Overwintering Geraniums Indoors
The popular zonal geranium is actually a pelargonium (Pelargonium x hortorum). It is sold as an annual, but in reality it is a tropical shrub and can live for decades where the winters are never cold (zones 10 and above). Therefore you can keep it from year to year if you protect it from freezing. You even have two options: put the plant into dormancy or grow it as a houseplant.
To stimulate dormancy, knock the soil off its roots and hang it upside down in a cold room. Or you can leave it in its pot, cut it back to about 4 inches (10 cm) high and cover it will a paper bag. In both cases, you’ll need a very cool room, about 45 to 50˚F (7-10˚C). This technique was popular during the time of our grandparents, when a cold room for storing vegetables was the norm, but today even an unheated basement is no longer all that common, so more and more gardeners use the alternative…
And the alternative is to grow the plant as a houseplant over the winter. Bring it indoors in autumn before it is damaged by frost and simply grow it in a pot like any other houseplant. (You can also bring in and root a cutting if you prefer overwintering a smaller plant). For the best bloom, give your pelargonium full sun or at least the best light you can. Otherwise it only needs typical houseplant care: water when the growing mix is dry to the touch, normal indoor temperatures, normal indoor humidity, reduced fertilization during the winter months, etc. In general, it won’t flower as generously as it did outdoors, but a plant with 3 or 4 stems will still probably be in bloom much of the winter.
Have fun with your pelargoniums this winter!