Several studies reported that rare earth elements (REEs) can exert both beneficial and harmful effects on plants, including improved seed germination, plant growth, and crop yield. However, these findings remain controversial and highly variable, likely due to differences in the specific REEs investigated, their concentrations, and the plant species studied. In our study we assessed the effects of increasing concentrations of gadolinium (Gd) and cerium (Ce), together with sodium citrate (used as chelating agent), on two important plant species: tomato ( Solanum lycopersicum ) and quinoa ( Chenopodium quinoa ). The aim was to establish the effects on seed germination and seedling growth, also detecting the presence of hormesis through the application of a statistical model. Seeds were placed in Petri dishes on filter paper soaked with the corresponding solution (from 10 μM to 150 μM; deionized water used as control). After germination, shoot and root length were measured and several indexes related to germination and initial seedling growth were calculated. The results highlighted species-specific and concentration-dependent responses. In quinoa roots, Ce induced a subtle biphasic dose–response trend characterized by a growth stimulation at low concentrations (10 μM) and inhibition at higher levels. Instead, tomato shoot length increased under both REEs, while high Gd doses negatively impacted on root growth. Low concentrations of REEs could be a successful approach to stimulate plant development, particularly in initial growth stages. Nevertheless, their mechanisms and the suitable dosage remain unclear, determining the need for further studies, especially to assess the long-term effects on ecosystems.