You can start the begonia tubers indoors in March/April. Plant the tubers hollow side up, no more then half an inch below the soil level, with adequate drainage holes. If you're using commercial potting mix, mix it with equal parts of peat moss and sand or perlite.
Water the tubers once really well to wake them up, and place them in a warm bright spot where the temperature will never fall below 60° F. Cover the freshly planted tubers with paper or polythene to promote growth, but remove the covering as soon as growth appears. Don't water again until you see some growth or the soil is quite dry; the begonias don't have any roots yet and you don't want to drown them. Some begonias will sprout right away others will take weeks; generally, expect to see growth shoots in five to six weeks.
Once They're Growing:
Once the shoots are showing, water regularly, never allowing the soil to dry out. Give maximum light, but shade from direct sun. Applying liquid plant food will improve your Begonias' growth; feed them with half strength balanced fertilizer the first time when their leaves and stems are about 3 inches tall, wait two weeks, and then start a regular feeding schedule. When the shoots are 6 inches tall, the Begonias are ready to be transplanted to the garden or outdoor containers. Don't put them outdoors, though, until all danger of frost has passed.
Tuberous Begonias thrive in partial to full shade and need well-drained soil. They need to be kept away from hot sun and drying winds. As they dislike dry conditions, they'll need to be watered generously, especially during hot weather. Keep their soil moist but not soggy; the tubers will rot if they get too much water. Always water the soil around the plants rather than the plants themselves, if possible, to avoid mildew, and at the first sign of a white patch on any of your begonias' leaves, apply a fungicide right away. Feed Begonias with a well-balanced plant food every 2 to 3 weeks.
Once the show is over in the fall, you'll be able to dig and store your tubers until spring rolls round again. Because Begonias are prone to rot if left in the ground until their tops fall off, I recommend bringing them indoors before frost. If you place them in a well- ventilated room and start withholding water, the foliage will die back and you'll be able to remove the stems and leaves without damaging the tubers. Clean them off, dust with soil dust, and cover the tubers with dry sand, peat moss or vermiculite; store them in a cool, well-ventilated, frost-free area such as a garage over winter. (55° F is ideal.)