We investigated the seasonality of the development of gametangia and sporophytes of Racomitrium lanuginosum at a snow-free site (ca. 645 m on Mt. Mihara) and compared it with a seasonally snow-covered site (ca. 2200 m on Mt. Fuji) to clarify the effects of the length of growth period regulated by snow cover. The number of inflorescences and the number, size and developmental stages of male and female gametangia and sporophytes were recorded once every month from March 2014 to February 2015. At the snow-free site, archegonia rapidly developed in spring, but antheridia took longer to develop from the late summer; sperm dispersal mainly occurred in winter. Spore dispersal occurred from winter to summer and fertilization occurred in spring. Some notable phenological patterns were observed at the snow-free site, such as the discordance of the timing of maturation between male and female gametangia, and the occurrence of spore dispersal in winter. These results suggest that the snow-free site is close to the limit of successful sexual reproduction and that snow cover is an essential factor to maintain the phenological patterns of this species.