Zonal geraniums are not considered hardy perenniasl in most of the United States, but you can safely winterize them if you follow a few simple directions. Zonal geraniums, ivy and scented geraniums are the most likely to thrive over the winter. Geraniums grown in pots are the easiest to winterize. In mid-autumn, simply move your pots indoors into a frost free room with plenty of light. Geraniums grown in pots enjoy being slightly root bound and tend to bloom better because of it.
If your geraniums are in the ground, dig them up in the fall. Try to do this before the first frost, but if you get them right after the first frost they will still most likely survive. Gently shake the geraniums to remove soil from the roots, and cut the stems back to about 4 inches, just above a series of outward facing nodes. Trim roots if necessary to make them fit into pots. Plant in trays or pots in potting mix. Save the parts you cut off to propagate for new plants.
Water your geraniums as soon as you bring them indoors or pot them. After that, only water when the soil is dry to avoid root rot. Try to keep them in a room with temperatures between 45 and 70 degrees Fahrenheit. Keep them nearly dry and feed them half strength fertilizer once a month. If you don't have a sunny room to keep your geraniums in, you can put them in the garage or a cold basement and let them go dormant. Cut them back before doing this and make sure they don't go completely dry.