Abstract Thyme is a unique plant with beneficial biological properties and is widely used in the healthcare and treatment sectors. This study was conducted to investigate the effect of horsetail extract on the morphological and growth characteristics of Zataria multiflora (ZM). For this purpose, foliar treatments with horsetail extract at concentrations of 0%, 0.5%, 1%, and 2% were applied to plants at the 7–9 leaf stage. The experiment followed a randomized complete block design with three replications. Evaluated traits included plant height, number of leaves per plant, number of sub-branches, leaf area index, plant fresh weight, plant dry weight, antioxidant activity, and essential oil composition. The results showed that the application of silicon-rich horsetail extract enhanced both growth and phytochemical traits. The 2% horsetail extract treatment significantly increased plant height, leaf length, leaf width, fresh weight, and dry weight. The highest antioxidant enzyme activity was observed under the 2% extract treatment. Additionally, essential oil content increased with higher extract concentrations. The main essential oil components identified were carvacrol (33.75–39.4%), thymol (12.81–14.35%), linalool (7.12–10.22%), and p-cymene (7.45–8.38%). The maximum levels of carvacrol (39.4%) and thymol (14.84%) were achieved at 1% and 2% extract concentrations, showing significant differences compared to the control group. Horsetail extract, rich in silicon, acts as a biostimulant and natural fertilizer, potentially improving the yield and essential oil production of Z. multiflora, particularly under organic cultivation systems.