Extending the daily light cycle during the final two weeks of cultivation may increase marijuana yields without diminishing cannabinoid content, according to a new study published in Industrial Crops and Products.
For the study, researchers at Wageningen University and Research conducted a detailed series of experiments using the THC-dominant ‘White Russian’ cultivar. The study aimed to determine whether increasing daily light exposure near the end of the flowering cycle could enhance biomass without interfering with flower development or cannabinoid production—a balance that has posed challenges in past research due to the plant’s photoperiod sensitivity.
Researchers point out that cannabis is a short-day plant that requires limited daylight to induce and maintain flowering. In commercial cultivation, plants typically undergo a vegetative phase under an 18-hour photoperiod, followed by 7 to 10 weeks of 12-hour days to promote inflorescence development. However, the authors note that “short photoperiods limit daily light integral (DLI), thereby restricting crop photosynthesis and plant growth.” They also found that extending the photoperiod during early or mid-flowering caused plants to revert to vegetative growth, with the emergence of new leaves atop inflorescences and delays in floral maturity.
Plant architecture of Cannabis sativa L. ‘White Russian’ after 8 weeks of generative phase. During 8 weeks of generative phase, the plants were grown under 8, 6, 4, or 2 weeks of SD followed by 0 (SD; control), 2, 4, and 6 weeks of blue extended photoperiod (LD-Blue) respectively. LD-Blue means 12 h of 600 µmol m−2s−1 white light (SD), extended with 6 h of 250 µmol m−2s−1 blue light. For visualization purpose, the fan leaves at the lower part of the canopy were removed.
To explore whether light could be safely extended after flowers had already formed, researchers tested various photoperiod extension schedules, light spectrums (blue, red, and white), and intensities (250, 600, and 800 µmol m−2s−1). They found that when the short-day cycle was extended too early—especially with low-intensity blue light for four or six weeks—plants reverted to vegetative growth. Inflorescences became underdeveloped, and new leaf growth emerged on top of flowering sites, compromising both flower structure and THC concentration.
In contrast, when the 18-hour light cycle was introduced only during the final two weeks before harvest, plants did not show signs of flower reversion. Regardless of the light spectrum used, flower development remained normal, and the overall inflorescence dry mass increased. At a higher light intensity of 600 µmol m−2s−1, the increase in yield reached 12.5 percent compared to plants kept under a constant 12-hour cycle. Importantly, cannabinoid concentrations, including THC and CBD, were not negatively affected when light was added only in the final two weeks.
Plant architecture of Cannabis sativa L. ‘White Russian’ after 8 weeks of generative phase (at final harvest). During the last 2 weeks before harvest, the plants were grown under SD, or extended photoperiod with blue, white, and red light. LD means 12 h of 600 µmol m−2s−1 white light (SD), extended by either blue (LD-Blue), white (LD-White), or red (LD-Red) for 6 h at 250 µmol m−2s−1. For visualization purpose, the fan leaves at the lower part of the canopy were removed.
The study also found that the spectrum of the extended light—whether blue, red, or white—had no significant impact on the final flower mass or cannabinoid content. Instead, it was the duration and intensity of added light that mattered most. At a modest intensity of 250 µmol m−2s−1, yield improvements were marginal. But when the light intensity matched that used during the regular flowering phase (600 or 800 µmol m−2s−1), plants were able to capitalize on the additional photons to boost flower growth without altering the chemical profile.
The findings offer a valuable cultivation strategy for indoor growers, suggesting that modest photoperiod extensions near harvest—applied at full light intensity—can increase yield without sacrificing quality. However, timing is critical. Exceeding two weeks of extended light or starting too early in the flowering cycle can lead to a reversal in growth phase and a decline in flower and cannabinoid development.
Researchers caution that the results may not apply uniformly to all cannabis genotypes, especially day-neutral varieties or CBD-dominant cultivars. They recommend further studies to assess strain-specific responses and to evaluate the potential effects on terpenoid profiles and secondary cannabinoids.
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