This was a study done in Crimea after the extreme 2005-06 winter.
Temperatures reached -13 F, -25 C and remained between 5 F to -4 F
(-15 to-20 C) for 3 days. Most kakis froze to the ground and some resprouts
did not survive the next growing season.
The hybrids that did best were F1 rossiyanka followed by F2 Nikitas Gift.
Both varieties produced fruit in 2006 although sharply reduced yields were
noted due to fruit drop. Nikitas Gift showed some dieback on one and two
year old wood.
For kaki the variety which performed best is named “conical” – "Конічна"
followed by varieties “Valentina” and “Abkhazia” in that order. Derevyanko
summarizes that since these 3 varieties showed some hardiness they can
likely be grown in areas outside of Crimea since similar winters are likely to
occur only once every 40-50 years.
F3 ‘Гора Говерла’ did not fare well with dieback to scaffold branching and
even the trunk resulting. They call rossiyanka F1 rossiyanka 18 for some reason,
I do believe its the regular one we are familiar with.