The Rex Cultorum group — whose members commonly are known as “rex begonias” — is aptly named. These “kings” of the Begonia world display wildly varied leaves streaked, bordered, spotted, and splotched by many colors. They also flower, but usually the flowers are overshadowed by the striking foliage.
Rex Cultorum begonias are derived from the species B. rex, whose dark leaves are marked with a prominent silvery-gray band. B. rex was discovered in Assam and introduced into cultivation in the 1850s. Its immediate and widespread use in hybridizing gave rise to countless cultivars that boosted foliage begonias as indoor plants.
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. Some varieties are upright and branching rather than creeping.
Some of the oldest rex cultivars, such as B. ‘Abel Carriere’ (1878) and B. ‘Louise Closson’ (1889), are still in cultivation. Other cultivars include B. ‘Fireworks’, B. ‘Helen Lewis’, B. ‘Lady Frances Jean’, B. ‘Merry Christmas’, B. ‘Purple Petticoats’, and B. ‘Woodriff’s Tricolor’. Several hundred named cultivars are grown today, and many more are hybridized and sold but never named. Rexes hybridize readily, and, of the several hundred seedlings that may grow from a single cross, no two will be alike.
Rexes are not “easy care” plants. 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. Rexes do best if day temperature hovers around 70 degrees F., and 60 degrees 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 don’t need pruning unless they are “upright rexes” 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.
The primary enemies of rex begonias are mildew and botrytis, both fungus diseases marked by white spores. The systemic fungicide benomyl is a good preventative. Many growers use a fungicide containing karathane to kill the diseases once they have started. As with all garden chemicals, follow label directions exactly.
Occasionally, the insect known as mealybug may appear as a small cottony-looking mass tucked in the joint where a leaf joins the leaf stem or the stem joins the rhizome. To kill the bugs, just dip a cotton swab in rubbing alcohol and touch it to each mealybug. A large infestation can be treated with malathion.
The best way to prevent insects and diseases is to keep a plant well groomed, removing dead leaves and any debris on the surface of the planting mix.
The Begonian of March 1980, a special issue called “The Regal Rexes,” is a source of detailed information on the Rex Cultorum group. Propagating information is contained in an article in the June 1980 Begonian.