Rex begonias like 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.
Airy, light, fast-draining soil is best for growing rex begonias. Their fine root systems prefer a well aerated peat-based potting soil in containers.
Rex begonias require regular water, but don't overwater them. The soil should feel slightly, yet evenly moist. Allow the soil surface to become dry to the touch before watering. Don't let the soil become overly dry or the plant will begin to wither. The plants thrive on humidity but, if misting leaves, ensure they don't stay too wet and encourage powdery mildew.
Temperature and Humidity
Rex begonias do best in moderate temperatures (60 to 70 degrees Fahrenheit) and moderately high humidity (around 50 percent). Growth will slow in freezing temperatures and some types of begonias go into a dormant period during the winter due to lack of light.3 As with other begonias, the rex and other foliage begonias prefer the same humid, gentle environment in which understory ferns thrive.