Panicle hydrangeas form buds on new wood (new stem growth) starting in spring, so deadheading in the fall won't harm these buds. You can even prune them in early spring before buds form.
However, as mophead hydrangeas require only a light touch when trimming, they can be safely pruned in spring without affecting that season’s flowering. The timing for pruning mophead hydrangeas ...
STEP 1: Deadhead any withered flowers that remain on the hydrangea bush. How to Prune Hydrangeas That Bloom on Old Wood How to Prune Hydrangeas That Bloom on Old Wood STEP 2: Remove branches that ...
but do not prune bigleaf and oakleaf hydrangeas, as you risk cutting off the next season’s blooms. In spring, give potted plants a jump start on the growing season by moving them to a sunny room ...