Phyllostachys nigravar.henonis, a monocarpic bamboo with a 120-year flowering interval, is next predicted to flower in Japan in the 2020s. Because a huge area of the country is presently covered by stands of this species, post-flowering dieback of these stands and ensuing drastic changes in land cover may cause serious social and/or environmental problems. No study on the regeneration of this bamboo species was conducted during the last flowering event in the 1900s, and the regeneration process of this species is thus still unknown. In 2020, we encountered a localized flowering ofP.nigravar.henonisin Japan and used this discovery as a rare opportunity to study the initial regeneration process of the species. Over 3 years, more than 80% of culms in the study site bloomed, but no seed was produced. In addition, no established seedlings were located. These facts strongly suggest thatP.nigravar.henonislacks the ability to produce seeds and cannot undergo sexual regeneration. Some bamboo culms were produced after flowering but died within 1 year of emergence. Small, weak culms (dwarf ramets) also appeared after flowering, but most died within 1 year as well. Three years after flowering, all culms had died, with no sign of regeneration detected. According to our 3 years of observation, this bamboo appears to be hard to regenerate—an idea completely contradicted by the fact that this species has long persisted in Japan. We thus considered other possible regeneration modes forP.nigravar.henonis.