Popular garden geraniums (genus Pelargonium) are traditionally considered an annual and are not frost hardy. However, with a bit of care, geraniums can be brought indoors in fall and overwintered successfully.
Garden geraniums prefer environments with moderate temperatures, ranging from 40 to 85 degrees Fahrenheit. Seeds can be started midwinter indoors at temperatures of 75 F. Indoors, geraniums thrive at daytime temperatures of 65 to 70 F and night-time temperatures of 55 F. Keeping geraniums in cold, drafty or excessively hot areas should be avoided. At temperature over 80 to 85 F, plants lose chlorophyll and suffer decreased growth.
Geraniums can be overwintered using one of three options. Repot plants indoors before the first frost and place by a sunny window, so they ideally receive 10 to 12 hours of light. Alternatively, preserve cuttings in rooting hormone and then plant indoors in sand, perlite or vermiculite until rooted. Lastly, geraniums can be put into dormant storage by removing all soil and hanging upside-down in a cool basement.
Because geraniums are not hardy, do not put replant overwintered plants outdoors until safely after your last frost date, typically in late May. Geraniums that suffer from cold damage have red foliage and produce little new growth. Also, do not confuse your garden geraniums with plants from the Geranium genus; although from the same family, the latter are hardy perennials.