Thanks for writing. There are several reasons why your hydrangeas may droop. Some varieties with very large flowers tend to be floppy because the stems have supporting those big blooms. Other times plants can get droppy if they're in too much shade --- the stems stretch, looking for more light, and become weaker. They could also develop strong stems if they're fertilized too much; fertilizer can encourage lots of fast growth which isn't as strong.
---Justin, Senior Garden Editor, BHG.com