Tuberous begonias produce colourful flowers on top of mounds of green foliage. Unlike most other summer flowers, begonias bloom best in areas that receive shade, making them a suitable addition under trees and shrubs, on the north side of buildings, and in other shaded areas of the garden. A tender perennial, tuberous begonias do not tolerate frost so they are dug up and stored indoors each fall. Start the begonia tubers indoors in late winter so they are ready to bloom when you plant them outdoors in late spring.
Fill a seed-starting flat with one part peat moss and one part coarse sand. Alternately, use a peat moss and vermiculite potting mixture.
Set the begonia tubers into the flat so the side of the tuber with the depression in it faces up. Cover the tubers with potting mix so that the top of the tuber sits 1/2 inch beneath the surface. Place the tubers 2 inches apart in the flat.
Water the potting mixture so it is evenly moist throughout but not soggy. Set the flat in a 65 to 70 degree F room where the tubers receive indirect sunlight. Water the mixture as necessary to keep it moist.
Transplant the tubers to individual 5-inch-diameter pots once their stems are 1 inch tall. Fill the pots with a mixture of three parts potting soil and one part humus. Water the soil until it is evenly moist throughout.
Lift the begonia tuber from the flat, taking care not to damage the roots. Plant the tuber in the pot at the same depth it was at in the flat and place it in a sunny windowsill. Water it as needed to keep the soil moist until you are ready to plant the begonia outside.
Make a seed-starting flat by cutting the bottom off a plastic milk jug, advises Washington State University Extension. Begonias can be transplanted from the flat to their permanent pot or bed if all frost danger is past, instead of into a second pot.
Do not allow water to collect in the depression on top the tuber. The water leads to rot so it is best to keep the depression dry.