Begonias grow from a tuberous root system, similar to that of a rhizome. Unlike rhizomes, though, a begonia root cannot be cut apart and divided when you desire new plants. Instead, grow new begonias from seed when you desire more of these flowering plants for the shady parts of your garden. Begonia seeds must be started in January if the plants are going to mature enough to bloom in the summer garden. Starting from seed is an inexpensive way to add these versatile perennials to your garden.
Fill a seed starting flat with a sterile potting mixture that is formulated for seed starting. Water the mixture until it is moist then allow it to drain for one hour prior to planting the begonia seeds.
Sprinkle the seeds sparsely across the top of the potting mixture. Begonia seeds are extremely fine so exact spacing is difficult. Generally you should strive to sprinkle one seed per square inch but over-seeding is likely unavoidable.
Cover the top of the seedling flat with a plastic bag. Set it in a warm (ideally, 70-degree F) room that receives bright, indirect sunlight to germinate. Germination takes approximately 10 days.
Remove the plastic once sprouts appear. Set the flat in a well-lit area and place a small room fan near the flat. Set the fan on low. As the fan blows over the seedlings the breeze causes them to grow strong stems. The breeze also dries out the soil surface and helps prevent fungal growth.
Thin the seedlings so they are spaced 2 inches apart in all directions. Pluck out the extra seedlings that sprouted in the tray or cut them off at soil level with a small pair of scissors to remove them.
Water the begonia as necessary to keep the soil moist but not soggy. Place water in the drip tray under the flat and let the flat sit in this water for one hour. The water is drawn up into the soil via the drainage holes in the flat. Empty the excess water out of the drip tray after one hour.
Transplant to 3 to 4 inch pots once the seedlings have produced their third set of leaves. Lift the begonia seedlings from the tray by their leaves then plant them at the same depth in the new pots. Water the soil in the pots lightly to ensure it is evenly moist throughout but not soggy.