I've had good results with leaving my dahlias in the ground every year, and they keep getting better and better. Biggest issue is powdery mildew in late summer.
www.rhs.org.uk/plants/dahli...
rhs.org.uk
How to grow dahlias / RHS Growing Guide
Find out how to grow dahlias, including choosing, buying, planting, watering, supporting, pruning, overwintering and propagating.