They should be planted out into the garden after the threat of frost has passed. They will flower 12 to 15 weeks from start-up. Starting Tubers Indoors 1. Select tubers that are firm to the touch. To get earlier blooms, start begonias growing indoors about six weeks before your planting-out date, the date in your area when the threat of night frosts is past.
Good drainage is essential.
Use a commercial potting soil mixed with equal parts peat moss and a drainage material such as sand or perlite. Place tubers in the soil mix with the rounded sides pressed gently onto the surface of the soil. Space the tubers about 2-3 inches apart. A warm humid setting is optimal for promoting growth. Keep soil moist, but not wet.