It's Actually Fine To Cut Panicle Hydrangeas In The Fall. Here's Why
By ZARA M.
Cutting panicle hydrangeas in the fall can have benefits for the plant because pruning can prevent snow and ice from damaging vulnerable, budless branches during the winter.
To prune, shorten back the branches by one-third of their total length, cutting above a bud. With branches that have several stems coming off them, remove each stem individually.
Alternatively, you can take the whole main branch back to a bud. When cutting back old stems, leave four pairs of buds on each one to encourage new growth.
To keep a more classic shrub shape, cut all shoots to a six-inch stub with about two dormant buds that look like raised dots under the bark. Your new shoots will sprout from there.