Dahlia bulbs can stand mild frosts, but they will not weather a deep winter freeze. If you live in an area where the ground stays below freezing for much of the winter, you will have to dig up and store your dahlia bulbs to preserve them until spring. You will probably lose a few of your bulbs, but with proper care you can keep most of them safe until the next growing season.
Dig up the tubers before the first hard frost freezes the ground solid. Some dahlia enthusiasts will wait until after a killing frost (a brief frost that blackens the plant but doesn't damage the bulb) but dig it up before the hard frost. Be aware that a killing frost can become a hard frost if it persists. When in doubt, dig up the dahlia bulbs earlier rather than later.
Cut the dahlia plant down so that only a 4 to 6 inch stalk is left.
Stick a spade deep into the ground about a foot from the plant and pull it back from the plant to loosen the dirt.
Repeatedly stick the spade in and pry, making a circle around the dahlia. Once you have completed a full circle, pull the handle back on the spade while pulling up on the stalk to remove the dahlia bulb.
Shake the dirt free from the root group and rinse the dahlia under a hose.
Preparing and Storing the Bulbs
Use a knife to cut back excess roots. Also trim away any rotten spots. When you are done, only the dahlia bulb should remain.
Cure your dahlias. Leave the dahlias in a cool, dry room to eliminate excess moisture. The amount of time it takes to cure dahlias will vary greatly depending on your climate, so ask local dahlia growers how long to wait. If the dahlias look wrinkled and dried out, you have let them dry for too long. Spray them with a bit of water to restore them.
Place your dahlia bulbs inside a plastic storage bin and cover them with vermiculite or peat moss.
Keep the dahlia bulbs in a cool, dry place. Ideally, you should store them somewhere where the temperature stays between 40 and 50 degrees F.
Open your dahlias about once a month and check the bulbs for rot. If any are rotten, either discard them or cut away the rotten parts.