Rex begonias (Begonia rex-cultorum) are a species of begonia grown primarily for their showy foliage. While the flowers are insignificant, according to the University of Illinois, the large leaves range widely in shape, color and variegation. Many cultivars are bi-colored. Others, such as "River Nile," have foliage with heavily ruffled edges. Rex begonias can be grown outdoors, depending on the climate, or indoors as a house plant.
Rex begonias are classified as tender perennials. This means that freezing temperatures will kill the plant. For that reason, they are often only grown outdoors as annuals, or they are grown only indoors. Rex begonias can grow year-round outdoors in tropical or subtropical climates. They are hardy in U.S. Department of Agriculture (USDA) growing zones 10 through 12, according to Michigan State University.
Humidity is very important for the growth of rex begonias. While outdoor rex begonias may come by enough humidity naturally, the drier air of indoor locations does not benefit these plants and may cause the edges of the leaves to turn brown and crispy, according to the University of Illinois. To remedy this, place your potted rex begonia on a shallow tray filled with pebbles. Add water to the tray so that the level of water reaches the bottom of the pebbles. The water will evaporate and add humidity to the air around your begonia.
Water and Soil
Although rex begonias need plenty of water in the air, this does not hold true for the soil. Too much water in the soil can cause root rot. Let the top layer of soil dry to the touch before watering your plant, but don't underwater so much that the leaves begin to wilt, warns the University of Florida. The soil should be light, loamy and well-draining.
Temperatures and Light
Indoor rex begonias do well when the temperatures are kept between 70 and 80 degrees F during the day, with a drop of 10 degrees at night. Outdoor plants benefit from morning sun and afternoon shade. Indoor plants should be placed by a window that presents bright but indirect or filtered sunlight.
Water left to sit on the leaves can cause them to develop leaf spot, which ruins the attractiveness of the foliage. Always water at the soil level and allow morning sunlight to dry the dew from the leaves of outdoor plants. A thick (3 to 4 inches) layer of mulch can protect in-ground plants in the cooler USDA zones during the fall and winter months, according to the University of Florida.