Zonal geraniums (Pelargonium x hortorum), hardy in U.S. Department of Agriculture plant hardiness zones 10 through 11, definitely do not grow as perennials in Wisconsin. Cheesehead geraniums must wait until late May to move outdoors and seldom live through October in Wisconsin’s continental climate, but they can come indoors before fall frosts, bloom throughout the winter and be ready for spring long before new plants appear in nurseries. Because of Wisconsin’s long winter and unpredictable spring, they’ll need lots of light and some extra attention.
1 Wash your potted geraniums' leaves with insecticidal soap and a fine mist of water as the weather cools. Take them indoors at least two weeks before the weather gets cold enough to turn the heat on in the house -- anytime from mid-August to mid-October, depending on which end of the state you inhabit. Flush the soil until water runs freely out of the bottom drainage holes to wash through any insect eggs.
2 Cut the geraniums back a quarter to a third of the way with a sharp knife or hand pruners to keep them compact. Remove all the flowers to compensate for the loss of sunlight.
3 Make new plants if desired by taking the top 3 to 4 inches of the trimmings. Clear all but the top few leaves off the branches and dip the cut ends in a bit of rooting hormone. Make a hole in moist potting mix with a pencil and put the cutting in so at least two growth nodes -- the thick places where leaves and branches start -- sit below soil level. Put three or four cuttings in a 4-inch pot.
4 Put the geraniums and any starts indoors and set them in an east-facing window. Keep the soil of the starts moist until they begin growing. Water parent plants and growing starts only when the top inch of soil becomes dry. Do not fertilize geraniums over the winter.
5 Prune the plants again when they begin flowering in late January through February. Use trimmings to start more new plants, if desired. Allow them to flower after that.
6 Wait until the last average frost date -- from late April to early June -- to move the geraniums outside during the day. Bring them in at night until nighttime temperatures stay above 40 degrees Fahrenheit.