Tuberous begonias have a root like a potato plant. But unlike a potato plant, you can't grow a tuberous begonia from the eye of the tuber. Tuberous begonias can be grown from divided tubers, but the tubers must be divided while the plant is actively growing. Unfortunately, many divided tubers suffer root rot and die when they are divided. The best method to propagate tuberous begonias is by rooting cuttings in a forsythe pot.
Prepare a forsythe pot: Place a cork in the drainage hole of a 3-inch-diameter unglazed clay pot; fill a 10-inch-diameter plastic container with vermiculite; and force the 3-inch clay pot down into the 10-inch-plastic container. There should be a 3-inch ring of vermiculite around the 3-inch container.
Moisten the vermiculite and fill the clay container with water.
Thin out 3-inch-tall stems from a begonia plant by cutting the stems at the base. Insert the stems into the moist vermiculite.
Place the container of cuttings in a clear plastic bag. Seal the bag and place the container in a sunny windowsill out of direct sunlight.
Check the container daily and fill the 3-inch clay pot reservoir any time it is low on water. The reservoir will keep the begonias watered. The begonias will take root within 2 weeks.
Open the bag a little bit each day to accustom the plants to the difference in humidity.
Fill a 4-inch container with potting soil. Hollow out a planting pocket in the center of the soil for your begonia. Place the roots of the begonia into the planting pocket and close up the soil behind the plant.
Water the soil so that it remains slightly damp to the touch.
Set the plant outdoors in the shade during daytime hours for 3 days to harden the plant off.