Should you cut back in the fall, and how much?
When Should You Prune Annabelle Hydrangeas
Certified GKH Gardening Expert
Generally you prune in the summer after they are done flowering.
You can also cut them back in winter or wait until early spring to trim back the old growth.
I personally trim the plants in the spring prior to leaf out and I am just south of you.
Here is a link with more information.