Grow Lights: What are Facultative Long-Day Plants?

October 25, 2018

Plants that do not require an exact photoperiod before they are triggered to flower, but will bloom faster or better with a specific photoperiod, are classified as quantitative, or facultative. Lettuce is a classic quantitative long-day plant.

Cosmos flowers and leafy lettuce growing side by side under the same lighting photoperiod.
PC: Leslie F. Halleck

While you might start your tiny lettuce seedlings under a long photoperiod of 16 hours, if you continue growing them indoors you must reduce that lighting period to 12 to 13 hours after you transplant the seedlings into their final container. If you continue growing them under 16 hours of light, the plants may bolt and go to flower too early, ending your harvest.

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