Begonias grow from a tuberous root system, similar in many ways to a bulb or rhizome. The tuberous root is not divided like bulbs and rhizomes, though. Begonia tubers are prone to rot and disease, so dividing them is difficult to do without losing both the old plant and the new one. Instead, the main method of propagating begonia is via root cuttings. Root cuttings grow into full plants within a season, so it is a quick way to bring more of these colorful perennials to your garden.
Fill a 10-inch diameter plastic flower pot with vermiculite. Vermiculite is a sterile, soil-less planting medium that is readily available at garden centers and is necessary for rooting cuttings.
Plug the bottom drainage hole in a 2 to 3 inch diameter clay pot with an appropriate-sized cork. Push the clay pot into the vermiculite-filled plastic pot until only the rim of the clay pot is above the vermiculite level.
Water the vermiculite until it is moist. Fill the clay pot with water. Water leaches from the clay pot and into the soil so that watering does not disturb the cuttings as they root. This is called a Forsythe pot (see Resources).
Cut full stems from established begonias in spring when the stems are approximately 3 inches tall. Cut the stem off where it joins the tuber, but avoid cutting into the tuber itself.
Set the cuttings into the soil just deep enough so they stand upright. Place the Forsythe pot into a plastic bag and place it in a warm room that receives bright, indirect sunlight.
Check the cuttings after two weeks for rooting. Tug on them lightly--if there is resistance they are rooted. If not, leave them in the plastic bag for an additional two weeks or until they root.
Open the plastic bag once the cuttings are rooted. Leave the bag open for two to three days, then remove the pot from the bag completely.