Answer: Most hydrangeas bloom on wood that grew the previous year, so pruning is best done immediately after it blooms. It is better to reach deep inside the plant and selectively remove some of the oldest, longest stems. Do not remove more than a quarter of the plant at a time.
Depending on the specific variety you have however, you may find that winter damage helps to maintain it at a relatively small size. In your climate it is possible it will suffer fairly extensive die back each winter due to the cold. Then it would require pruning in spring to remove the resulting dead stems. Wait until late spring to be sure stems are trulycdead before you remove them -- this type of winter/spring will reduce flowering.
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