What do I trim back, hydrangeas?

Often, the earlier you get it done after bloom, the quicker the shrub can recover, producing more and larger blooms next season. Don't prune these hydrangeas to the ground in late fall. Doing so removes all of next year's flower buds.

Here you go:

https://www.wikihow.com/Prune-Hydrangeas

My Grandmother would cut them straight off level (like a military buzz cut) about 3 foot off the ground. They would bloom out with new stalks EVERY year. She has some blue, pink & purple ones in the back yard & in the front yard. NEVER cut them down TO the ground because they take a year to grow back up to over 3 feet. Her way, they would always bloom every year. She had a green thumb with anything.

I only shape mine and give most flowers away so they have been cut. I leave it alone unless I shape it.