Bioactive Compounds in Hawthorn Leaves (Crataegus spp.)—Extraction, Functionality, and Future Perspectives: From Waste to Wealth

The transition to a circular bioeconomy enhances the valorization of agricultural by-products. Hawthorn leaves (Crataegus spp.), generated in large quantities from orchard maintenance, represent a promising yet underutilized biomass. This comprehensive narrative review synthesizes recent advances regarding their bioactive compounds, extraction methods, and applications. A systematic literature search was conducted to identify relevant studies. The analysis reveals that hawthorn leaves are rich in polyphenols (e.g., flavonoids, procyanidins), with their content often exceeding that found in fruits. Modern “green” extraction techniques (e.g., ultrasound- and microwave-assisted) demonstrate superior efficiency in recovering these thermolabile compounds compared to conventional methods. The broad spectrum of associated biological activities—including antioxidant, cardioprotective, neuroprotective, antimicrobial, and insecticidal effects—underpins their potential in nutraceuticals, cosmetics, and functional foods. Crucially, this review highlights the significant promise of hawthorn leaf extracts as a source for developing natural, plant-based biopesticides, aligning with sustainable agriculture and integrated pest management principles. To fully realize this “waste-to-wealth” potential, future research should prioritize the scaling of eco-friendly extraction, field trials for crop protection efficacy, and the standardization of extracts.