Hydrangeas that bloom in the summer should be pruned after the flowers fade, and those that bloom in the fall should be pruned while they are dormant in the late fall or winter. Annual pruning is not required to produce healthy blooms, according to Fine Gardening magazine.
Summer-blooming hydrangea bushes usually produce flowers on old wood, which are stems that exist from previous seasons. Once the plant flowers, the stems start to produce next year's buds. Pruning the hydrangea shortly after the flowers fade prevents the accidental cutting of new growth because it is easy to see the fresh buds.