A double perovskite-type substrate of La2MgGeO6 (LMGO) was successfully synthesized via a high-temperature solid-state reaction method and was codoped with Mn4+ and Dy3+ to form a new deep-red phosphor (LMGO:Mn4+, Dy3+) for artificial plant growth light-emitting diodes (LEDs). This extraordinary phosphor can exhibit strong far-red emission with a maximum peak at 708 nm between 650 and 750 nm, which can be ascribed to the E-2(g) -> (2)A(2) g spin-forbidden transition of Mn4+. The X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) clarified that the La3+ sites in the host were partly replaced by Dy3+ ions. Moreover, we discovered energy transfers from Dy3+ to Mn4+ by directly observing the significant overlap of the excitation spectrum of Mn4+ and the emission spectrum of Dy3+ as well as the systematic relative decline and growth of the emission bands of Dy3+ and Mn4+, respectively. With the increase in the activator (Mn4+) concentration, the relationship between the luminescence decay time and the energy transfer efficiency of the sensitizer (Dy3+) was studied in detail. Finally, an LED device was fabricated using a 460 nm blue chip, and the as-obtained far-red emitting LMGO:Mn4+, Dy3+ phosphors for Wedelia chinensis cultivation. As expected, the as-fabricated plant growth LED-treated Wedelia chinensis cultured in the artificial climate box with overhead LEDs demonstrated that after 28 days of irradiation, the average plant growth rate and the total chlorophyll content were better than those of specimens cultured using the commercial R-B LED lamps, indicating that the as-prepared phosphor could have a potential application in the agricultural industry.