I have used sand, peat moss and vermiculite over a decade for winterizing the dahlias. Would using shredded paper, not newsprint, also a good medium for winterizing. I live in Zone 8B.
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The American Dahlia Society recommends storing tubers in vermiculite.
Since you are in zone 8b, you can leave your tubers in the ground and after frost kills the foliage, cut the plants back and cover with 3 to 4 inches of mulch.