Hydrangea paniculata.

Definitely a Hydrangea but I don't think it's H. paniculata.

DSC03332.JPG (1959×1469) (coocan.jp) What else could it be with those leaves? Not all paniculata flowers look like most of the sterile type.

Agree with H. paniculata.

Thank you all very much. I can't imagine how it got there. Or, if it was planted by a previous owner, why it took 25 years to bloom. Just one of the many garden mysteries, I guess. Eg: how did a frog wind up in my rain barrel when I have a screen over the top?

Lack of sun = no flowers.

Thank you, Carol! Any ideas about the frog in my rain barrel?!

Thanks! I never knew there were paniculatas that looked like this, I assumed they all looked like 'Limelight' etc.

I just looked it up online and saw that it's a climber. Now I remember getting some climbing plant many years ago in trade from someone in the State Dept. Lost track of it for years, obviously. And we recently had some trees trimmed, so it's getting a bit more light.

Hydrangea paniculata are not climbers - mostly large growing shrubs or trained into tree-like forms. The only climbing hydrangea for your zone would be H. petiolaris and after 25 years it would be massive in size even in lots of shade. And while they can take their own sweet time about flowering, that too would have taken place long before now. Also they produce a quite flat lacecap type flower, not the domed inflorescence in your photo and that should have happened several months ago.....June rather than late August.

I think previous suggestions of an overlooked paniculata are correct.

No, it isn't H. petiolaris. Entirely different flowers and leaves. And it would be attached to the wall.

Everyone agrees your plant is H. paniculata. The only climbing Hydrangea I know of is Hydrangea anomala subsp. petiolaris. Perhaps you will find it too...and discover how the frog got into your rain barrel lol!

There is the native Hydrangea barbara which I grow and is hardy further north. Hydrangea barbara (Climbing Hydrangea, Woodvamp) | North Carolina Extension Gardener Plant Toolbox (ncsu.edu)

So there is more than one climbing Hydrangea!

That is actually Decumaria barbara. In the Hydrangeacea family but not a true hydrangea. And the flowers look nothing like those in the OP's photo.

There are other climbing hydrangeas but they are evergreen, tropical in origin and uncommon in commerce.