How to Prune begonia 'Lucerna'?
No pruning required

begonia 'Lucerna' · Begonia 'Lucerna'
Grow outdoors in well-drained humus-rich, neutral to acidic soil. Grow in good but not direct light in a frost-free area. Under glass grow in well drained loam-less or loam-based compost. Will not tolerate water logging. See Begonias: outdoors for further advice.
No pruning required