Optimizing sowing window and planting density for enhanced growth, productivity and biochemical quality of senna (Cassia aungustifoli Vahl.)

Background Cassia aungustifoli Vahl. (senna) is an economically important medicinal plant cultivated for its sennoside-rich leaves. Crop establishment practices, particularly sowing time and plant density, regulate canopy development, nutrient acquisition, and biomass production; however, their interactive effects on growth, nutrient-use efficiency, and leaf quality under semi-arid conditions remain poorly understood. Results Field experiments were conducted during the Rabi seasons of 2023–24 and 2024–25 using a split-plot design comprising five sowing windows and three planting densities. Early sowing during September II fortnight (S₁) and October I fortnight (S₂) significantly enhanced plant height, canopy spread, branching, leaf area, leaf area index, SPAD chlorophyll, carotenoid content, nutrient uptake, and fresh and dry leaf yields compared with delayed sowing. The 45 × 30 cm planting density (D₂) produced the highest leaf yield, crude protein content, nutrient uptake, and economic returns while the results of sennoside yield remained non-significant, whereas 60 × 30 cm (D₃) promoted greater individual plant growth. The S₁D₂ interaction consistently recorded the highest biomass production, nutrient uptake, nutrient-use efficiency, and benefit–cost ratio. Early sowing also reduced residual soil nitrogen, phosphorus, and potassium, indicating greater resource acquisition and utilization by the crop. Conclusions Early sowing, particularly the September II fortnight, improved vegetative growth, physiological traits, nutrient uptake and leaf biomass, while 45 × 30 cm spacing maximized dry leaf yield per unit area and economic returns. Wider spacing (60 × 30 cm) favoured individual-plant growth, but did not maximize yield per hectare. Sennoside concentration was statistically unaffected by the tested sowing windows and planting densities. Under the conditions of this two-year study, September II or October I sowing combined with 45 × 30 cm spacing provided a practical strategy for high leaf biomass and economic productivity.