Bruce asked this on the Geranium phaeum thread and I thought it might get missed
Can I take hydrangeas cuttings now and winter them in the greenhouse ready for next year?
This info from the RHS should help - https://www.rhs.org.uk/advice/profile?pid=122
All my hydrangeas are paniculata types as they flower on new wood and get pruned back in spring to encourage new stems and more flowers. They're all in flower now so will be left alone but next spring I shall prune them and use those stems as cuttings.
I don't grow the others as they flower on old wood and that usually gets frozen to bits in my winters so I end up with dead 'uns. I expect a few cuttings taken now form non-flowering stems may well take over winter.