The growing season for most annuals comes to an end with the first frost in fall. However, you don't have to let the first frost destroy your geraniums. Geraniums can be overwintered indoors by potting up individual plants, taking cuttings, or storing bare-root plants in a cool, dry place. Regardless of the method, the plants should be removed from the garden prior to the first frost.
Carefully dig up each plant and place in a large pot. Water each plant thoroughly, and then place the geraniums in a bright, sunny window or under artificial lighting. Geraniums prefer cool indoor temperatures. Daytime temperatures of 65 to 70 degrees Fahrenheit and slightly cooler night temperatures are ideal. During their stay indoors, water the plants thoroughly when the soil becomes dry. Geraniums are likely to become tall and lanky by late winter. In March, prune back the plants. Cut the geraniums back by one-half to two-thirds. The geraniums will begin to grow again within a few days and should develop into nice specimens by May.
Using a sharp knife, take 3- to 4-inch stem cuttings from the terminal ends of the shoots. Pinch off the lower leaves and dip the base of each cutting in a rooting hormone. Stick the cuttings into a rooting medium of vermiculite or a mixture of perlite and sphagnum peat moss. Pots and flats with drainage holes in the bottom are suitable rooting containers. Insert the cuttings into the medium just far enough to be self-supporting. After all the cuttings are inserted, water the rooting medium. Allow the medium to drain for a few minutes, then place a clear plastic bag or dome over the cuttings to prevent the foliage from wilting. Finally, place the cuttings in bright light, but not direct sunlight. The cuttings should root in 6 to 8 weeks. When the cuttings have good root systems, remove them from the rooting medium and plant each rooted cutting in its own pot. Place the potted plants in a sunny window or under artificial lighting until spring.
Bare Root Plants
Dig the geraniums and carefully shake all the soil from their roots. Then place 1 or 2 plants in a large paper grocery bag and store in a cool (45 to 55 degree Fahrenheit), dry location. An unheated bedroom or indoor porch might be a suitable location. An alternate (somewhat messier) method is to hang the plants upside down in cool, dry location. The foliage and the shoot tips will eventually die. In March, prune or cut back each plant. Remove all shriveled, dead material. Prune back to solid, green, live stem tissue. After pruning, pot up the plants and water thoroughly. Place the potted geraniums in a sunny window or under artificial lighting. Geraniums that are pruned and potted in March should develop into attractive plants that can be planted outdoors in May.
The overwintered geraniums can be planted outdoors in May (after the danger of frost is past). Before planting, harden or acclimate the geraniums outdoors for several days. Initially, place the geraniums in a shady, protected location and then gradually expose the plants to longer periods of sunlight. Plant the geraniums in the garden after the plants have been properly hardened.