Many herbaceous ornamental plants exhibit a photoperiodic flowering response. Under ambient short days (SDs), lighting from incandescent (INC) lamps during the night inhibits flowering of SD plants (SDPs) and promotes flowering of long-day plants (LDPs). INC lamps are inexpensive, easy to install, and emit an effective spectrum for controlling flowering, but they also are energy inefficient and are being phased out of production. Three annual SDPs and six annual LDPs were grown under a 9-h day with a 4-h night interruption (NI) with different red-to-far-red ratios (R:FR) from light-emitting diodes (LEDs). A moderate to high R:FR (≥0.66) was most effective at interrupting long nights and inhibiting flowering of SD plants. For LDPs, a mixture of R and FR light (R:FR= 0.28 to 1.07) most effectively promoted flowering. For both SDPs and LDPs, flowering percentage and flowering time were similar under SDs and the NI with only FR, indicating that NI with only FR was perceived as an SD. Therefore, some threshold amount of R light is required to inhibit flowering in SDPs and promote flowering in LDPs. A second experiment determined whether the sensitivity to the R:FR changed during a 15-hour night. Lamps with R:FR ratios of 0.65 to 2.38 were all effective at promoting flowering, regardless of whether the photoperiodic treatment was delivered at the end of the natural photoperiod or during the middle of the night. In these experiments, plant responses were similar under INC or R+FR LEDs and thus, LEDs can provide an alternative to INC lamps for photoperiodic lighting.
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