Medicinal Plant-Derived Biostimulants for Crop Improvement: Phytochemistry, Mechanisms, Agronomic Evidence, and Future Prospects

Medicinal and aromatic plants represent a phytochemically rich yet underexplored source of botanical biostimulants. Most research and commercial products focus on macroalgal extracts, humic substances, protein hydrolysates, and microbial inoculants; medicinal plant extracts remain comparatively under-validated mechanistically and agronomically. This review examines 37 experimental studies published up to July 2026, covering cereals, legumes, vegetables, and horticultural crops. It explores the phytochemical composition of medicinal plant extracts, their physiological and molecular effects, extraction and formulation technologies, and regulatory challenges. Extracts of Moringa oleifera, Aloe vera, and Glycyrrhiza glabra, containing diverse phenolics, flavonoids, terpenoids, alkaloids, and saponins, have shown potential to improve seed germination, seedling vigour, crop yield, and tolerance to drought, salinity, and heavy-metal stress. Although plant species differ considerably in phytochemical composition, their extracts may produce similar physiological responses in plants. This suggests that chemically distinct extracts can influence growth through common regulatory pathways. These effects may involve reactive oxygen species (ROS) signalling, enhanced antioxidant defence, and direct free radical scavenging by phenolic compounds. However, responses vary among species, and some metabolite-rich medicinal plant extracts show no measurable benefits. Phytochemical variability, non-standardised extraction, formulation instability, and fragmented regulatory frameworks continue to constrain translation into agricultural practice. Future research should prioritise green extraction technologies, nano-enabled delivery systems, multi-omics integration, and predictive modelling to strengthen mechanistic understanding and identify promising candidates. Standardised extraction and multi-location field trials are essential to ensure consistent efficacy and establish medicinal and aromatic plants as a reliable source of biostimulants.