I like tomato cages for Dahlias, I found them to work quite well. Some folks turn them upside down, and cut the legs off so there aren't dangerous poking things sticking up, and then make hooks to anchor them to the ground. I've just used them as for tomatoes, but I think some prefer the cage to be smaller at the top for Dahlias. Considering the shape of the mature plant, I guess that makes sense.
