Wait a few days after the foliage is blackened by frost before digging the tubers to store for the winter. If plants are in a frost-free area, dig by mid-November. Cut the stalk to 4–6″ tall, rinse off the soil, and allow the clump to air dry under cover for 24 hours. Line cardboard boxes or terra-cotta pots with newspaper and layer tubers with barely moist sawdust, sand, or peat. Do not store in plastic. Keep boxes cool (40–50°F) and dry for the winter in a dark spot Check for rot or shriveling on a monthly basis; if shriveling occurs, mist the packing material lightly with water. Remove all old foliage from the garden area. Early Spring Dahlia tubers may be divided now. Plant tubers 6″ deep when soil has warmed after frost or start early indoors in pots. Water tubers sparingly once after planting and then do not water until new growth appears. Fertilize with 5-10-10 when growth reaches 2″ tall.
Here're the recommendations from the dahlia society... https://dahlia.org/docsinfo/articles/digging-dividing-and-storing-tubers/
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