|When and how should I prune my hydrangea?|
|It depends on which species you have, actually. Does your shrub produce blooms on new growth, or growth from the previous year? For instance, H. macrophylla blooms on the previous year's growth and need time to form new flower buds. In your climate, it's best to leave the old flower heads in place to protect next year's flower buds from the cold. In early spring, cut flowering stems back to the first set of fat flower buds. My secret to having lots of flowers and still keeping the plant in shape is to prune about a third of it back to the ground each year, keeping two-thirds of it in bloom. This is great for rejuvenating an over-grown shrub. It may take a couple of years, but you can whip your plant into shape without sacrificing all of the blooms at any one time.
Hydrangea paniculata and H. arborescens flower on the current season's wood, so cut the shrub back in summer after they bloom, either to the ground, or to leave 2-4 buds per shoot.
Good luck with your pruning project!