Let the seed pods try on the plant. I like to put a small piece of tin foil around the stem so the pods dont fall off on the ground. After it is completely brown, bring it indoors and dry in a cool, dry place. Then you can break it open and harvest the seeds.

keep in mind that begonia seeds are very fine--i took seed pods, stem and all, and put them in a plastic baggie and just shook them up. the brown seeds on B. subvillosa just shook out as a dust-like powder.

-ming