Used often as winter-blooming houseplants or floral gifts, Rieger begonias were developed from hybrid hiemalis-group begonias in the 1950s. This new strain of begonias was easier to grow, displayed more disease resistance and produced much more flowers — perfect for commercial production.
Genetic origins of Rieger begonias contain traits of Begonia socotrana and the tuber-root hybrids known as Begonia x tuberhybrida, the tuberous begonia. These were first dubbed hiemalis-type begonias from which Rieger begonia were developed. Thus, Reiger begonias grow from tuber-like structures that combine traits of thick stems and fibrous and tuberous roots.
With such a complex, muddy lineage, Rieger begonias grow from modified or semi-tubers. For the sake of marketing or simplicity, nurseries and growers may colloquially call them bulbs, although not true bulbs in the botanical definition.
Literature and discussion about Rieger begonias often finds them grouped with tuberous begonias when it comes to culture and plant structure. Nonetheless, it strengthens the position that Rieger begonias grow from tubers.