They produce flower buds sometime after leaf out (2-4 weeks afterwards, depending how you "measure" the time) so you can either (a) prune to the ground as soon as possible or (b) prune to the ground after they have bloomed. But this guess only works if you have a normal winter.

Weeeell, this year, who knows exactly what 'normal' will be. This winter weather has been all wacko! Normally, I would expect to be able to prune safely -usually- by mid-May perhaps. That's here. And if you live north of me then normally, early-to-mid June. But it is hard to tell this year; Annabelle's leaf out happened in late February. God! If yours has not leafed out then you can prune now. If you can post your location, others in your area might be able to share their guesses.

If this winter's impact on plants makes it difficult to guess when to prune, play it safe: enjoy the blooms and prune the stems in the Fall? It would be much later than pruning now but, it still would be done this year.

I would also try to find a spot for the hydrangeas where pruning will not be necessary. Consider transplanting them to a location where they can grow without forcing you to regularly prune. If possible. And don't forget to check for suckers.

You could also prune (this year) the 1/3 largest stems all the way to the ground or almost all the way down. Then next year, you could prune the next 1/3 longest ones. And on year three, you could prune the remaining 1/3.

Where are you? Where I am, nothing has started growing other than bulbs, so it would be fine to prune them.

I don't have Incrediball, but I do grow Annabelle, which is another Hydrangea arborescens which has a similar growth pattern to Incrediball. It is planted where the snow dumps off the roof, so some winters all the branches break under the weight of snow. I then prune it to near the ground and it regrows to its full 4' height in just a few weeks and blooms as usual. So I am not sure that it will be easy or even possible to control height with pruning without removing flowers.

Hi,

I'm in NW Illinois, Zone 5.

In zone 5 you prune them down to 6" now or so without any ill effects. IME they will have fewer larger flowers when hard pruned than if not, but they won't be any shorter.