I'm fairly new to northern gardening and absolutely LOVE hydrangeas, but have a few questions if you could help.

I planted 3 "Bonnie Blue" hydrangeas 2 years ago which have seemed to freeze to the ground this past winter and in 2014. They are coming back, however; I have 3 other unknown hydrangeas (they were here when we bought our house) that lose their leaves in the winter, but the stems don't die and they come back at that height each spring. I also have one other established hydrangea (unknown) that also freezes to the ground.

Why is it that some hydrangeas come back on last year's stems, but others freeze all the way to the ground? It doesn't seem to matter where they are located, as some are close to the house and others are not, so protection from cold might not play into this.

Are all hydrangeas supposed to come back on last year's stems? If so, does that mean I should be protecting the ones that die down to the ground from the winter cold?