I'm in Zone 5. I've not tried any of the current catalog listings, but some years ago I planted out B. 'Torsa', a cross of B. grandis basically selfed. It didn't survive, put it on the NW side of the house, I have since thought I might have tried a southern location, or mulched it better, or had deeper snowfall for protection, etc.,etc., but I am just skeptical that there is such a thing as a begonia hardy to Zone 5. Big Dipper Farms in Washington State will list begonias, but only as 'informational', won't sell them anymore, in other words. More than 25 years ago an English friend told me Kew was experimenting with B.formosana for hardiness, apparently nothing to that after so many years.