Yes, and you have to dig deep in some places to get below the frost with your foundation. In those places a basement makes sense because you’re digging that far either way. Texas frosts don’t get very deep, so you’re able to have a shallower foundation making a basement just an extra cost.
The op has it right. Definitely dahlias. I can see why you’d think zinnias though. It’s interesting how different (or similar) we can make plants look with dedicated breeding efforts.