Hemerocallis citrina Baroni (Daylily): A Comprehensive Review of Flavonoids and Phenolic Compounds—Extraction, Phytochemistry, Biological Activities, and Food Applications (2000–2026)

Hemerocallis citrina Baroni, commonly known as daylily or “Huang Hua Cai,” is a well-established traditional medicinal and edible plant widely cultivated in East Asia for more than two millennia. Its flower buds, stems, leaves, and roots are rich in flavonoids and phenolic compounds, which constitute the principal bioactive phytochemicals responsible for a wide range of pharmacological effects, including antioxidant, anti-inflammatory, neuroprotective, antidepressant, and sleep-improving activities. Although several studies have investigated the chemical composition of Hemerocallis citrina Baroni, a comprehensive and up-to-date review specifically focused on the flavonoids and phenolic compounds of this plant remains lacking. In this review, we systematically summarize the research progress from 2000 to 2026 on the extraction methods, structural identification, phytochemical profiles, and biological activities of flavonoids and phenolic compounds from H. citrina flower buds, stems, and leaves. Advanced analytical techniques, such as UPLC-PDA/MS, UHPLC-QQQ-MS/MS, and HPLC-Q-TOF-MS, have revealed a diverse array of these compounds, including quercetin, kaempferol, and isorhamnetin glycosides, as well as various caffeoyl- and p-coumaroylquinic acids. The health-promoting effects of these polyphenols, together with current knowledge on their mechanisms of action and potential applications in functional foods and traditional Chinese medicine, are critically discussed. By consolidating recent phytochemical and pharmacological findings, this review provides a scientific foundation for the further development and utilization of H. citrina flavonoids and phenolic compounds as valuable resources for functional foods, nutraceuticals, and natural therapeutics.