Tuberous begonias, Begonia tuberhybrida, are best propagated by stem cuttings. Propagation by root division is often unsuccessful and sometimes damages the mother plant. Stem cuttings can be made from excess stems, and they root quite well in ordinary sand or soil. For best success, take the cuttings during the spring or summer while the plant is in an active growth phase.
Prepare a small cup or pot of sand to receive the cutting. Moisten the sand well, but do not allow standing water.
Cut 3-inch pieces of stems using clean sharp scissors or a clean knife. Take the cutting from the tip of a stem.
Poke a hole in the sand with a small twig or pencil and insert the stem. Press the sand in around the stem to support it.
Place the cup with the cutting into a plastic bag. Seal the bag, leaving it inflated to hold it away from the plant.
Maintain the cuttings in a cool room, 60 to 70 degrees Fahrenheit, and out of direct sunlight.
Check the cuttings daily. Make sure they are not too warm in the bag, and that the sand stays moist. The cuttings should be well rooted within two to four weeks with roots that are at least 1 inch long.
Open the plastic bag as soon as the cuttings are rooted. Open only a little at first, opening more each day until the plants are acclimated to normal humidity.
Transplant the new plants to pots filled with potting mix, or harden them off if they will be moved outdoors.
Harden off the plants for outdoor planting by moving the plants outdoors to a sheltered spot for a little time each day. Increase the time outdoors daily until the plants are spending the entire day outdoors. Once they are spending the day outdoors, transplant them into the landscape.