When should I start tuberous begonias indoors?
Plant tuberous begonia tubers indoors 8 to 10 weeks before the average last spring frost in your area. (Flowering typically begins 12 to 14 weeks after planting.) Start tuberous begonias in pots or other suitable containers. All containers should have drainage holes in the bottom. Use a well-drained potting mix. When planting the tubers, place the concave or indented side upward. The rounded side is the bottom. Cover the tubers with ½ to 1 inch of potting soil. Water well. Then place the containers in a warm, 70ºF location. Since the tubers are susceptible to rotting, keep the potting soil moist, but not wet. Once the tubers sprout, move the plants to a sunny window or place under fluorescent lights. Fertilize the plants with a dilute fertilizer solution once every 2 or 3 weeks. Harden (acclimate) the plants outdoors for several days before planting. Initially, place the plants in a shady, protected location. Then gradually expose the plants to longer periods of early morning or late afternoon sun. Plant tuberous begonias outdoors when the danger of frost is past.