The main joy of a Rieger begonia is its lovely flowers, so look for new plants with lots of unopened, tight, and healthy buds. Never let your Rieger begonia sit in water or spray the leaves directly with water. Both of these can encourage diseases that will kill the plant.
Light
Reiger begonias need bright, indirect light year-round. They are photoperiod bloomers, meaning the bloom is stimulated by the number of hours of light they receive.
Water
Keep the top 1/2 inch of your Rieger begonia dry between waterings.
Temperature
Reiger begonias need average to warm (and humid) temps. They can go as low as degrees Fahrenheit at night in the fall to help stimulate a bloom.
Soil
Airy, light, fast-draining soil is perfect for these begonias.
Fertilizer
Use liquid fertilizer weekly at quarter strength or biweekly at half strength. In the fall, switch to a high-phosphorous fertilizer to encourage bigger blooms.
Pruning
As old flowers are spent, pinch off the flowers to encourage new blooms to open. After the plant is done blooming, cut it back and let it dry out, then move it to a bright place and begin watering again when the weather warms up.
Propogating Rieger Begonias
Most growers recommend that people throw out their Rieger begonia after the first bloom is over. And from a financial point of view, that makes perfect sense for the growers. Like other begonias, however, the Rieger’s begonia readily propagates from leaf-tip cuttings, so it seems a shame to throw away a plant that wants only for a short dormant period and can be quickly and easily propagated into a host of new plants. The best time to take cuttings is after the bloom is over and the plant has been cut back when the new growth begins to emerge in the spring. Use a seed-starting mix and keep the cuttings evenly moist.
How to Grow Rieger Begonias From Seed
Sow begonia seeds in potting mix, in a seed tray in left a cool place with indirect light. Press the seeds lightly in the soil, water with a spray bottle to dampen. Cover with plastic. Check and mist every few days till seeds germinate. Increase the sunlight. Once seedlings have leaves, replant in larger pots.
Potting and Repotting Rieger Begonias
Even people who keep Rieger begonias frequently don’t repot the parent plant. Instead, after the dormant period is over and the plant has begun to regrow, you can cut off the new leaves and propagate them as leaf-tip cuttings, then discard the parent plant. Alternatively, you can only take some of the new growth for propagation and let the parent plant grow out again (its bloom the following year might not be as spectacular). When repotting a Rieger begonia, only increase the pot size as much as necessary. These plants like to be slightly pot-bound.
Common Pests/Diseases
Watch out for spider mites, whiteflies, aphids, mealybugs, and thrips; all love this beautiful flowering begonia.
