Begonias (Begonia spp.) are difficult to grow from seed, but the seeds themselves are quite easy to retrieve from plants at the end of the flowering period. These annual plants can flower throughout summer, but produce seeds in small seed pods toward the end of the growing season. Annuals do not return from their roots in the following growing season, so you must retrieve the seeds in fall and store them until spring. The many different begonia species can be used for borders, container gardens or grown indoors as houseplants.
Cut the seed pods from behind the female flower heads when the pods swell and begin to dry, a short time after the flowers expire. Let the seed pods dry as much as possible on the plant, but remove them before they split open and disperse seeds on the ground. Use a pair of scissors or bypass pruners to cut the seed pods off the plant.
Spread the seed heads and any seeds that have burst from the pods in a single layer in a shallow cardboard box lined with newspaper. Store in a cool, dry place for about one week to ensure the seeds and pods are totally dry.
Lay a piece of white paper, such as heavy printer paper or cardstock, on a flat work surface. White paper makes it easy to see the small dark begonia seeds.
Hold a seed pod in your hand over the white paper. Use your thumbs to split the seed pod open and empty the seeds onto the paper. Work with only one or two seed pods at once.
Fold the paper in half and gently shake the paper over the cardboard box so viable seeds fall into the box. Guide the seeds with a finger to hold back the seed chaff and other plant litter so that only the seeds fall into the box. If necessary, return the seeds to the sheet of paper and sift through the chaff a second time.
Store the seeds in envelopes or small jars until you're ready to start them in late winter or early spring. Baby food jars work especially well for seed storage. Label each envelope or jar with the year of retrieval and begonia species and cultivar.