As rare earth elements gain strategic importance, knowledge of their environmental pathways becomes increasingly needed. In particular, mechanistic insight into plant uptake of rare earth elements informs both risk assessment and mitigation strategies in case of environmental contaminations and modern green applications such as biomining. In this study, we addressed the mobility, speciation and deposition of Eu(III), serving as surrogate for trivalent lanthanides, within the Poaceae Sand oat (Avena strigosa) from both microscopic and macroscopic perspectives. Using hydroponic bioassociation and extraction experiments, we tracked the metal’s pathway within the plant. A combination of (micro)spectroscopic and chromatographic techniques, mass spectrometry, autoradiography and iterative factor analysis enabled us to develop a comprehensive understanding of Eu(III) speciation and its influence on translocation of lanthanides within plants. The results show that Eu(III) is absorbed by epidermal cells and root tips, but predominantly the apoplast, in which Eu(III) is subjected to cell wall binding and phosphate precipitation. Internalized Eu(III) is bound to organophosphate ligands in the cell interior. Xylem loading occurs within less than one hour and translocation to the shoots is achieved by complexes with oxalate, citrate and malate. The use of radioactive 152Eu(III) as tracer revealed that the majority of the metal remains in the roots, while a minor portion is deposited uniformly in the non-vascular tissue of both young and mature leaf lamina. These findings advance our mechanistic comprehension of rare earth element transport, the chemical binding environments encountered in plants and lay the foundation for environmental risk assessments and phytomanagement for metal-enriched areas.