You can deadhead - not prune - hydrangeas whenever the mood strikes :-)) I don't grow paniculatas but unless I deem them very unsightly, I leave the old flowerheads on my mops intact through winter.

With the mops, you just want to remove the old flowerhead, cutting down no further than the first set of leaves beneath the flower. With the panicles, you can cut back as little or as much as you prefer.

As mentioned, you can do it now if you like or wait until late winter/early spring clean up.

It may be due to circumstances. It causes no harm to let them drop on their own; deadhead as soon as the bloom completely browns out; deadhead in late winter/early spring so you can enjoy the blooms for winter interest. It does not affect future blooming either for spring 2026. You can also make a habit to deadhead by your average date of first frost if your area gets a lot of snow or gets lake effect snow as the stems may bent, break or flop in winter due to the excessive amounts of snow in some years. Cut above the first pair of leaves nearest the bloom; if the stem is leafless when you deadhead later on, cut the string(s) that attaches the bloom to the stem.

Diane, I’m curious to know what you decided to do about deadheading. I’m South of you & though in same zone it’s warmer here I’m sure. I’m in Vancouver, WA. We’ve had such a mild winter so far that plants are confused. My hardy fuchia all still have their leaves & ’berries’ from flowers! I’ve usually cut them back by now. I don’t cut off any flowers on either my paniculatas or mopheads until usually early spring. But, I did just do lots of cleanup the other day as it was high 50’s & sunny! I noted that my Little Lime Paniculata has green leaves unfurling at the bottom & the buds on top branches are fattening up, Same thing on my Pia mophead. It’ll be too bad if we have a hard frost in February which isn’t unheard of. A few years ago we had a heavy wet snow in April & February ice storms aren’t unheard of. I have 8 Bobo paniculatas in my front yard & I haven’t touched their flower heads at all. They are still very tidy looking & get much more sun than my backyard hydrangeas do, I want to leave their flower heads on to maybe protect the buds if we have one of those late freezes. I’m pretty sure that if I cleaned them up now & we did have a freeze that I could just cut off the dead tops but I don’t want to do the work twice & my guess is that would cause later blooming.