Maximizing crop yield is essential for the economic viability of indoor vertical farming, where operational costs are high. Among the many factors influencing productivity, planting density stands out as a manageable and cost-effective variable. Optimizing planting density offers a practical approach to improving yields without requiring major structural or technological changes. In this study, we evaluated the effects of planting density on lettuce (Lactuca sativa) yield and individual plant growth characteristics. Two cultivars with contrasting growth habits were used: butterhead lettuce ‘Rex’, known for its compact form, and green leaf lettuce ‘Fusion’, which exhibits an upright growth habit. Plants were cultivated for 24 days after transplanting in a controlled indoor environment maintained at 22 °C, under a photosynthetic photon flux density of 200 μmol∙m-2∙s-1 with an 18-hour photoperiod. Using a deep-water culture hydroponic system, we tested five planting densities: 21, 42, 82, 109 and 131 plants∙m-2. The nutrient solution was prepared with deionized water and a water-soluble fertilizer (12N–1.75P–13.3K; Jack’s Nutrients FeED 12–4–16 RO), providing 150 mg∙L⁻¹ of nitrogen. As planting density increased from 21 to 131 plants∙m-2, total shoot fresh mass per unit growing area rose from 1.6 to 7.2 kg∙m-2 in ‘Rex’ and from 2.6 to 10.9 kg∙m⁻² in ‘Fusion’. However, in ‘Fusion’, increasing density led to a 29% reduction in plant diameter, 19% in leaf number, 25% in leaf area, 33% in shoot fresh mass, and 5% in root fresh mass. Similarly, in ‘Rex’, leaf area and shoot fresh mass decreased by 23% and 26%, respectively, while root fresh mass, plant diameter and leaf number remained relatively consistent across densities. Our results suggest that while increasing planting density from 21 to 131 plants∙m-2 reduces individual plant growth, it increases overall lettuce crop yield per growing area in indoor lettuce production.
