Increasing biomass and essential oil production in four cultivars of Thymus vulgaris L. under rainfed cultivation through optimizing of planting density and manure application

In a field experiment, under rainfed cultivation, four cultivars of Thymus vulgaris L. were grown under two manure levels (no manure and manure application at 50 t ha⁻¹) and three plant densities (8, 6, and 4 plants m⁻²) in a factorial split-plot based on a RCBD experiment during 2020–2022 crop years. Tukey’s HSD test ( P ≤ 0.05) revealed significant differences among treatment combinations for most measured traits. Biomass production responded strongly to manure application and plant density. The highest fresh yield (10,362.4 kg ha⁻¹), dry yield (5,260.6 kg ha⁻¹), essential oil content (3.46%), and essential oil yield (182.17 kg ha⁻¹) were achieved by cultivar 1 under manure application at 50 t ha⁻¹ and 8 plants m⁻². In contrast, the lowest fresh yield (5,339.5 kg ha⁻¹), dry yield (1,697.1 kg ha⁻¹), and essential oil yield (26.24 kg ha⁻¹) were observed under non-manured treatments at low density. The highest thymol concentration (49.49%) was obtained from cultivar 4 receiving manure at 50 t ha⁻¹ and grown at 4 plants m⁻², whereas the maximum carvacrol content (5.37%) was observed in cultivar 2 without manure at 6 plants m⁻². The greatest p-cymene (16.54%) and terpinene-4-ol (0.44%) concentrations were recorded in cultivar 4, while cultivar 2 exhibited the highest γ-terpinene (17.26%). PCA and Cluster analysis confirmed the results of means comparison and revealed substantial differentiation among cultivars, manure, and plant-density combinations. The results indicated that manure application substantially improves biomass accumulation and essential oil production, while lower plant density favors accumulation of major aromatic constituents. Under the conditions of this rainfed experiment, cultivar 1 combined with 50 t ha⁻¹ manure and a density of 8 plants m⁻² produced the highest fresh and dry biomass, EO concentration, and EO yield. This treatment therefore represented the treatment combination with the highest overall biomass and essential oil productivity under the experimental conditions. In contrast, cultivar 4 at lower density tended to favor the accumulation of major phenolic constituents, particularly thymol. Clinical trial number: Not applicable.