im in zone 8b in western oregon , and I bought some cheap ( dollor store ) dailias 2 years ago and they stayed in the ground with no problem in heavy clay soil even though last winter was the coldest in 40 years (-10f)

I just go some show quality dahlias and am planing to leave them in the ground my house to to warm to store them over the winter and do not have any frost free outbuildings

\I dug the holes deeper and put a few inches of lava rock under them to improve the drainage in winter when it rains non stop , and mixed a bunch of manure rich compost and leaf much and peat into the soil that I willed the holes with and planted them 5-6" deep

I plan on covering them with plastic this winter to keep them drier and cover the plastic with leaves and hay to keep them warmer

anyone have any better ideas