Germinating the seeds
If you have germinated other Begonias before, you may use the same technique that worked for you. The tips below reflect what has worked for me, although other methods can certainly work.
Getting started -- Use about 6 to 10 small pots or cups that are 2-4 inches (5-10 cm) tall. The pots should have drainage holes.
A typical soil mix for germinating them is 3 parts potting soil to 2 parts perlite or coarse sand. An alternate mix is equal parts of coir fiber (or peat moss) and perlite, with some slow-release fertilizer added. Fill the containers with the mix and water it until it is evenly moist, but not soggy.
The seeds are very tiny, so work in a well-lit area. Sprinkle several seeds evenly across the surface of each pot. An easy way to pick up the seeds is by breathing on your finger tip to lightly moisten it, then dabbing the seeds with it. After sprinkling them on the soil surface, drip a few drops of water over the seeds, to spread them down into the cracks of the soil, which will keep them moist, but still give them light. Do not bury the seeds with soil like you would with larger seeds.
Until the seeds sprout, ensure that the soil surface never dries out. You can maintain high humidity by enclosing the pots in a plastic container or bag - just leave it open a crack to let in fresh air. Once or twice a day, drip a few drops of water on the surface to keep it moist.
The seeds germinate well at about 65-74 degrees F (18-23°C). I don't have experience germinating them outside this temperature range. I recommend placing a minimum/maximum thermometer near the pots, especially if using a heating mat.
The seeds need light to germinate, so place the containers in a bright spot out of direct sun. A fluorescent bulb kept 4 inches (10 cm) above them provides the right amount of light.
The seeds should start sprouting in 2-3 weeks, but may take longer at cooler temperatures. Continue dripping water on the soil surface after they sprout, since young seedlings have a small root system.
When the seedlings are a month old, you may carefully pull them and transplant them, however i recommend simply cutting off any slower ones, and leaving 1 strong seedlings in each pot.
Fertilizing -- If your potting soil contains fertilizer (check the package) your seedlings shouldn't need feeding the first 3-4 weeks. Then feed about every 2 months with a slow-release (pelleted or organic) fertilizer.
On older plants, it's normal for an older leaf to occasionally turn yellow and drop, but if it seems excessive, the plant may need more nitrogen fertilizer. The soil could also be too dry down in the root zone.
Watering -- Once the seedlings are a month old, you may stop keeping the soil surface moist at all times. Aim to keep the rest of the soil evenly moist (but not soggy).
Lighting -- The plant likes filtered sun or bright shade + morning sun. Protect it from strong afternoon sun.
Transplanting -- When the seedlings are 2 months old, repot to a larger container about 1 quart (1 liter) in size. Water the soil before repotting, to keep the soil ball from breaking apart, which can damage the roots. Protect from direct sun the first week after repotting.
Climate -- Begonia fusca comes from mid-elevations in Mexico, where the climate is mild all year. It has handled temperatures in the mid-80s (30°C) - use caution in warmer temperatures. It reportedly can re-sprout from the roots in Zone 9, but i recommend protecting it from freezing temperatures. Indoors, keep the humidity above about 40-50%, perhaps with the use of an ultrasonic humidifier if necessary.
If you have any questions or problems, please email me.
Enjoy your plants!
Strange Wonderful Things