The Rieger begonia, also referred to as winter flowering begonia, is a hybrid developed by crossing a tuberous and wax begonia. Rieger begonias have a long blooming period and have low maintenance requirements. Plant the Rieger begonia in a container for use indoors or out doors or in a garden bed as a tender annual. Rieger begonias do not respond well to bright sunlight or over-watering that creates wet soil around the root system.
Plant the Rieger begonia in a container that has bottom drainage holes and is 2 to 3 inches wider in diameter than the root ball. Fill the container with a well-draining soil that includes vermiculite or perlite.
Create a hole that is slightly larger than the root ball and set the plant in. Pack soil around the root ball to hold the plant in place. Apply water to the container after planting until it drains from the bottom holes.
Set the container in a shaded location that receives only dappled sunlight during the early morning or late afternoon.
Water the soil once a week if the top layer feels dry to the touch. Apply water to the soil until it runs out the bottom holes. Do not allow water to saturate the foliage or flowers as this increases the risk of powdery mildew.
Fertilize the Rieger begonia every other week during the flowering season with a 15-30-15 water-soluble fertilizer diluted to half strength.
Deadhead the plant by removing spent flowers to stimulate new flower growth. Cut back stems that appear leggy or long to a length of 2 to 3 inches. Use caution while removing flowers as the stems are brittle and break easily.
Move the Rieger begonia indoors prior to the expected first frost in fall. Grow the plant indoors until there is no longer a risk of frost in the spring.