Dots
Dahlia corms don't come back in this zone because they aren't hardy below 5 degrees, I think (I'm not exactly sure how much cold they can stand. The ones I had were next to the wall, facing east, and next to the dryer vent. They lasted 2 or 3 years there.) So if it goes to zero, they die.

My sister's house is next to an alley, and people used to go by and cut off the tops of flowers all the time, until she decided not to plant them there any more.