The first thing you must know before propagating begonias is that there are three basic types: fibrous, rhizomatous and tuberous. Fibrous begonias have roots like most plants. Rhizomatous begonias have similar roots to normal plants but produce large puffy rhizome out of which the roots grow. Tuberous begonias produce a fleshy tuber at the base of the stem, similar to a bulb.
Propagate begonia cuttings
Find a healthy, 10 cm section of the begonia to use for propagation. The section must be from a non-flowering stem. Make a cut directly above a leaf node and dip the cutting in rooting hormone powder.
Make a potting mix of half peat moss and half perlite or coarse sand. Moisten the potting mix to prepare it for the begonia propagation.
Insert the begonia cutting into a 7.5 cm pot filled with the moistened potting mix. Place the entire pot into a clear plastic bag and place it in bright, but filtered, light.
Watch for signs the propagation is successful. Successful propagation has taken place if there is new growth on the plant. This generally takes three to six weeks. At this time, remove the plant from the plastic bag and treat the plant like a mature begonia.
Propagate begonia tubers
Wait for the foliage of the tuberous begonia to die in the autumn. The foliage of this type of begonia dies in the autumn. At this time, the plant will produce new, small tubers which shoot out of the main tuber.
Remove one of the main tubers from the begonia. This is what you'll use for propagation.
Store the tuber in a jar or other sealed container in a cool, dark place until spring. The ideal temperature for storing tubers is about 13C.
Plant the begonia tuber in a 7.5 cm pot in the spring using a standard potting mix.
Water the begonia propagation only enough to keep it from completely drying out at first. Gradually increase the amount of water you give it until you are watering it as you would a mature tuberous begonia.