In the subalpine zone on Mt. Hakusan, Japan, P. hakusanensis Koidz., a native alpine species and Plantago asiatica L., regarded as a domestic alien plant grow synpatrically along with their hybrids. Here, we observed their flowering phenology on Mt. Hakusan from 2011 to 2014 in order to clarify the factors of natural hybridization. We established 1, 3 and 6 m2 plots (total of 18 plots) in the natural habitat, recorded the number of flowering inflorescences of P. hakusanensis, P. asiatica and their hybrids. In addition, we calculated the effective cumulative temperature for the first flowering from snow disappeared, and investigated their number of seeds production per plant. In P. hakusanensis, many plants bloomed a lot of inflorescences immediately after the timing of snow disappearance every year, and in some years a part of the plants bloomed a few of inflorescences again after 37.8 ± 8.1 days and 302.4 ± 67.2 degree-days from snow disappearance. In the second flowering time of P. hakusanensis, P. asiatica started flowering after 56.4 ± 13.1 days and 403.0 ± 96.7 degree-days from snow disappearance, so their flowering times overlapped. P. asiatica was able to produce many seeds in the warm year. The flowering time of their natural hybrids overlapped with P. hakusanensis. Moreover they produced seeds every year and their death rate was low. Therefore, they may remain and pollinate with P. hakusanensis.