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.