Place the tuberous begonias in a flat container with the hollow indent facing upward. Pour peat moss over the tubers so it just covers them. Moisten the peat moss with water so it is moist but not wet. Place the flat where the tubers will get indirect light and the temperature is 70 degrees Fahrenheit.
Pour potting soil into the contains for the begonias and fill them 3/4 full.
Remove the tubers from the flats when the stems are 1 inch tall. Place the tubers into the containers. Place one tuber in each container.
Cover the begonia tubers with soil and pat it down. Be sure the stem sticks all the way out of the soil.
Water the tuberous pastel begonias well until water drips out of the bottom of the container. Place the begonias in a bright location so they grow thick, sturdy stems.