Hydrangea bushes have become overgrown, how far down should I cut them to make sure they flower next summer?
Certified GKH Gardening Expert
You can cut them down completely if they become overgrown, but this will stop their flowering for a year or two as they recover. If you want to thin it out some, then just take a few branches out here and there, then on what you leave, you will have to cut the spent bloom only past the bloom.
Here is an article with more information: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm