We aimed to investigate the control effect of season and time of mowing on Coreopsis lanceolata. To this end, we conducted a 2-year mowing experiment on the vegetation of gravely beds of Kiso River. Mowing was conducted either once, twice, or thrice a year, in June, October, and/or February, since 2007. We observed that the number of flowering shoots decreased in the plot mowed in February (once) ; in plots mowed in June and February and in October and February (twice) ; and in plots mowed in June, October, and February (thrice). In addition, overall, mowing had no effect of reducing the number of individuals, but the number decreased in plots selectively-removed only in June. This experiment shows that mowing in February is effective in decreasing the flowering of Coreopsis lanceolata, and mowing more than once (in February and another season) enhances this effectiveness. Moreover, in this 2-year experiment, we found that mowing does not decrease the number of individuals of Coreopsis lanceolata.