Digging, Dividing, and Storing
Here in North Central Washington our dahlias need to be removed from the ground in order to save them and replant them in the spring. Tubers left in the ground will freeze and will die. We dig up our tubers starting in mid-October with the goal of keeping them in the ground for as long as possible to keep them happy, but not so long that they freeze beneath the ground.
Tubers that know winter is coming will store better than those on plants still producing flowers. Sometimes we get a "killing frost" that turns the foliage black, but most of the time we cut our plants down to about 6" and let them sit in the ground for about a week before digging. This allows the tubers to harden off, and makes the eyes (next years stalks) easier to see when you divide.
Digging and Washing
Sometime around Halloween, depending on the weather, it's time to dig. Loosen the soil 18" or so around the base of the plant then lift. We find a pitchfork works best to avoid damaging tubers. Be prepared to divide and store your dahlias within 24 hrs. Tubers left on concrete garage floors too long will feel soft, which means they are drying out. Think of a carrot left on the kitchen counter overnight. If you choose not to divide right away, putting clumps on a tarp or in garbage bags can buy you time.