Effect of Spacing and Daily Light Integral on Growth of Sweet Basil (Ocimum basilicum)

Basil (Ocimum L. spp.) is a commonly produced fresh culinary herb. Hydroponic greenhouse producers of fresh-cut culinary herbs want to maximize yield per unit area. One of the easiest ways to increase yield is to use high-density spacing. However, specific recommendations for spacing plants grown under high and low daily light integrals (DLIs) is lacking. Our objective was to quantify the effect of plant density and light intensity on productivity of sweet basil. Seeds of Ocimum basilicum L. ‘Nufar’ were sown into 162-count sheets of phenolic foam hydroponic propagation cubes, placed in a growth chamber with a target air temperature of 24 °C, and irrigated with deionized water or a nutrient solution. Two weeks after sowing, seedlings were transplanted into deep-flow technique (DFT) hydroponic systems with plants spaced 10, 15, 20, 25, or 30 cm apart. The mean greenhouse temperature was 22.1 °C, with mean DLIs of 5.0−6.3 (low) and 14.2−15.2 (high) mol∙m⁻²∙d⁻¹. Nutrient solutions were maintained at a target pH of 6.0 and electrical conductivity of 1.6 mS∙cm⁻¹. Three weeks after transplanting, 10 plants from each system were selected and height, node number, and branch number were recorded. Plants were harvested and the total fresh mass of each DFT system was recorded. Shoots were then placed in a forced air drier maintained at 67 °C for 3 d then weighed and dry mass was recorded. Fresh mass per m² increased with increasing plant density. In the high DLI house, decreasing spacing from 30 to 10 cm between plants increased fresh mass by 747 g∙m⁻², though fresh mass per plant slightly decreased. A similar trend was observed under low-DLI conditions, where increasing density increased fresh mass by 489 g∙m⁻². Fresh mass per plant also decreased under low DLI. Though trends are similar in low and high DLI conditions, yield per m² varied between the two different DLIs. For example, when basil was spaced on 10-cm centers, fresh mass was 334 g greater under high DLI compared to low DLI conditions. Increasing the DLI and density of plants are useful practices to increase fresh mass per unit area in hydroponic systems.