Vertical farming for lettuce production in limited space: a case study in Northern Thailand

Greenhouse vertical farming under natural sunlight is an alternative farming technique that grows crops in a stacking column and extends in a vertical direction. Sunlight availability is one of the crucial factors for crop development in vertical farming. Therefore, this investigation aimed to examine the effect of sunlight availability on lettuce growth and yields at different levels of vertical shelves.

Six shelves were constructed with three levels: upper, middle and lower levels. Lettuces (Lactuca sativa L.) as 'Baby Cos' and 'Green Oak' at 14 days after sowing were planted on the three levels. The photosynthetic photon flux density (PPFD) was recorded, and the PPFD values were then converted to the daily light integral (DLI). Plant height and canopy width were measured three times at 14, 21 and 28 days after transplanting. At maturity, fresh weight (FW) was directly monitored after harvest.

The results showed that the highest PPFD and DLI values were found at the upper level (PPFD 697 μmol m^-2 s^-1 and DLI 29 mol m^-2 d^-1) in comparison to the middle (PPFD 391 μmol m^-2 s^-1 and DLI 16 mol m^-2 d^-1) and lower (PPFD 322 μmol m^-2 s^-1 and DLI 13 mol m^-2 d^-1) levels. The lowest plant height and canopy width values were observed on the upper levels for both lettuce varieties during the three measurement dates. The middle ('Baby Cos' = 123.8 g plant^-1 and 'Green Oak' = 190.7 g plant^-1) and lower ('Baby Cos' = 92.9 g plant^-1 and 'Green Oak' = 203.7 g plant^-1) levels had the higher values of FW in comparison to the upper level ('Baby Cos' = 84.5 g plant^-1 and 'Green Oak' = 97.3 g plant^-1). The values of light use efficiency (LUE) showed an increased trend from the upper to lower levels in both varieties, with values of 'Baby Cos' of 0.10 g mol^-1 in the upper level, 0.28 g mol^-1 in the middle level and 0.26 g mol^-1 in the lower level and 'Green Oak' of 0.12 g mol^-1 in the upper level, 0.44 g mol^-1 in the middle level and 0.57 g mol^-1 in the lower level. The findings of the study indicated the viability of utilizing vertical shelves for lettuce production.