Depending on how much light you get will determine how your dahlias will perform. In my experience they need at least 6 hours of full sun a day, morning light is the best as they tend to wilt during the heat of the day. Dahlias originate from Mexico so yes zone 8 is fine, I am in zone 7 and I leave my dahlias in the ground every year with a heavy winter mulch of leaves. I also have morning sun & dappled afternoon shade. Be careful, I started with 50 plants & now I have over a 1,000. Get a few and see how they grow. Lots of luck!