I've overwintered Dahlia 'Union Jack' from Old House Gardens here in zone 6
(East Tennessee) the winter of 2009 and it grew just fine in 2010. It's
overwintering again, due to me forgetting to dig it up, but this winter/spring
is much wetter than normal. We've had so much rain in the last month. I hope
it'll be OK.
We're zone 6 but we usually only get 5 or less nights in the single digits (F)
Susan
E. Tennessee, USA
zone 6