Dahlia Imperialis, and they are fun to watch grow if nothing else. In zone 5 you will have a challenge to get them to bloom, but it's sure fun trying.

Here are sources from the Big List
* TREE DAHLIA Alpen Gardens
* TREE DAHLIA 3 VARIETIES Corralitos Gardens
* TREE DAHLIA 3 VARIETIES Plant Delights Nursery
* TREE DAHLIA IMPERIALIS Alpen Gardens
* TREE DAHLIA IMPERIALIS Dahlia Dandies
* TREE DAHLIA IMPERIALIS Eagle Dahlia Farm
* TREE DAHLIA IMPERIALIS Love House Dahlias

Here is a link that might be useful: Find them all at the Big List