Probably hold the horses. Here in Texas, a plant that has not leafed out from the base by mid May is probably not going to. I have one Annabelle that may get the boot after this rough winter. It has not leafed out and shows no new growth from the base.

I also started pruning dead looking stems from mopheads that had new growth only from the base 1 or 2 weekends ago and found that lots of them were green so I stopped cutting them and decided to wait until later in the month (to see if they would finally leaf out). You may want to wait to prune them until the end of month since you are so far up north.

If by month's end you have no greenery of any type (no leaf out from the stems or from the base), the chances of a recovery are slim. If the stems do not leaf out by month end, I see no way that the stems will recover in another year. If the plants does not produce new growth from the base by month end either then I see no way that the plant will recover and leaf out on another year.

In the meantime, maintain mulching operations to keep 3-4" of mulch year around and maintain the soil moist as constant as possible.

For those who are eager to find out what will happen with the dead looking sticks, one thing you could do is prune the dead looking sticks in 1" increments from the ends to the base until you either get to the bottom or until you hit green. The problem with this idea however, is that while pruning, you could cut off flower buds that certain hydrangeas produce at the end of the stems. But if your hydrangeas bloom on new wood or your hydrangeas rebloom, you may still get bloomage in the summer.

Luis

3' is a fine height to prune. But a bad enough winter unprotected would whack them back to the ground either way, no matter what the original height. I'm in zone 6b and am afraid all ~50 of my macs got it.

Replace any with no growth at all, but going forward consider mulching heavily up to 2+ feet in height before the brunt of winter, with heavy leaves, straw, or whatever else your landscaper can get his hands on. If you can get buds as high as 1' to perform and get a lot of crown shoots going, the plants can survive longer and recover quicker.