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.
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. 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.
The primary enemies of rex begonias are mildew and botrytis, both fungus diseases marked by white spores. A systemic fungicide may be used commercially as a preventative but the home gardener's best defense is to remove dead leaves promptly and provide good air circulation. Occasionally, 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 the plant well groomed, removing dead leaves and any debris on the surface of the planting mix.
Best used as a container or basket plant. The rex begonia 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.