How to Grow Rex Begonia Use a porous potting mix in a relatively shallow pot, and feed the plant regularly. Keep the plant moist, but avoid overwatering. Be especially careful of directly spraying the leaves, as standing water will encourage powdery mildew, which is a disaster on a plant that is cultivated for its beautiful foliage.
It can be challenging to find just the right conditions for rex begonia to display its best leaf color. Ideal daytime temperatures are around 70 degrees Fahrenheit, nighttime temperatures around 60 degrees, and a constant humidity level around 50 percent. Provide the plant with plenty of bright indirect sun, but keep it out of direct sunlight. Rotate the plant frequently to expose it to equal light on all sides.
Light 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.
Soil Airy, light, fast-draining soil is best for rex begonias.
Water Rex begonias require regular water, but be careful not to over-water them. They also thrive on humidity but don't like direct misting, which encourages powdery mildew. Let the soil surface become dry to the touch before watering.
Temperature and Humidity Rex begonias do best in moderate temperatures (60 to 70 degrees Fahrenheit) and moderately high humidity (around 50 percent). They cannot tolerate freezing, and some types of rex begonias go into a dormant period during the fall. As with other begonias, the rex and other foliage begonias prefer the same humid, gentle environment in which understory ferns thrive.
Potting and Repotting 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. If the rhizome still has room to grow in its pot, the begonia should be considered happily potted. When the rhizome begins to butt up against the sides of the pot, however, it's time to repot into a fresh pot with fresh soil. Divide the rhizome when repotting 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.
Varieties of Rex Begonia Begonia rex is the basis for many hundreds of hybrids, and its forms are too numerous to count.