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My bergenia (Bergenia cordifolia), it survived this winter with a low in the -30C range, more few dozen days below -20C. (no cover no other protection, ~60cm snowfell this winter and what did the job )

Hi,
You should give a chance to Phyllostachys bissetii, it is cold hardy down to ~ -25°C!
(some of them down to -30°C, if -30°C freezing cold do not last too long)