Most medical cannabis cultivars are short-day plants, meaning they require limited light exposure to trigger flowering. That restriction also limits photosynthesis, and ultimately, growth and yield.
A recent study explored whether extending the photoperiod after flowering has already begun could help increase inflorescence yield without negatively affecting cannabinoid levels.
Researchers ran several trials in a controlled environment, extending the standard 12-hour photoperiod to 18 hours using 6 hours of supplemental blue light at 250 μmol m−2s−1. When applied from week 2 or week 4 of the flowering stage, this caused plants to revert to vegetative growth, delaying flower maturity and producing unwanted leaf growth on top of the buds.
However, when the same blue-light extension was applied only during the final two weeks of flowering—starting at week 6—plants showed no negative impact. Yield, development, and cannabinoid content were comparable to those grown under a consistent 12-hour light schedule.
The team then tested different light spectra (blue, red, and white) during that final two-week period and found no significant differences in yield or cannabinoid content. That said, light intensity appeared to play a major role. In a follow-up trial, the final two weeks of flowering were supplemented with white light at either 600 or 800 μmol m−2s−1. Yield increased in both cases, but the improvement was statistically significant only at 600 μmol m−2s−1.
These findings suggest the potential to increase inflorescence yield without negative effect on cannabinoids by extending the photoperiod, but it appears effective only during the last 2 weeks before harvest and with a substantial increase in amount of light.
Wannida Sae-Tang, Ep Heuvelink, Iris F. Kappers, Willy Contreras-Avilés, Donis Bernal Cortes, Hein Groen, Jesús Marin Gomez, Céline C.S. Nicole, Leo F.M. Marcelis,
Long-days during the last two weeks before harvest applied to short-day medicinal cannabis can improve inflorescence yield, Industrial Crops and Products, Volume 233, 2025,
121442, ISSN 0926-6690
Source: Science Direct
