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Longer Photoperiod Substantially Increases Indoor-Grown Cannabis’ Yield and Quality: A Study of Two High-THC Cultivars Grown under 12 h vs. 13 h Days


Ashleigh Ahrens

David Llewellyn

Youbin Zheng

School of Environmental Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada

Author to whom correspondence should be addressed.

Plants 2024, 13(3), 433;

Submission received: 7 January 2024 / Revised: 26 January 2024 / Accepted: 30 January 2024 / Published: 1 February 2024

(This article belongs to the Special Issue Cannabis sativa: Advances in Biology and Cultivation)

The publication by Ahrens, Llewellyn, and Zheng presents findings from a study on the effects of extending the daily photoperiod from 12 hours to 13 hours on the yield and quality of indoor-grown cannabis, specifically focusing on two high-THC cultivars, ‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG). The study demonstrates that a longer photoperiod substantially increases both the yield and quality of cannabis without delaying flowering initiation time in GG, though IM showed a slight delay.

Key findings include:

  1. Increased Yield: Extending the daily photoperiod to 13 hours resulted in significantly higher inflorescence yields for both cultivars compared to a 12-hour photoperiod. The yields were 1.35 times higher for IM and 1.50 times higher for GG, which is 4 to 6 times higher than the relative increase in daily light integrals (DLIs).

  2. Quality of Inflorescences: The concentrations of major cannabinoids in the 13-hour treatment were either higher or not significantly different from those in the 12-hour treatment for both cultivars, indicating that the longer photoperiod did not negatively affect the chemical quality of the cannabis produced.

  3. Vegetative Growth: The vegetative growth of IM plants was greater and more robust under the 13-hour photoperiod, suggesting that a longer photoperiod can also enhance the physical growth and development of the plants.

  4. Flowering Initiation and Development: There was no delay in the flowering initiation for GG, but IM experienced a minor delay under the 13-hour photoperiod. Despite this, the extended photoperiod led to increased yields and did not negatively impact the overall flowering process.

  5. Commercial Implications: These findings suggest substantial commercial benefits for using photoperiods longer than 12 hours to increase yields without decreasing cannabinoid concentrations for certain cannabis cultivars grown indoors.

This study highlights the potential for optimizing indoor cannabis cultivation practices through adjustments in photoperiod length, providing evidence that a moderately extended photoperiod can lead to significant increases in both yield and quality for certain high-THC cannabis cultivars.


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