I've always wandered this myself. Thanks for the post Jim. When I first planted my hydrangea outside it grew nicely. When the winter ended, and spring came, I was about to chop off all those dead looking branches, but noticed growth from them. Good thing I left it alone.

I would leave them until later in summer when they are about to bloom and by then, you may find that you may not need to cut the cane all the way down, thus allowing you to increase the overall height.

As I understand it, different kinds of hydrangea have different fall pruning needs.

The classic old-fashioned Pee Gees and Annabelles bloom on new wood and need to be cut back almost to the ground so they don't get too floppy and big.

The fancy japanese hybrids bloom on old wood and shouldn't be pruned back hard because you'll cut off next year's buds.

And I don't prune my oakleaf hydrangeas at all.

I planted a bed of mixed hydrangeas three years ago and pruned like this for the first time last fall. So far the results look good.