Begonias are native to tropical and subtropical regions and die if left outside in climates where winters are cold. Unless you plant your begonias in pots that you can take into the house after the summer, plan to dig them out every year. When you do, you'll notice the plants have an underground stem that looks swollen (the tubers) from which the roots develop. You'll need to store them properly through the winter to keep them alive and ready to grow again come spring.
Cut the begonia's above-ground stems. After the leaves die in the fall and before you dig your plant out, trim all the branches to ground level.
Dig the tubers out. Use a trowel to remove them from the soil, taking care to not pluck the roots growing from them. Leave any soil clinging to the plant on.
Lay the tubers on newspaper. Put them side by side and not in a pile. For the next three weeks, keep them in a dry and dark area to let the external moisture evaporate. If you live in a dry climate, this may take as little as just one week. When the soil on the roots turns crumbly, shake it off and trim any remaining stems.
Put some vermiculite or peat moss in a clean bag and add the tubers. Either one helps them conserve internal moisture. Store the bag away from sun and artificial light in an area that stays at about 10 degrees Celsius. Every two weeks, check your tubers. If you feel any soft spots, cut the affected part of the plant, as it might have a fungal infection that will spread. In spring, replant your tubers after the last frost.