Seasonal Dynamics of Leaf Phenolic Compounds in Rhododendron tomentosum Harmaja Growing in a Coniferous Swamp Forest

Rhododendron tomentosum Harmaja is an evergreen Ericaceae species recognized as a rich source of biologically active phenolic compounds. However, information on seasonal variation in its non-volatile phenolics throughout the annual phenological cycle remains limited. Therefore, this study investigated the influence of phenological development on the phenolic composition of R. tomentosum leaves collected from a natural population in Lithuania between October 2023 and October 2024. Phenolic profiles were determined using a validated HPLC–PDA method. A total of 25 phenolic compounds were identified, comprising phenolic acids, flavan-3-ols, procyanidins, flavonoids, and coumarin derivatives. The highest total phenolic content was observed during autumn–winter dormancy, whereas the lowest levels occurred during fruit development. Procyanidins and phenolic acids predominated during dormancy, while flavonoids, particularly quercitrin, hyperoside, and isoquercitrin, accumulated during flowering. PCA revealed clear phenological grouping of samples and demonstrated distinct associations between dormancy and procyanidin-rich profiles and flowering and flavonoid accumulation. These findings highlight phenological stage as a key factor influencing phenolic composition and indicate optimal harvesting periods depending on the target phenolic compounds.