Photoperiod

Photoperiod is the 24-hour ratio of light to darkness perceived by a plant; in cannabis cultivation it denotes the light regimen used to drive developmental transitions. Most Cannabis sativa cultivars are facultative short-day (long-night) plants, initiating inflorescence development when the daily photoperiod falls below a cultivar-specific critical threshold (Small 2017; Spitzer-Rimon et al. 2019). Indoor growers almost universally use 18 h light / 6 h dark (or 24/0) to maintain vegetative growth and then abruptly shift to 12 h light / 12 h dark ("12/12") to trigger flowering (Potter 2014). Zhang et al. (2021) demonstrated critical photoperiods of 13 h 45 min to 14 h in many hemp cultivars, with substantial cultivar-dependent variation; Ahrens et al. (2023, 2024) showed some drug-type cultivars flower robustly at 13 h with yield gains. Light leaks as low as ~2 μmol·m⁻²·s⁻¹ during the dark period can delay or disrupt flowering (Zhang et al. 2021). Etymology: Greek phōs/phōtos ("light") + periodos ("circuit, cycle"). Synonyms: daylength, light cycle, 12/12. → See also: Vegetative, Flowering, Auto-flowering.