Three Categories of Plant Photoperiods

December 5, 2018

Photoperiodically, plants fall into three primary categories: long-day, short-day, and day-neutral. Plants that are not photoperiodic are day-neutral. In other words, long-day plants initiate flower buds when the days grow longer than their critical daylength. Short-day plants will initiate flowering when the days become shorter than their critical daylength.

Fall-blooming garden mums respond to short days (long nights) to flower.
PC: Ball Horticultural Company

It’s not the length of daytime, or light, the plants are responding to, but rather the period of uninterrupted darkness. The plant measures the amount of PR phytochrome in its system after a length of darkness. Therefore, long-day plants need short nights to flower, and short-day plants need long nights.

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