Supplemental But Not Photoperiodic Lighting Increased Seedling Quality and Reduced Production Time of Annual Bedding Plants

In northern latitudes, the photosynthetic daily light integral can be less than 5 mol·m –2 ·d –1 , necessitating the use of supplemental lighting (SL) to reduce bedding plant seedling production time and increase quality. Our objectives were 1) to quantify seedling quality and production time under continuous 16-h or instantaneous threshold SL, continuous low-intensity photoperiodic lighting (PL) for 16 or 24 hours with and without far-red light, or no electric lighting; and 2) to determine whether the described lighting treatments during propagation impact finished plant quality or flowering. Seeds of begonia ( Begonia × semperflorens ) ‘Bada Bing Scarlet’, gerbera ( Gerbera jamesonii ) ‘Jaguar Deep Orange’, impatiens ( Impatiens walleriana ) ‘Accent Premium Salmon’, petunia ( Petunia × hybrida ) ‘Ramblin Peach Glo’, and tuberous begonia ( Begonia × tuberosa ) ‘Nonstop Rose Petticoat’ were sown in 128-cell trays and grown under either SL, PL, or no electric lighting (control). SL treatments consisted of high-intensity light-emitting diode (LED) or high-pressure sodium (HPS) lamps providing a photosynthetic photon flux density ( PPFD ) of either 70 µmol·m –2 ·s –1 on continuously for 16 h·d –1 or 90 µmol·m –2 ·s –1 based on an instantaneous threshold. PL treatments consisted of low-intensity red:white (R:W) or red:white:far-red (R:W:FR) lamps for 16 h·d –1 or R:W:FR lamps for 24 h·d –1 . Seedlings of gerbera, impatiens, and petunia from each treatment were subsequently transplanted and finished in a common greenhouse environment. The highest quality seedlings were grown under SL compared with PL or control conditions. When comparing SL treatments, seedlings produced under HPS or LED SL using an instantaneous threshold were of equal or greater quality compared with those under continuous SL with a 16-h photoperiod. Although the greater leaf area and internode elongation under PL may give growers the perception that seedling production time is reduced, PL did not increase biomass accumulation and seedling quality. Petunia seedlings propagated under HPS lamps using an instantaneous threshold flowered 4 to 11 days earlier compared with the other SL treatments. In addition, petunia propagated under R:W:FR PL for 16 h·d –1 flowered 5 to 7 days earlier compared with LED SL and the other PL treatments.