After digging up your dahlia tubers, remove all soil from them, mark each tuber according to its variety, coat each tuber with fungicide, place the tubers in produce bags lined with vermiculite and store them in a cool, dark place. Check your stored tubers about once a month, as some tubers inevitably rot and require removal.
Make sure that your produce bags have small holes that allow air circulation. Using solid plastic bags with no holes encourages fungal growth, even on tubers coated in fungicide.
Sphagnum peat moss, wood shavings, sawdust and coco coir are suitable alternatives to vermiculite. Any inert material that readily absorbs moisture to prevent fungi from growing makes a good dahlia storage material.
Keep dahlia tubers in a room that stays cool but that does not get extremely cold. While high temperatures provide a hospitable environment for fungi and other microscopic pathogens, freezing temperatures can ruin tubers by rupturing cell walls and killing the plant. An ideal temperature range for dahlias is between 35 and 50 degrees Fahrenheit. Wide swings in temperature can also lead to dead or rotten tubers.
Darkness is key for preventing rotten tubers. When keeping tubers in a room with regular light exposure, store them in an opaque container. These precautions are not necessary when storing tubers in a permanently dark room, such as a crawlspace or basement.