Much beloved in Victorian times when homes tended to be chilly and drafty, begonias make excellent houseplants even today. The easy-to-grow plants flourish on an average, well-lighted windowsill and practically thrive on neglect -- but they are as rewarding as they are easy.
Although there are thousands of varieties of begonias, most fall into one of three main groups based on the plants' differing root structures. Tuberous begonias, grown from tubers, are known for their large roselike flowers that bloom from summer to fall in outdoor displays and pots. Rhizomatous, which include rex begonias, are so named for their thick, fleshy stems, called rhizomes, which either spread over the soil or rise up in short gnarled trunks; because new leaves can grow from every joint of the rhizome, the foliage forms a solid canopy. Fibrous begonias have ordinary roots and include angel-wing types, renowned for their graceful winglike leaves and canelike stems, and wax (semperflorens) begonias which have waxy leaves and charming flowers year-round.
BEGONIAS FOR INDOORS
Both fibrous and rhizomatous begonias make stunning houseplants because of the amazing colors, shapes, sizes, and textures of their foliage. Leaves can measure half an inch across or more than a full foot. They can be rounded, star-shaped, or resemble bird wings. Their texture can be smooth and glossy or soft and fuzzy. And although the foliage is often green and red, it may also be splashed with silver, purple, or pink. Rhizomatous begonias flower in winter, but compared to the extravagant foliage, the blooms are modest. Many varieties of wax begonias bloom throughout the year.
Begonias are remarkably pest resistant, and they thrive in the warm temperatures and bright, indirect light found in most homes. These tropical and subtropical natives tend to prefer environments similar to their native habitats: Keep the plants in a warm (58 degrees to 72 degrees), humid spot with bright light, such as strong, indirect sunlight.
Begonias prefer good-quality, light, well-drained potting soil. Unglazed clay pots are best. Begonias, especially rhizomatous ones, are happiest when their roots are slightly pot bound, or constricted, so repot them only when roots emerge from the drainage hole or when they require more frequent watering, and even then move the plant into a new pot no more than an inch greater in diameter than the old one. Before repotting, observe whether rhizomes spread over or just below the soil surface -- both patterns of growth are common -- so that you can set the plant at the same level in its new pot.
CARING FOR BEGONIAS
Water plants in the morning, and avoid splashing the leaves. With larger-leaved begonias, wait to water until the foliage starts to flag just a little, a sign that the plants need a drink. Because rhizomatous types store up quite a bit of moisture, avoid overwatering them. Keep your begonias in a well-aerated spot to reduce the chances of disease.
Feed plants with a balanced, water-soluble houseplant fertilizer every 2 to 4 weeks during active growth, generally from midspring to midfall.
Because their leaves are rich in oxalic acid, a natural insect repellent, begonias resist pests well. But the foliage is prone to the fungal diseases powdery mildew and botrytis, which mark the leaves with whitish spots or gray mold and can kill plants. Keeping the foliage dry is the best protection. Spray leaves at the first sign of infection with a solution of one tablespoon of baking soda and a few drops of mild liquid detergent in a quart of water.