what is the best way to grow Rieger begonia?
Your begonia is Begonia x hiemalis, a tuberous type, sometimes called Rieger or winter-flowering begonia.
As a houseplant, they prefer a bright area, which is necessary for continued flowering. Keep them near a sunny window, but avoid direct sunlight. Soil should be kept moderately moist. Water thoroughly when soil surface is dry to the touch, but be sure not to let the plant stand in water. Rieger begonias prefer moderate temperatures, 65 - 70F during the daytime and 60 - 65F at night. Avoid excessively hot or cold locations. Occasional misting of the foliage is okay, but do not mist late in the day. You could also stand pots on trays of moist pebbles to increase humidity around the plant. Use high-potash, liquid fertilizer for actively growing plants every two weeks.
Many people find Rieger begonias difficult to propagate (but not impossible), so most begonias are discarded after flowering. They can be restarted by cutting them back to within 3 inches of the crown, reducing water and keeping them in a cool location. Increase water in spring. New shoots may be used as cuttings.
Take cuttings from a 5 inch stem. Cut a 4-inch section of growth just above a leaf node on a non-flowering stem.
Create a mixture of rooting soil by mixing equal portions of peat moss and perlite. Moisten the mixture to a level where water cannot be squeezed out of the soil. Fill a 3-inch pot with the rooting soil and stick the cut end of the stem into the soil. Cover the pot by placing it in a clear plastic bag, making sure the plastic does not touch the cutting (hold it up with something like chop sticks). Monitor the cutting to prevent the soil from drying. Lightly mist the soil with water if necessary. Be careful to not get water directly on the cuttings.
Watch for new plant growth. This is a sign the propagation was successful! Remove the plastic covering and continue to grow the cuttings until the root structure is strong enough to be transplanted.
Hope this helps.
Courtesy of the NYBG Plant Information Service