To provide a grass form controlling method freely promoting or suppressing flower formation according to a purpose while suppressing elongation of main stems.

This grass form controlling method includes controlling flower formation while suppressing elongation of main stems through irradiating plants with red light (R; 600-700 nm) adjusted in optical power. Alternatively, the method includes controlling flower formation while suppressing main stem elongation of plants through irradiating the plants with red light (R; 600-700 nm) adjusted in optical power, and irradiating the plants with blue light so as to promote main stem elongation and flower formation. Alternatively, the method includes controlling flower formation while suppressing main stem elongation through irradiating plants with blue light to promote main stem elongation and flower formation, and irradiating the plants with red light (R; 600-700 nm) adjusted in optical power.