It all depends on what kind of hydrangea you have. There are several different types of hydrangeas grown as ornamentals here, and some of them bloom on new wood while others bloom on old wood. Pruning has to be timed to the particular way the one you're growing sets buds. Otherwise, by pruning at the wrong time for that variety, you could be cutting off all the buds that have formed for the next year, which then results in you not having any blooms that year.

The types of hydrangeas grown here include H. quercifolia (oakleaf hydrangea), H. arborescens (smooth hydrangeas like "Annabelle" or "Snowball"), and H. macrophylia (lacecap and mophead types).

If you are growing an oakleaf, mophead or lacecap type, these normally bloom on buds formed during the previous late summer and autumn. So, for these types, you prune them after their bloom period has ended in spring.

If you are growing a panicle or smooth hydrangea type, they generally bloom on the current year's new growth. If they need to be pruned, you can do it anytime from late summer to very early spring before new growth appears. If your plants of this type are older and are large, well-established and very dense, they would benefit from you pruning away up to 1/3 of the stems each year, cutting them back right above the ground. This keeps them from becoming unruly monsters. If you can remember each year exactly which 1/3 you cut previously, then you can cut a different 1/3 in year 2, and the final 1/3 in year 3, so that the entire plant is renewed by pruning during every 3-year period.

Hope this helps,

Dawn