To complicate the issue, different types of hydrangeas need pruning at different times. You will have to know what type your plant is. Hopefully you saved the label. If not, an educated guess can be made by looking at its foliage and flowers.
Pruning Instructions by Type of HydrangeaBigleaf, Mophead or Florist Hydrangea (Hydrangea macrophylla) is sometimes easy to recognize because it is the one whose flower color changes with the soil pH: blue in acid soil, pink in alkaline. There are a few varieties that simply stay white, making it much harder to categorize from the flowers. The leaves are coarsely serrated and glossy, dark green. H. macrophylla also include the Lacecap hydrangeas, whose flowers look like a circle of unopened buds surrounded by open petals. In reality, the unopened buds are the fertile flowers with pollen and the outer flashy petals are sterile and are just there to attract bees. This is true of most hydrangeas, so don’t become frustrated waiting for all the buds to open.
Bigleaf type hydrangea set their flower buds at the ends of the upright or lateral branches, during late summer to early fall. Pruning Bigleaf hydrangea in the spring or even late fall, after the buds have been set, will remove the flower buds and any chance of getting flowers that season.
Bigleaf hydrangea should be pruned as soon as the flowers have faded. You should begin to see new growth coming in from the base of the plant. To keep the plant vigorous, selectively prune out the dead and weaker stems, both old and new. Don’t prune out all the old wood, since this is what will keep flowering as the new growth matures.
Bigleaf hydrangeas are the variety most susceptible to winter bud injury. If you live in an area with severe winters or your plant is exposed to winter winds, you might need to offer it some winter protection, to protect the flower buds. Tying the branches together and wrapping them with burlap isn’t pretty, but it could mean winter survival. Remove the burlap when the buds begin to swell.