Wisconsin’s chilly climate, which falls into U.S. Department of Agriculture plant hardiness zones 3 through 5, overpowers all but the cold-hardiest of plants, making winterizing most ornamentals a must. Container gardening lends itself well to overwintering most tender perennials, such as zonal geraniums (Pelargonium x hortorum), which thrive in USDA zones 10 through 11 -- in colder zones, you simply take them inside when the temperature drops. However, zonal geraniums with a lot of leggy growth overwinter better as cuttings, since you have to prune the plants back by one-third to one-half before bringing them indoors. The state’s first and last frost dates, which range from September 21st to May 10th, make overwintering geraniums in Wisconsin a bit different than overwintering them in other locales.
Cut several 3- to 4-inch-long pieces of the healthiest stem tips from the healthiest geraniums just before the first frost of the season, using sterilized pruning shears for the task. Make the cut on each stem tip just below a leaf node, the bump from which leaves emerge. Pull away the leaves from the lower half of the cutting.
Spread 1 tablespoon of rooting hormone powder containing IBA (Indole-3-butyric acid) on a plate in an even layer. Dip the cut end, or basal end, of each cutting into the root hormone powder, and shake off the excess.
Insert the bottom half of each cutting in a 6-inch-deep pot filled with equal parts sphagnum peat moss, perlite and coarse river sand. Water the growing medium until water drains from the pots, and cover each with a plastic bag. Wrap a rubber band or tie a piece of twine around the pot to hold the bag a few inches above the top of the cutting so that it has room to grow.
Place the geranium pots in a bright windowsill, ideally one that faces east so that the cutting gets the morning sun. Spray a fine mist of water on the growing medium a few times a day with room-temperature water from a spray bottle to keep it moist at all times. Remove the bag after two to three weeks.
Dig the geranium cutting up after six to eight weeks, and transplant it to an 8-inch-deep pot, this time filled with a mixture of equal parts compost, sterilized loam and perlite. The addition of compost and loam helps condition the geranium to outdoor soil.
Place the geranium outside for a few hours each day during the week leading up to the last frost of spring, a technique referred to as "hardening off." Hardening off geraniums helps them make the transition from inside to outside and lessens the stress the move places on them. Place the pot outside to stay after the threat of frost passes in your area.
Dig up the geraniums, and shake the soil from the roots just before the first expected frost of fall. Trim back any diseased, damaged or dead roots to healthy ones. Tie one end of a piece of heavy-duty twine around the roots of each geranium.
Tie the other end of the twine to a rafter or beam in a cellar, basement or attic that maintains a constant temperature between 45 and 50 degrees Fahrenheit. Tie the twine so that the geraniums hang upside-down, suspended in the air. Keep the light off in the room at all times.
Take the geraniums down from the rafter or beams two or three times during winter, and soak the roots in room-temperature water for an hour or two. Hang the geraniums after soaking.
Take the geraniums down after the threat of frost passes in spring. Trim back all the dead or withered stems and leaves, using sterilized pruning shears for the task, and plant the geraniums outside.
Prune back geraniums in containers to one-third to one-half of their total height, using sterilized pruning shears for the job, just before the first frost of the season. Pruning minimizes the leggy growth that occurs inside during winter.
Place the geraniums in a sunny spot in room with a temperature that stays consistently between 60 and 75 degrees Fahrenheit.
Pinch back new growth to shape the geranium and keep any weak stems in check. Water the geranium to a depth of about 1/2 inch when the soil feels dry to the touch. Take the geranium back outside after the threat of frost passes in spring.