We purchased a Dahlia imperialis more than three years ago, as an experiment. It blooms in mid to late November. In past years, we have used it as a barometer to know when we should receive our first frost of the year, since the plant has set buds as early as Halloween and we know the frosts come soon after that. They did not make it past the first frosts for the first couple of years.
This year by comparison, now in late November, we have not had any freezing temperatures as of yet, and our Dahlia is, for the first time since we bought it, in bloom!