Annie, "then the dahlias took over..."
These things happen!
Do you have to dig and store your dahlias for the winter, or can you leave them in the ground? I love dahlias, but didn't do very well with storing them, I'm afraid -- they came through a couple of winters stored in our crawl space, then the next year I went to get them out and they were full (and I mean FULL) of some kind of bugs, although I don't remember clearly anymore what kind, just remember they were about crawling away. Yuck! Last time I grew dahlias!