Because dahlias aren't winter-hardy, they must be removed from the ground prior to winter, then prepared and stored until they can be replanted the following spring. Dig tubers right after the first frost; if they are too green, they won't store well. Label either the box or the dahlia tuber so that you will remember the variety when it comes time for spring planting.
Cut the stalks of the dahlia plants down to 4 inches in height.
Begin digging into the soil, using the gardening fork, one foot away from the plant, in a circle around the plant. When you have dug all the way around the plant, insert the gardening fork into the soil and gently lift, or pry, the tuber from the soil.
Cut the stalk the rest of the way, down to the tuber. Rinse the soil from the dahlia tuber, gently rubbing if needed, but being careful not to rub the skin from the tuber.
Hang the tubers upside down until completely dry. This could take up to 3 weeks.
Moisten, just slightly, the coarse sand. You don't want the sand to be wet, just sprinkle a small amount of water over it.
Line the cardboard box with newspaper. Place a 2-inch layer of sand on the bottom of the box and place some tubers on top of the sand. Don't allow them to touch one another. Pour another layer of sand over the tubers and continue layering tubers and sand. Cover the box and place it in a dark area with a temperature that will remain between 40 to 45 degrees F.
Check the dahlia tubers once a month. If they appear to be shriveling, mist them lightly with water and allow to air-dry before placing them back in the sand.