Rieger begonias (Begonia hiemalis) are extended-blooming begonias that are extremely cold-sensitive and usually grown as houseplants. In some mild-winter regions, Rieger begonias can be grown in the ground outdoors. These perennial plants are hybrids of wax and tuberous begonias. The Rieger begonia typically blooms during winter but can bloom any time of year when it's exposed to short daylight hours. Rieger begonias enjoy warm but not hot temperatures and can bloom for several months. These begonias typically grow 12 to 18 inches tall and wide.
Place your Rieger begonia in bright indirect light, such as in an east-facing window. Maintain daytime air temperatures around your Rieger begonia of 65 to 70 degrees F and slightly cooler to about 60 to 65 degrees F at night.
Water your Rieger begonia evenly and moderately until the water just begins to drain from the bottom of the pot. Water the begonia only frequently enough to keep the potting soil slightly moistened and to keep the soil from drying out completely.
Fertilize your Rieger begonia with a water-soluble houseplant or flower fertilizer while it's actively growing. Follow the dosage instructions on the fertilizer label.
Cut back the Rieger begonia to about 3 inches beyond the crown after it finishes flowering. This will help the begonia to grow vigorously instead of declining after blooming.