Dahlias work great in containers and borders!
Even though few border/container dahlias appear in the exhibition circuit, they have quite the draw with the general public, and their demand appears to be increasing exponentially.
These dahlias are usually 12-20 inches high and require very little if any support. Outstanding new fully double dwarf dahlias are making it to market. And there are many other low growing dahlias that are well suited for the border or the container.
Use the dahlia, especially the color, as the focus and other plants as a complementary medium. For many, containers are the only way a dahlia enthusiasts can have a garden. Using containers can enable a gardener to have plants on the roof of an apartment house in Manhattan.
A number of commercial growers offer border/container dahlias. Find them online or at your local garden center.