Zonal and ivy geraniums (Pelargonium spp.) can survive a light frost, but when temperatures drop below 26 to 30 degrees Fahrenheit, significant leaf and stem damage occurs. Geraniums are native to southern Africa where they grow in sunny locales in a moist to dry, well-drained soil.
If you grow a particularly showy geranium that you like or spent a fair amount of money to obtain, digging and re-potting the plant indoors before a killing fall frost arrives makes sense. Conversely, if the geranium is common and inexpensive to replace, it may not be worth the time and effort to bring plants indoors to overwinter. Just plant newly purchased seedlings next spring.
There are three options for overwintering geranium plants according to Cooperative Extension publications from both Illinois and Iowa State Universities. Dig the entire plant up and plant it in a container to bring indoors; take stem cuttings and root them in small containers to grow over the winter months; or dig up plants, wash off all soil and hang them to air-dry in a cool, dry place. The last option is the most difficult to complete successfully for healthy plants come spring.
In order for geranium plants to remain healthy over the winter when brought indoors they must have as much direct sunlight as possible. A bright window in a cool, unheated room with temperatures of 50 to 60 degrees Fahrenheit is ideal. Warmer rooms lead to "leggy" plants. Also, do not over-water geranium plants in the winter–keep the soil on the dry side.