Just so as not to discourage cold-zoners, I'd just like to mention out that D. orientale is hardy to zone 2 (my mother grows it on the cold prairies ;) and D. pardalianches is hardy to zone 3. (I grow both species here in zone 3.) I can attest, too, to their longevity.
For the record, the hardiness zones I mention in my articles are based upon the RHS and several other popular garden encyclopedia...often these books have a tendency to be cautious in their hardiness ratings. Perhaps it is because the books are written more for Europe or the US than Canada...the writers are probbaly not fully aware of just how hardy some of these plants are. However, in my articles I would rather stay on the side of caution than be too optimistic...my brother in Calgary grows D. pardalianches with great success...Calgary is rated zone 3. Who am I to disagree with the 'experts', hence I stuck with the hardiness rating of zone 4 for D. pardalianches. It is great to hear of others who have found a particular plant is hardier than documented.
Another factor to take into account is just how accurate the hardiess zone for a given area really is. As an example, St. John's (my home town) is rated zone 6 in the USDA stsyem but zone 5b in Canada (a reminder than most of the encyclopedia follow the USDA system)...zone 6 can drop to -10 F...I cannot recall it ever getting that cold in St. John's. In reality we are closer to a USDA zone 7 (in fact I grow several plants that are rated to that zone). It IS possible that those who grow a given plant is an area colder than that rated for that plant MAY be in a warmer microclimate...there are probably pockets of zone 4 within Calgary's zone 3...just food for thought!