Among the various types of begonias, the B. Rex are some of the most beautiful and stunning plants. Sometimes called painted-leaf begonias or fancy-leaf begonias, these plants are known for their showy, sometimes jaw-dropping leaf coloration. They frequently have large leaves (up to 6 in. long) that are brightly colored in various shades of green, red, silver, and even purple. These plants are grown almost exclusively for their foliage—their blooms tend to be small and less showy, so many growers pinch off blooms to maintain breathtaking leaf displays.
Some types of Rex begonias go into dormancy during the winter.
Light: Bright, indirect light year-round. Because they are not aggressive bloomers, they can tolerate less light than other begonias and will thrive under fluorescent lights.
Water: Rex begonias like even water, but hate being overwatered, and they thrive on humidity, but don't like direct misting, which encourages powdery mildew.
Temperature: Average to warm (and humid). They cannot tolerate freezing, and some types of Rex begonias go into a dormant period during the fall.
Soil: Airy, light, fast-draining soil.
Fertilizer: Use liquid fertilizer weekly at quarter strength or biweekly at half strength.
The vast majority of begonias offered in the trade are hybrids and thus cannot be accurately propagated from seed. Rhizomatous begonias can be easily propagated by rhizome division during repotting, while almost all begonia species will readily sprout from leaf-tip cuttings.
A rooting hormone can help the cuttings sprout. Rex begonias can also be propagated by pinning a leaf down to the bedding mix and making small incisions in the leaf veins, or by inserting a leaf (with a petiole) directly into soil.
Rex begonias are rhizomatous plants that grow from a shallow and knobby rhizome.
As such, they do best in large, relatively shallow pots where the rhizome has room to spread. As long as there is still growing room in the pot, the begonia should be considered happily potted. When the rhizome begins to butt up against the pot side, however, it's time to repot into a fresh pot with fresh soil. Divide the rhizome at potting time to increase your plant stock. As with all begonias, don't soak the soil of newly potted plants, but do keep it lightly moist and warm.
Begonia rex is the basis for many hundreds of hybrids, and its forms are too numerous to count. The resulting hybrids are often known in the trade only by fancy trade names dreamed up by growers looking to market their plants. As a result, you might find the same crosses named two different things in different garden centers! One interesting and beautiful begonia that's often included with the Rex begonias as a foliage begonia is the B. masoniana, or iron cross begonia. This beautiful plant was introduced to cultivation in 1952 and has puckered leaves with a dark cross in the middle. It's available in various colors.
Because these are primarily foliage plants, careful cultivation is a must to really realize their full beauty.
As with other begonias, the Rex and foliage begonias enjoy the same humid, gentle environment of understory ferns. Be especially careful of directly spraying the leaves, as standing water will encourage powdery mildew, which is a disaster on a plant meant to be viewed. Even the best Rex specimen can only be expected to thrive for a few years, but because they propagate so easily, it's a simple matter to keep a steady supply of healthy plants by taking new ones every year.