Many flowering plants use the pigment phytochrome to sense seasonal changes in day length, which they take as signals to flower. This sensitivity to day length is termed photoperiodism. Short day plants flower when the length of daylight falls below a certain critical level.
It is not actually the period of light exposure that limits flowering. Rather, a short day plant requires a minimal length of uninterrupted darkness in each 24-hour period (a short daylength) before floral development can begin.
Plants make use of the phytochrome system to sense day length or photoperiod. This fact is used by florists and greenhouse gardeners to control and induce flowering out of season, such as with Poinsettia.