Objective This study elucidates the flowering characteristics and breeding system of S. apetala, both aims at understanding its reproductive regularity, to achieve population control and effective utilization.

Methods The floral characteristics, flowering process, flowering phenology were observed. The changes of pollen viability and stigma receptivity during the whole flowering process of single flowers were measured by potassium iodide and benzidine hydrogen peroxide methods. The pollen ovule ratio and hybridization index were calculated to determine the breeding system of S. apetala. Various pollination experiments were carried out and the visiting insects and visiting behaviors were observed.

Results :(1) The collective flowering period of S. apetala is from May to June and from September to October, which present a double peak Mass-flowering pattern, and the single flowering period is 41.8 ± 4.6 h.(2) Inflorescence of S. apetala is racemes, apetalous, flowers with scented and nectar, stigmas elongate above pistils;(3) Pollen viability and stigma acceptability performed a trend increasing first and then decreased with the flowering progress, and both reached the peak at the full～flowering period;(4) The P/O ratio was 1843.13 and the outcrossing index was 4. Combined with the results of different pollination experiment, it showed that the S. apetala belongs to outcrossing type and self-～compatible plant.(5) S. apetala is mainly pollinated by insects and supplemented by wind, its effective pollinating insects include Apis cerana, Oecophylla smaragdina, Lathyrophthalmus arvorum and Trabala vishnou.

Conclusion This study showed that the flower characteristics of S. apetala were compatible with its breeding system of outcrossing type and self-compatibility. Pollen viability and stigma acceptability were higher in general, and it has two pollination mechanisms: wind pollination and insect pollination. These are the key reasons for its rapid adaptation to new environments, successful introduction and population expansion.