Horticulturists with Iowa State University Extension and Outreach advise that perennials, such as gladioli and dahlias, must be stored indoors during winter to survive. For dahlias, it is recommended to cut the plants to within 2 to 4 inches of the ground within 3 or 4 days of a killing frost and leave them in the ground for 6 to 7 more days. Next, dig the dahlias with a spade or shovel, gently shake off the soil and cut the stems back to within one inch of the crown. Then, wash the dahlia clumps to remove soil and allow to dry for 24 hours.
For more information regarding winter storage of perennials, contact the Hortline at 515-294-3108 or email firstname.lastname@example.org. Horticulturists with Iowa State University Extension and Outreach offer cultural and winter storage requirements for several commonly grown tender perennials.