It is still fine to plant in this zone. Make sure to leep well watered until the ground freezes (but don't over-water) and mulch the area well. I don't use root stimulators, so can't offer any advice regarding that.

What type of hydrangea are they? Reason this is important to note is regarding need for winter protection. If they are paniculata type or Annabelle, there is no need for winter protection, they bloom on new wood.

If they are macrophylla type, you probably want to winter protect, as they bloom on old wood - lots of threads on winter protection on this forum.

If they are oakleaf types, I advise winter protecting for the first season or two, after that (fully established), they don't require winter protection in my experience.

Yea, it's raining again here today - pretty hard, too. Fall rains are good for the garden, I look at it that way :0)

I planted the Tardiva today, the Quick Fire still needs to go in the ground. It was been a horrible Fall, seems like it rains every weekend. All the macs were planted or moved over a month ago (I don't have any macs that bloom only on new wood, waste of time here).