Tender begonias bloom from spring until later summer, but do not tolerate winter freezing. The tubers are usually dug up in the fall and replanted each spring. If you need to add more of these shade-tolerant flowers to your garden, you can't divide them as you would other tuberous flowers. Instead, collect seeds from the healthiest and largest plants and start them indoors to add more blooms to your landscape the following year.
Collect the seed pods once the stems have begun to dry and shrivel, usually after the first fall frost has killed back the plants and around the time you would normally dig up the roots for storage. Pods form where the flower previously was located, and they are tan to dark brown, orb-shaped pods.
Place the pods in a shallow bowl and set it in a warm, well-ventilated room to finish drying. Leave the pods to dry for approximately seven days, or until the pods begin to crack open.
Split a pod open with your fingers, holding it over a sheet of typing paper. Shake the seeds out onto the paper. Begonia seeds are extremely small.
Fold the typing paper in half, allowing the seeds to collect in the crease. Tilt the paper slightly over a bowl, allowing the seed to roll into the bowl. Bad, withered seed and plant material from the pod won't roll off, as these items are not spherical.
Label an envelope with the begonia variety and year harvested. Store in a cool, dry place until you are ready to replant. Alternately, store in a sealed jar in the refrigerator for up to ten years.