Dahlia bulbs are best overwintered in a basement if there is one. The room must be as cool and dry as possible. If you don't have a basement, a garage or garden shed might do the trick. It is important that the tubers of georgines do not get frost.
The dahlia should be wintered dry and cool in the basement
Overwinter in a cool, dry cellar
A basement that does not get warmer than four to eight degrees and that is very dry is ideal for overwintering dahlias or georgines.
Do not place dahlia bulbs too close together on wire racks or in uncoated wooden boxes.
Alternatively, you can put the tubers in plastic bags and hang them up. Cut large holes in the bags beforehand so that the dahlia bulbs get enough air.
tips and tricks
Allow the dahlia tubers to dry well before storing, especially if you have a damp basement. Dry tubers do not rot so quickly.