I am not aware of any variety or cultivar of Hydrangea quercifolia considered to be any more winter hardy than another. Virtually every source you will find, including Dirr, lists them as a zone 5 plant regardless of selection, but not lower.

That is not to say that the siting of your Ruby Slippers isn't being influenced by whatever microclimate may be available in that location that is allowing it to survive. But I couldn't speak to the same degree of success with any other placements nor if your current Ruby Slippers will continue to survive your winters and thrive. Maybe.......maybe not :-)

And maybe your zone is not as severe as you think or is undergoing some modification that is making winters less harsh. But at best I would consider it to a be a marginal plant for your area.

The best thing about oakleafs growing in colder climates is that they will almost always flower. Even though flowers are produced on old wood, winter cold does not appear to have the same damaging effect on latent flower buds as it does with macs. Stem dieback from cold is a different matter - that can and does happen in colder locations - but if the stems survive the winter (no dieback), chances are very good that they will produce flowers.

Thanks that's helpful.

I think the zone is still zone 4, but it may be that our microclimate is more tolerant. We're basically on the side of a gentle 1-2 mile slope, so we don't get mountaintop or hollow conditions.

I was hopeful that the ruby slippers survived this past winter. I did mulch it well. However we had a terrible cold snap in January (worse than average)- it was below 0 for days and easily got below -20 several times. The plant had only been in the ground since the fall. I really like the plant, and figured that if it could survive those conditions it may be that I can plant more.

I have no idea what kind I have but we are on year 4 with them -- did not flower until the third year and this summer (year 4) I only got to blooms. They appear to me to be kind of delicate plant (thin branches, prone to breakage). I am in SE MI -- I am going to wrap them in burlap for the winter this year and see if that makes a difference next summer. I find they are very slow growing -- the landscaper that put them in said they get 8-10 feet -- well, I hope I still there live and get to see it. Here is a pic from this summer.

This is the ruby slippers I put in last fall that has less than a year in the ground. The comments here and on the post I started about a new shrub border are making me think either I got lucky and found a sheltered spot- or maybe ruby slippers is the answer to my original question.