Root rot or Verticillium wilt caused by a soil-borne fungus might the culprit behind the hydrangea's deteriorating condition. This condition is commonly caused by overwatering or poorly draining soil, which creates an environment that allows fungus to thrive and attack the plant's roots.

Thanks, but how do I know which one? I need to water one side more or less :D

"Jordan" once one see the wilt in the leaves is there any coming back from it?

I used to have a beautiful 5 ft tall hydrangea paniculata 'White Diamonds'. One spring, three or four years ago, it started to wilt like that. In a few weeks, the whole plant was dead. I never found any logical explanation for its demise. Needless to say, I didn't put another hydrangea in that spot. I still have two other 'White Diamond's at that end of my backyard that are doing fine. However, since they are in shade, they don't bloom as well as the one that died. Practically broke my heart.

Your condition, given that grouped stems are affected together is more likely indicative of root rot. The leaves will remain wilted even if the plant has been

watered thoroughly.