Sapindus mukorossi Gaertn. is a traditional and important virescent tree species in southern China with peel and seed oil rich in saponins. This tree species is one of the newly developed woody-oil species approved by the State Forestry Administration of China. To investigate the flowering phenology and characteristics of this multipurpose tree species, we observed and compared 9-yr-old plantations of S. mukorossi at population, individual, inflorescence, and single-flower levels in Zhejiang Province, Tiantai County, from 2012 to 2013, based on the first flowering date, relative flowering intensity, flowering synchrony, and so on. The results showed that the flower color of S. mukorossi was yellowish white and the flowers were either male or bisexual. The male flower (5.09 mm in diameter) was larger than the bisexual flower (3.72 mm in diameter). There were usually eight stamens in the male flower; however, a few flowers had seven, nine, or ten. The pistil of the male flower was degenerated and only a slight, light-green protuberance was preserved. The calyx of the bisexual flower folded together lightly and the anthers were under the perianthes. There were generally eight stamens in the bisexual flower, but some also had seven or nine. The chapiter was higher than the anther and was outside the bud. Generally, the ovary had three chambers, but had two or four in rare cases. S. mukorossi flowered earlier in 2012 than in 2013, and the flowering period of the test populations was from the middle of May to late June. The development of the single flower was 8-9 d long from the flower bud to wilting stages. In 2012 and 2013, the florescences of the population, individuals, and inflorescences were approximately 30, 20, and 11 d, and 28, 19, and 13 d, respectively. The flowering amplitude of individuals and inflorescences showed a single peak, sharing a high similarity from year to year. The temporal patterns of flowering of S. mukorossi were very similar at the population level, with a well-defined peak and marked synchrony. The index of synchronicity was about 0.868. The relative flowering intensity was distributed over a wide range, but was concentrated between 30% and 40%. The flowering intensity showed an extremely significant (P=0.01) correlation annually and over different years. The range of variation of flowering synchrony was 0.81-0.97 and 0.70-0.98 in 2012 and 2013, respectively. The flowering synchrony correlation was extremely significant (P=0.01) annually, but not in different years. This suggested that there were temporal and spatial differences in the allocation of S. mukorossi reproductive resources, with longer flowering periods and higher flowering synchronization. Long flowering periods can alleviate the interference of illegal pollen and maintain population gene diversity. Heredity is the main factor affecting flowering phenology in S. mukorossi populations. The effects of ecological and environmental factors on flowering phenology need to be researched further.