Culture
Begonias are mostly frost tender, they are treated as annuals outdoors or grown indoors in cool climates. They are hardy to Zones 10 – 12.
Rex begonias are not easy care plants and are mainly grown indoors, especially in the St. Louis area. They require high humidity (more than 50% -- some cultivars want more than others), porous planting mix, a shallow pot, heavy fertilization during growth, and care to avoid overwatering. Once you hit the proper combination of growing conditions, the stunning color display will make it worthwhile. Your watering technique should permit the soil surface to become almost dry between waterings. Stick a finger into the planting mix to check. In spring, when new growth has started, a balanced complete fertilizer (23-19-14) or similar formula) should be applied quarter strength every two weeks. Or a controlled-release fertilizer can be applied every three months. Taper off in fall and stop in winter.
Provide plenty of light without putting the plants in direct, hot sun. Spring morning sun or filtered sunlight may be acceptable in mild areas. If light comes from one side, give each plant a quarter turn weekly. Rex begonias do best if day temperatures hovers around 70°F and 60° at night. If it is cooler, they usually will survive but growth will be slow. In fall or winter, unless grown under lights, many cultivars enter dormancy – they stop growing and might even drop some or all of their leaves. If this happens, water only sparingly until spring, when new leaves will emerge.
Most rex begonias don’t need pruning unless they are “upright” cultivars or the rhizome has grown too long for its container and has unsightly bare sections. Pruning is simple: just cut the rhizome back. It will develop new leaves and may even branch. You can root the rhizome cutting and grow another plant. Tip pinching earlier will result in beneficial branching.
Noteworthy Characteristics
Begonia rex-cultorum is a name applied to a group of cultivated begonias known as rex begonias. All rex begonias include the wild species B. rex in their parentage. This rhizomatous perennial is native to rocky, forested valleys and slopes in northeastern India, southern China, and Vietnam. The leaves have are sparsely hairy, have prominent veins, and can reach up to 12" long and 8" wide. The upper surfaces of the leaves have a dark green center and dark green margin surrounding a silvery center. The undersides of the leaves are red. Small, pink flowers are held in loose clusters atop upright flowering stalks reaching up to 12" long.
Rex begonias are widely hybridized and cultivated for their showy foliage. The leaves range widely in color, texture, and size. Most rex begonias grow from a thickened stem structure called a rhizome. They are not classed with other rhizomatous begonias, however, because of their bold leaves and more exacting growing requirements.
Genus name honors Michael Begon (1638-1710), Governor of French Canada.
The specific epithet rex-cultorum combines the specific epithet of the parent species Begonia rex with cultorum meaning "cultivated". This name refers to cultivated selections of rex begonias.
'Super Curl' is a rex begonia selection that features colorful foliage and a dense, mounding habit. The spiral leaves have wavy, maroon-black margins, silvery centers, and prominent, maroon-black veins. The small, pale pink flowers are not showy. Mature plants will reach up to 1.5' tall with a similar spread.
Problems
Rex begonias are susceptible to mildew and botrytis fungal infections. A systemic fungicide may be used commercially as a preventative but it is best to remove dead leaves promptly and provide good air circulation. Mealy bugs can also be problematic. The best way to prevent insects and diseases is to keep the plant well groomed, removing dead leaves and any debris on the surface of the planting mix.
Uses
Best used as a container or basket plant. Rex begonias makes a wonderful addition to a shaded deck or patio garden. It is also a good plant for small spaces such as a roof-top garden or on a balcony. Can be an attractive indoor plant if provided the proper growing environment.