Gardeners raise Begonia rex-cultorum hybrids — commonly called rex begonias — for their distinctive, heavily textured foliage. Often, people call them king or painted leaf begonias because of the very decorative leaves. Some cultivars have uniquely spiraled leaves, but many have heart-shaped leaves. Typically, they are medium to large plants, although some hybrids or cultivars are dwarf versions. No matter what the size, these plants often delight their owners with their colorful foliage.
Many botanists believe that the original Begonia rex species no longer exists in cultivation. Growers have crossbred various begonia species so much that simple classifications are extremely difficult. Officially, rex begonias belong to the genus Begonia of the family Begoniaceae. There are 1,300 to 2,000 species in this genus, adding to the confusing classifications.
People usually raise a rex begonia as a houseplant. Many rex cultivars thrive under fluorescent lights, such as in an office setting. They are more colorful and vigorous if they are 6 to 10 inches (about 15 to 25 cm) below the bulb, and heat from the light may help the plant produce colors that are more radiant. When planted in the garden, they appreciate filtered light, but often scorch in direct sunlight. If planted outdoors in temperate or colder climates, growers normally dig up the root system and store the plant indoors for the winter.
Growers choose these plants for their kaleidoscope of colors, including various greens, silvers or grays, and maroon to blackish maroon. One of the more popular hybrids is known as Merry Christmas, rhurthal, or ruhrtal, which typically has a distinctive band pattern of maroon-red fading to pinkish silver and green. These colors radiate from the base of the leaf outward.
There is a great variety of leaf shapes and colors for gardeners. Helen Lewis is a rex begonia cultorum that has very dark maroon to wine-colored leaves that are marked with a band of silver that mimics the outline of the leaf edge. The underside of the leaf is a bright red, as are the leaf's veins. Escargot has spiraled leaves that have a wide band of silver accenting the green leaf top and sporting a red underside. Helen Teupel has a slightly heart-shaped leaf that has very pointed, jagged edges.
Rex begonias can grow quite large, and may produce leaves that are more than 12 inches (30.5 cm) long and more than 6 inches (15 cm) wide. Emerald giant has leaves that may measure up to 18 inches (about 45 cm) long. Other begonias may have smaller leaves, such as duartei, with leaves that are less than 7 inches (18 cm) long. Mini merry lives up to its name, as its 3-inch (8-cm) long leaves usually mimic the much larger Merry Christmas' begonia coloring.
Begonia flowers usually have both female and male flowers on the same plant. Generally, rex begonia flowers are not showy like the tuberous begonia flowers. Often, they measure 0.5 to 1 inch (1.5 to 2.5 cm) across. Most of the flowers are pink, though they may be white or orangish pink, depending upon the cultivar or hybrid. Many growers pick the flowers off to keep the leaves at their prime.