Geraniums will die if left outdoors during the winter, although they can sometimes be brought in, saved and used next year. But unless they are in really good condition, it may not be worth the effort. You'll be better off buying new plants in the spring.
There are two methods of saving geraniums. One is to dig the plants, cut them way back and plant the in pots. The other is to start plants from cuttings. The second is better because young plants are more vigorous and attractive than old ones. Moreover, an old plant will take up as much space (in a pot) as several young ones. On the other hand, the old plant will produce more flowers indoors during the winter than a young one.
If you want to keep the old plants you will have to prune them heavily when you dig them. Even when a plant is carefully lifted, at least half of its active roots remain in the soil. To balance the large top and this abbreviated root system, severe pruning is necessary.
Remove the weak growth entirely, reduce the strong main stems to three or five, and cut these back so that only two to four bunds are left on each. Young branches will develop from these buds and form a compact plant.
If you are going to take cuttings, it should be done as soon as possible now. The part of the plant that has been exposed to the most sun will provide the best cuttings.
With a sharp knife, cut off tops of young branches, 4 to 5 inches long, slicing straight across the stem just below a node, where the leaf joins the stem.
Remove the lower leaves from the cuttings, retaining only two or three at the top. If a flower bud is present, remove it.
If you wish to use a root-promoting substance such as Rootone or Hormodin, it may speed up rooting: however, good results usually are possible without the hormone. If it is used, instructions that come with it should be followed closely.
Plant the cuttings in 2 or 3-inch pots of good potting soil and water them thoroughly. Place the pots in partial shade. Water only when the soil becomes rather dry and then do a thorough job.
After about eight to 10 weeks in the small pots, that is, four to six weeks after rooting, the plants will need larger pots.
After the cuttings roots, in about 4 weeks, they will need plenty of sunlight. As soon as the tops begin to grow, pinch out the tip, to encourage side shoots to develop.