Hi, MFIX. The stems in cold zones will tend to dry out during normal winters. These can be left 'as is' in Spring until mid-May when all leafout should have happened. Any stems without leaves by then can then be pruned.

Many factors contribute to 6' tall hydrangeas. The length of the growing season, the amount of nitrogen in the soil and the genes of the hydrangea variety are some.

In zone 5, your growing season will be limited so it will be hard for a shrub that starts growing from the base to attain a height of 6'... but somewhere between 4-5' may be doable.

Since the stems will usually dry out in winter and begin to grow from the crown in Spring, you can safely prune when you notice that the plants are exceeding 4'. Yes, flower buds created in July-August will get cut but the stem will dry out during winter... therefore no harm done.

This pruning is however, an annual chore that I would not like to have to do every year in the middle of the summer here. If you get tired of that, make sure that you do not add too much nitrogen-rich fertilizers and use varieties that will stay 4' high or so instead.

Luis