Just a note to agree with Uli, we've grown Dahlias for 20 years now, and some just die for no particular reason. The tubers look fine, but nothing happens. We used to store them quite carefully, and that helped. Due to shortage of time the last few years we've just had them on a table in the garage, and lost more of them. The ones that survive seem to be the older kinds without names that we've been given by other gardeners. Generally speaking Dahlias do very well up here in Sweden. /Neil