Optimizing Photoperiod and PPFD at Fixed DLI in Vertical Farms
Researchers from the University of Bologna, in collaboration with the Swedish Environmental Research Institute, evaluated how extending the photoperiod duration while proportionally reducing photosynthetic photon flux density affects energy efficiency and yield in leafy greens. The experiment was conducted on a commercial vertical farm equipped with full-spectrum white grow lights.
Experimental parameters and studied regimes
The primary condition of the trials was maintaining a fixed daily light integral (DLI) at 9.75 ± 0.15 mol/(m²·day). The study compared three combinations of photoperiod and PAR intensity (PPFD):
- 14 hours of lighting per day at a PPFD of 190 μmol/(m²·s);
- 18 hours of lighting per day at a PPFD of 150 μmol/(m²·s);
- 24 hours of continuous lighting at a PPFD of 115 μmol/(m²·s).
The experiment covered four crops: crisphead lettuce (Lactuca sativa L.), canasta lettuce (L. sativa L.), kale (Brassica oleracea L.), and pak choi (Brassica rapa L.). All plants were grown in a closed multi-tier module under controlled temperature, relative air humidity, and continuous hydroponic mineral fertigation.
Energy efficiency and plant morphology
Reducing lighting intensity while simultaneously extending daylength increased lighting energy use efficiency (L-EUE) approximately two-fold for canasta lettuce, kale, and pak choi without reducing crop mass. For crisphead lettuce, L-EUE values did not differ significantly between the three regimes, but the lowest energy efficiency for the remaining crops was recorded under the short photoperiod (14 hours) and maximum PPFD (190 μmol/(m²·s)).
The efficiency gain was driven by the fact that under moderate PPFD levels, the photosynthetic apparatus is not overloaded with an excess photon flux and operates in the linear region of the light response curve. As a result, virtually every absorbed photon is utilized for biomass accumulation, whereas at 190 μmol/(m²·s), a portion of the energy is dissipated as heat.
Plant morphological responses proved to be species-specific:
- Both lettuce cultivars showed maximum plant height under continuous lighting (24 hours, 115 μmol/(m²·s));
- Pak choi formed a broader leaf lamina and demonstrated higher light use efficiency (LUE) under the 14-hour regime at 190 μmol/(m²·s).
The increased lettuce height under continuous lighting did not compromise commercial appearance or head compactness, while for kale, daylength extension was accompanied by uniform rosette development without signs of chlorosis or tipburn.
Implications for indoor cultivation facility design
For vertical farms and facilities without natural light, the balance between installed LED fixture capacity and operating hours determines capital and operational expenditures. Transitioning to a longer photoperiod at reduced PPFD decreases peak electrical demand on the power distribution network and reduces peak thermal output from luminaires, which is essential for sizing HVAC systems.
When a lighting installation delivers 115 μmol/(m²·s) instead of 190 μmol/(m²·s), fewer diodes are required or luminaires can operate at lower drive currents. This lowers LED junction temperatures, enhances efficacy (μmol/J), and extends fixture service life without spectral degradation.
In facilities that feature independent lighting scenario control, extending the photoperiod allows daily energy consumption to be leveled across the 24-hour cycle, reducing cable gauge requirements, circuit breaker ratings, and power supply capacity. In addition, steady around-the-clock power draw makes it possible to take advantage of off-peak and shoulder electricity tariffs without sharp load spikes on the cooling infrastructure.
Study limitations
The trial was conducted at a single commercial facility using full-spectrum white LEDs at a DLI of 9.75 mol/(m²·day). The study evaluated four leafy green species strictly during the vegetative stage. Applying these findings to commercial greenhouse operations with natural light or higher-irradiance systems will require additional trials at elevated DLI levels and alternative spectral recipes.
Practical takeaways
When engineering lighting schemes for vertical farm leafy green production lines, it is advisable to consider continuous or extended photoperiods (18–24 hours) with moderate PPFD (115–150 μmol/(m²·s)). This enables a reduction in nominal installed LED capacity without biomass loss, significantly lowering initial CAPEX on climate control equipment.
For crops such as pak choi, where leaf morphology and rosette compactness are critical for market quality, maintaining a moderately short photoperiod with higher radiation intensity remains preferable. The optimal lighting strategy should be selected individually based on cultivar traits and crop management protocols.
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