Grow Lights: What are Qualitative and Quantitative Photoperiods?

July 26, 2018

Photoperiodic plants fall into two subcategories: qualitative and quantitative. Plants that require a specific critical daylength to flower are classified as qualitative, or obligate. Without that exact amount (or more) of uninterrupted darkness, the plant will never flower.

The flowers and bracts of this ‘Peterstar White’ poinsettia are the result of exposure to a specific short-day (long-night) photoperiod.
PC: Ball Horticultural Company

Poinsettia plants are a classic example of a qualitative short-day plant. They will not initiate flower buds until the dark period reaches around 11 hours and 45 minutes, depending on the cultivar. Flower initiation is optimal with a 14-hour period of darkness.

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