Rieger begonias (also known as Hiemalis begonias) are popular with gardeners because of their long-season blooming, glossy leaves and need for little attention. Rieger begonias are a cross between wax begonias and tuberous begonias, according to the University of Illinois Extension. They can be grown as houseplants or outdoors as tender annuals.
Plant your begonia in an area that receives filtered sunlight or part shade. If grown inside, place your begonia in an east window. If the foliage turns copper, the plant is receiving too much light.
Water once a week when the top of the soil is dry. Water your plant at the soil line to avoid wetting the foliage, which can lead to powdery mildew. Rieger begonias can put up with less water than normal, but they will not survive having their roots constantly wet.
Fertilize once a month with a water-soluble 15-30-15 fertilizer. According to Michigan State University Extension, over-fertilization will cause blue green foliage that curls at the ends
Deadhead spent blooms, and pinch back long stems to keep the plant compact. If the entire plant gets too leggy, cut it back by half.
Bring your begonia inside around a week before the earliest frost date for your area. Reduce the amount of watering. Let the plant rest for two weeks without water, then prune it and begin watering once more. Return the plant to the garden in the spring after the final frost date.