To save geraniums over winter, dig the plants up in the autumn before the first frost. Shake the dirt off the roots, and hang the plant upside-down on a rafter in a location with a temperature above 50 F, such as a basement. Soak the roots in water several times over the course of the winter. Allow the roots to dry slightly before rehanging the plants. Plant the geraniums after the last expected frost the following spring.
Before storing the plants, spray them to remove pests, and cut away black or rotted roots. If black or rotted roots develop over winter, cut the damaged roots away as needed. To quickly store geraniums when rafters aren't available, dig the plants up, and place them on the floor in an out-of-the-way location.
Maintain temperatures above 50 F, and water the roots lightly during winter. In spring, slowly introduce the geraniums to normal watering and light before planting.
Alternately, take 4-inch cuttings from the geranium in autumn. Dip the cuttings in rooting hormone before planting them in an appropriate growing medium. Drape plastic over the cutting, and store the cutting in a warm location with indirect sunlight until it is well-established. Transplant the cuttings to the garden after the last frost of the season.