@driftless here are my thoughts.
For 6a i think JT-02 is the #1 safest option. Tony had success with it but eventually heavy losses at Omaha, you might want some americans like Prok or Geneva Long in those borderline locations.
Also, i read of a eastern european test that Pamjat Pasenkova is hardier than any of the Gora series which were all killed in that test, but Pamjat Pasenkova did fine i would trust it in 2012 6b maybe into 6a, though JT-02, JBT-06, and NB-02 when established are what i would prefer first in 6a.
Never believe those nurseries when they say "zone 6 hardy for kaki and nikitas gift etc…
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