瑜伽師地論略纂 11.164

11.164

論云「如優曇鉢花時乃出現」，謂西方有樹，常不出花，但於枝岐間生，若輪王出世，方始生花，先云優曇鉢花也。

When the treatise says "appearing only occasionally like an udumbara flower", it means that in the western regions there is a tree that ordinarily does not produce flowers, which only grow between its bifurcated branches; only when a wheel-turning king appears in the world does it finally begin to flower. In earlier times it was called the udumbara flower.