How do I over-winter dahlias?
Cut back the plants to within 2 to 4 inches of the ground within three or four days of a killing frost. After cutting back the plants, leave the dahlias in the ground for an additional six or seven days to “cure.” Then carefully dig up the dahlias with a spade or shovel. Gently shake off the soil, then cut the stems back to the crown. (The dahlia crown is located at the base of the stem where the tuberous roots are attached.) Carefully wash the dahlia clumps to remove any remaining soil. Allow the dahlias to dry for 24 hours. When dry, place the dahlia clumps upside down in cardboard boxes and cover them with vermiculite, peat moss, or wood shavings. Store the dahlias in a cool (40 to 50 degree Fahrenheit), dry location.