Known for sizable blooms, the hydrangea is a large, lush plant which many gardeners shy away from pruning because several varieties only bloom on the previous year's growth and they don't want to cut future flowers off. There are, however, some which bloom on new growth, as well as others which bloom on both the old and new growth making pruning a little easier. You can safely prune in the fall if your hydrangea varieties are Pee Gee, Annabelle, Grandiflora, Penny Mac, All Summer Beauty, Endless Summer, Limelights and Oakleaf.
Wait until the plant has finished blooming for the year. You can trim in the early fall or wait for the leaves to drop so you can see the hydrangea's branches better without leaves in the way.
Select older, woody branches to cut first. They can be trimmed away or just shortened by one-third or more. Make each cut just above a side branch which looks to be growing healthily and trim the selected branch horizontally across the entire stem.
Trim away only the weaker of the new shoots if more thinning of a crowded plant is needed. Stems which have already flowered are ideal for further trimming because you know they are on their second year.
Leave young, tender shoots alone as they will be the branches which bloom in the following year.