That does look like it could be Dahlia.
The trick for survival for the Z8 plants (plants with Z8 as their lowest reliable survival zone) is to prevent ground freeze. If you do that, the actual weather doesn't matter. Called zone-cheating, zone-pushing, exploiting a microclimate. There are a LOT of individual tactics that can be used to try to accomplish this.
In Z7 and below, the ground freezes. In Z8 the ground does not freeze. Not a guaranteed pattern every winter, but something we can use to cheat higher zone plants in lower zone places. This is the most dramatic dividing line between temperate zones, IMVHO. Once a plant requires Z9+, it's not going to live through frequent frosts that go on for 2-3 months, and a spell of nights cold enough to make the dog's water bowl turn to ice, even if that's not enough to freeze the ground. Winter here usually has at least 1 of those spells. So it's much easier for you to use Z8 plants than it is for me to try Z9 plants. Look for other Z8 plants if you've discovered a microclimate &/or want to experiment with zone-cheat techniques.
