Hi,

These are usually pruned in late winter or spring. You can cut them back in fall if you want.

From the University of Minnesota:

Hydrangea paniculata, panicle hydrangea

Prune back stems to just above a fat bud — called a heading cut — in fall, late winter or spring.

These plants have conical-shaped flower heads. The dry, tan flower heads on the plant can provide some winter interest in your landscape, so wait to prune these until late winter or spring.

Good luck.