Tuberous begonias are tender perennials that add color to shade gardens and other areas with limited light. Unlike other tuberous-rooted plants, begonias do not respond well to divisions as they are easily prone to disease once the tubers are damaged. Begonias can be propagated by seed or by cuttings instead. Root cuttings are one of the quickest methods to grow new plants that is also the least likely to damage the existing begonia. Cut the begonia tubers at any time during the growing season and have plants that are ready for the garden by the following spring.
Prepare a rooting pot for the begonia cuttings. Fill a 10-inch diameter plastic pot with vermiculite then place a cork in the drainage hole of a 2-inch clay pot. Push the clay pot into the vermiculite in the larger pot so that only a half inch of the clay pot's rim is above the vermiculite surface. Water the vermiculite in the larger pot until it is moist, then fill the clay pot with water. The clay pot continues to leech water into the vermiculite, ensuring it remains evenly moist throughout the rooting process.
Cut off a 3-inch length from an actively growing stem from an established begonia plant with a clean pair of shears. Take at least three of these cuttings for each new plant you wish to grow, as they may not all root successfully. Avoid cutting into the actual root of the begonia when taking the cuttings, as this can lead to disease problems.
Fill a small bowl with a rooting compound, available at garden centers and from florists. Dust the cut end of each stem cutting in the rooting hormone. Rooting hormones help encourage healthy root development.
Push the cut end of each cutting deep enough into the vermiculite so that it is supported upright. Space the cuttings at least 1 inch apart.
Set the rooting pot inside a clear plastic bag and seal the bag closed. Place the pot in a warm room in bright, indirect sunlight.
Open the bag after two to four weeks, but leave the pot inside it. Add additional water to the clay pot if necessary. Give each cutting gentle tug to verify they have rooted. Rooted cuttings offer resistance when tugged on.
Transplant the cuttings to pots once they are actively growing again, usually within one month of successfully rooting. Plant the cuttings at the same depth in a permanent pot that they were at in the cutting pot.