Dahlia imperialis is the tallest of the genus. In Europe it is mainly grown for its very decorative leaves similar to a tomato plant, and also because of its imposing height which can reach from 3 to 5m in one growing season once the plant is established. This humongous height gies it its common name of "tree dahlia". Indeed in its native Mexico and Latin America it is a tree that can grow up to 6m in height forming woody trunks. 
 
In Europe however it behaves as a dieback perennial regrowing from the ground stump every season. 
 
This has proven hardy in my nursery showgarden up to -10°C with heavy mulch and protected against winter wet. Protection against field voles is necessary as they seem to love this species.