Rex begonias are normally grown for their foliage and not their flowers. There are many varieties with large variegated leaves of white and green, red and green, and other combinations. Rex begonias are known to be picky and difficult to grow, but the fact is they don't like extreme heat or wet feet. They may require more care than other foliage plants but the bright foliage that brighten your home in the winter is well worth it.
Water only after the top of the soil has dried out. Over watering will cause fungal diseases that will kill the plant. Some rex begonias will go dormant in the winter while others flower. If the plant is going dormant, allow the soil to dry out to an inch down before watering.
Keep the plant in 65 to 70 degree F temperatures during the night and 70 to 80 degrees during the day. Begonias will require filtered light from a south-facing window in the winter as days are shorter and the sun is not as bright.
Fill a tray with pebbles and water. Place the plant on top of the pebbles, but do not allow the bottom of the pot to sit in the water. Begonias require humidity and in the winter while heat is on and windows are closed the air becomes very dry. Another way to provide humidity is to place a humidifier in the room near the plant.
Apply a houseplant fertilizer if the plant is starting to bloom or not losing its leaves. If the begonia is going into dormancy, wait to see new growth before adding fertilizer. Refer to manufacturer's directions as to the amount to use.
Spread a shallow layer of light organic mulch over the soil in the winter to add nutrients to the soil and help add humidity around the plant.