Answer: In the Pacific Northwest, Dahlia tubers are dug in the fall and stored because they will rot in water soaked soils. Since you're in eastern Washington, the "dry" side of the mountains, you can leave them in the ground all winter, as long as you know the soil drains well. They should be just fine, especially with a few inches of mulch over the top of the soil.
Best wishes with your landscape!
Q&A Library Searching Tips