I left mine in the greenhouse uncovered when it was 0°C outside and probably 1 °C in the greenhouse and it started growing anyway in march.
I too found it was cold hardy until -7/8 to -10 °C and I really hope that is true because so far it has been my most favorite plant, it barely lost anything compared to a pursha a,d mine is now also flowering (and still growing new branches and leaves all over the place)
I bought a second one because this seems to be one I can easily work with
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Tested this winter yes down to -8C it can survive although autumn growth dies.
Almost all the nurseries in Europe sell Shikuawasha on Alemow rootstock. The rootstock will die at -2C to 0C or before most likely because of root rot. I usually toss to compost or give to someone those plants with Alemow, they all get root problems after few years.
I have my plants on PT.
This year I will graft few plants on Shikuwasha seedlings and test it for hardiness as well. Although on its own it is not resistant to root rot.