Plant biostimulants are used to promote plant growth by increasing tolerance to abiotic stressors and improving the efficiency of natural resource use. In the present two-year research (2022–2023 and 2023–2024), the effects of biostimulant foliar applications on the morphological parameters, fresh and dry yields, antioxidant capacity, total phenolic content, and chemical composition of the essential oil of thyme (Thymus vulgaris L.) were studied. For this purpose, four commercial biostimulants, Biostimol Plus + Peptamin-V Plus^®, Acadian MPE^®, Megafol^®, and BlueN^®, were evaluated on thyme cultivated in field conditions. The experiment was laid out in a randomized block design with five treatments and with three replications. During the second growing season, the plants treated with BlueN^®, composed of the bacteria Methylobacterium symbioticum SB23, showed the highest plant weight (152.1 g plant⁻¹), fresh biomass yield (501.9 g m⁻²), and dry yield (172.2 g m⁻²). BlueN^® was the biostimulant that also obtained the highest essential oil yield in both years (0.47 and 0.53%), and for all biostimulants, the amount of thymol and carvacrol increased in the second year, especially with Megafol^® (63.75 and 3.16%). The antioxidant capacity was enhanced in the second year by all biostimulants, according to the ABTS assay, but in particular, by BlueN^® and BPPVP (26.97 μmol/g and 25.01 μmol/g), while the phenolic content was higher in the first year, especially with BlueN^® (65.98 mg GAE/g Extract). The other biostimulants had less intense effects. In conclusion, the biostimulants influenced some characteristics of the essential oil, but the greatest influencers were BlueN^®, Megafol^®, and BPPVP.

During spring–summer 2023 and 2024, an Artemisia annua crop was carried out, using two planting densities (20D = 5.0 plants m⁻²; 40D = 2.5 plants m⁻²). Morphological traits were measured, including height, stem number, diameter, weight, and dry above-ground biomass. The aerial parts were hydro-distilled, and the essential oil (EO) yield increased from the 1st to 2nd year, from 0.117 to 0.439% for 20D and from 0.157 to 0.550% for 40D. There were significant variations in chemical composition between the years, with an increase in the presence of oxygenated monoterpenes in the 2nd year and the disappearance of oxygenated sesquiterpenes. In the 1st year, sesquiterpene hydrocarbons were the main class, while in the 2nd, oxygenated monoterpenes predominated. The main components were artemisia ketone (8.05–65.77%), eucalyptol (4.70–13.14%), and β-selinene (5.38–37.53%), present in all the EOs, and trans-caryophyllene (11.65%), present only in the 1st year EOs. The possible phytotoxicity of the EOs on seeds of plants found in the A. annua crops was evaluated. The most susceptible seeds were Sinapis alba, Papaver rhoeas, and Portulaca oleracea. The phytotoxicity was greater in the 2nd year, with more marked effects on the germination of P. rhoeas and P. oleracea (up to 100%). The inhibition of root elongation reached 100% for those at the higher concentrations tested.