To overwinter a geranium means to bring the plant indoors where it remains alive throughout the winter. It may or may not bloom during the winter, but either way it should thrive, and you’ll be able to replant it in the spring.
When overwintering geraniums, bring them inside, cut the stems back by about 6 inches and put the potted plants in a cool, dark place or put the plants in a netted bag that can be hung. You can also put the potted plants in a sunny location, where they may bloom later in the winter.
If you notice over the winter that your geranium is starting to mummify, immerse it in tepid water, which will rehydrate it. If your plant is in a dark spot, start exposing it to sun in February.
At this time, you may want to introduce some fertilizer. Fertilize once a month, beginning in February, until you take the geranium back outside when the chance of frost has passed.
Turkey Creek Lane recommends that when you bring your geraniums up from the basement or from the dark area where they’ve been overwintering, do not take them outside immediately. Put them in your garage for two days or somewhere in the house with sun exposure so that they can gradually acclimate to the sun.
After two days, take them outside and put them next to a fence or some kind of sheltered structure. If the geraniums are exposed to too much sun too quickly they may not survive.
You may want to re-pot your plants before taking them back outside, using new, all-purpose potting soil. If your plants are spindly and leggy, cut them back some more.