Gardeners buy geraniums by the six-pack every spring, then throw them out in the fall. Unlike petunias and impatiens, however, geraniums are actually perennials, which means that their life cycle lasts for more than just one or two years. They're too tender to stay outside in freezing weather, but if you bring them inside, you can keep them alive for many years.
Bring potted geraniums indoors before the weather turns consistently cold. If your geraniums are in pots, bring them in as is. If they've been in the ground, dig them up and shake the dirt off their roots, then pot the plants, using soil mixed for container plants. Cut each plant back by a third to a half.
Place the pots on a sunny windowsill in a cool room (a daily temperature range between 12.7 and 18.3 degrees C is best). If you don't have any south-facing windows, use a grow light in the evenings.
Water plants only after the top inch of the soil has dried out. If plants start getting "leggy" (too much stem between pairs of leaves), pinch the stem back to right above a pair of leaves.
Keep the pots indoors till the time the local nurseries start selling new geraniums, then return them outdoors to their summer home.
Rather than keeping whole plants, take cuttings of your favourites and root them for the following year.
Take 3-to-4 inch sections of stems, removing any leaves from the bottom 2 inches. Dip the stem in rooting hormone, then place the cutting in a small pot of perlite. Water the plant and then place each pot in a plastic bag. This creates a "mini-greenhouse" that will keep the cutting moist. Open the bag occasionally to make sure the perlite is still moist, and add a little water, if necessary.
The cuttings will root in six to eight weeks. When they do, move them to regular potting soil and put them in a sunny spot, supplemented with grow lights if necessary, until it's warm enough to move them outside.
If you don't want to keep geraniums as actively-growing plants, let them go dormant. You'll need a cool space (7.22 to 10.0 degrees C), like a basement or attached garage.
Remove the plants from the soil, knocking most of the soil off the roots. Place the bare-root plants in open paper bags or hang them from a rafter, root-side up.
Once a month, take all the bare-root plants out and inspect them, discarding any with stems that have shrivelled. Put them in a bucket of water for about two hours, then shake off the extra water and return the plants to their resting spots.
In early April, pot up the plants in potting soil mixed for containers. Pinch off the ends of the stems, water thoroughly and put them in a sunny spot to resume growth. It may take several weeks before you see signs of new growth.