The tuberous begonia (Begonia x tuberhybrida) is a showy garden flower whose waxy, succulent petals are edible — crisp and vividly citrus-tart, tasting almost like a sharp green apple or lemony sorrel. The petals brighten salads, drinks and plated desserts, but they are high in oxalic acid and so are used sparingly as a garnish rather than eaten by the handful.