition for nutrients. So, you might try reducing the number of flower buds before they open to help direct nutrients to the remaining flower buds. Best wishes with your hydrangeas!
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