Oxide perovskite phosphors (ABO₃-type) are promising light-conversion materials for controlled-environment agriculture, but their large indirect bandgaps (3.0–4.7 eV) and strong thermal quenching limit radiative efficiency and stability. In this study, a dual-site engineering strategy is applied to LaMg_0.5Sn_0.5O₃, where partial substitution of Ge⁴⁺ for Sn⁴⁺ yields a wide-bandgap LaMg_0.5Ge_0.3Sn_0.2O₃ host (E_g ≈ 3.7 eV). Density functional theory (DFT) and experimental characterizations confirm that Mn⁴⁺ doping introduces mid-gap states, enabling efficient, host-decoupled far-red emission (~703 nm) via the ²E → ⁴A₂ transition. Ba²⁺ co-doping further enhances the emission intensity by 1.67-fold via charge compensation. The resulting LaMg_0.5Ge_0.3Sn_0.2O₃:Mn⁴⁺, Ba²⁺ phosphor exhibits intense and narrow far-red emission (FWHM = 20 nm), a peak internal quantum efficiency of 98.8%, and thermal stability with 78.9% emission retention at 150 ℃. Its emission spectrum aligns well with the absorption profiles of phytochrome Pfr and chlorophyll b, facilitating enhanced photosynthetic response. In proof-of-concept garlic cultivation trials, the fabricated far-red phosphor-converted LEDs demonstrated the capability to effectively promote biomass accumulation and morphological development under a time-segmented lighting schedule. This dual-doping approach provides a versatile design framework for high-performance far-red phosphors in sustainable agriculture lighting.