光周期是指植物每天接受光照的时长。许多植物利用光周期作为触发开花、休眠或营养生长的信号。室内种植者通过设置生长灯上的定时器来控制光周期，从而控制植物的生长，而无需考虑季节。
关键事实
- Long-day plants flower when light exceeds a critical threshold (typically 14–16 h)
- Short-day plants flower when darkness exceeds a critical threshold (typically 12+ h of dark)
- Day-neutral plants flower regardless of photoperiod (e.g. cucumbers, tomatoes)
- Indoor growers use 18/6 (light/dark) for vegetative growth and 12/12 for flowering
- Even brief light interruptions during the dark period can delay or prevent flowering
- LEDs and timers give indoor growers precise, season-independent control