A plant's response to the relative lengths of day and night periods, which triggers seasonal behaviors like flowering, dormancy, or leaf drop. Plants are classified as short-day, long-day, or day-neutral based on their photoperiodic requirements.
From Greek 'photos' meaning 'light' and 'periodos' meaning 'period' or 'cycle.' The concept was discovered in the 1920s by botanists Garner and Allard, who found that plants measure night length rather than day length to time their seasonal responses.
Photoperiodism is why you can't grow short-day plants like poinsettias year-round under normal conditions - they need long nights to flower! Amazingly, even a brief flash of light during the night can reset their internal timer and prevent blooming.
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