Rex begonias are considered the "kings" among the wide variety of begonias because of their strikingly colorful foliage. Rex begonia leaves vary in texture, size and shape, streaked and veined with several colors. Hardy to USDA zones 10-12, these plants are best grown indoors and require high humidity and careful tending.
Rex begonias thrive in day temperatures of about 70 degrees and 60 degrees at night. They're grown more for their large, beautiful leaves than their flowers, which are tiny.
Rex begonias grow from rhizomes, or underground stems or tubers. Place your plant or bury the rhizome in a shallow container filled with rich, aerated plant mix. The rex begonia requires at least 50 perecnet humidity and careful watering that keeps the soil moist but not too wet. Begonia rhizomes are sensitive to soggy soil or overwatering and may rot.
Provide good indirect light (morning or filtered sunlight is best). Some rex begonias do well in shade gardens but still need indirect light.
Ongoing care for your rex begonia includes watering that allows the soil to nearly dry but retain moisture. Every three months, apply a controlled release fertilizer. Another option is to use a complete fertilizer (23-19-14 is a good balance) in the spring every two weeks at 1/4 strength. Decrease fertilizing when fall arrives and stop in winter.
In winter, rex begonias may go dormant (stop growing and/or drop leaves) unless you choose to add fluorescent lighting. If they go dormant, water sparingly until spring when new leaves appear.
To maintain high humidity, add a room humidifier or place your rex begonia pot on a tray of wet pebbles and keep the stones wet. Remove any withered or dead leaves and any debris around the rex begonia.
The only pruning required is to the rhizome if it outgrows the pot. Just trim it back; it will grow back and produce new leaves. The rhizome cutting can be planted to start a new plant.