On February 16, 2011, Page Engelhart held a workshop where members made wedge cuttings of Begonias. They were Rhizomatous. All parts of the leaf are capable of rooting and forming a new plant. The only requirement is that the leaf portion contain a main vein.
You planted your leaf in potting soil, and by now should have watered the wedge thoroughly FROM BELOW. Watering from above can dampen the entire wedge and cause rot.
Tent the pot with your baggy, or place it in a plastic propagation box, to keep the humidity high. If using a baggy, check the pot regularly to make sure condensation doesn’t become so great the the dew drops fall and rot your leaf. Keep the baggy off your wedge with chopsticks, skewers, or related tool.
Your plant should be kept at room temperature (65 to 75 degrees), ideally under fluorescent lights for 14 to 16 hours a day. If you do not have lights, place your plant in an East-facing window to ENJOY LIGHT BUT NOT HEAT. You should not need to water your plant (after the initial soaking) for a few weeks if you keep it enclosed. This is more true with the plastic pots than the degradable ones, which will dry out faster.
Keep the plant where you can see it daily. It’s easy to forget things that don’t cry out!
The cuttings will root within 2 to 3 weeks (or they will die) but probably won’t show new growth until a couple of weeks after that.
When you see new leaves coming, start opening up your plastic baggy/propagation box to acclimate your tender babies to the drier air. Then water as needed but keep in mind that begonias like to dry between waterings.
Below are the titles of a couple of great books on propagating as well as a thorough website on propagating begonias.
Toogood, Alan. Plant propagation. NY NY: DK Publishing, 1999 Druse, Ken. Making More Plants. NY NY: Clarkson Potter Publishers, 2000