You can overwinter dahlia tubers in the ground over winter in mild areas and in very free-draining soil.
After the dahlia flowering season is finished, leave the stems to go black from the first frost for approximately a fortnight to allow the sugars to go back down the stems to feed the tuber. Then cut the stems off just below ground level, mound up the soil or add compost and mulch with straw or bracken to protect the tubers over winter.