Comparing Supplemental and Sole-source Lighting for Bedding Plant Seedling Production

In order to produce uniform and high-quality annual bedding plant seedlings or plugs in late winter through early spring, supplemental lighting (SL) must be used to maintain a photosynthetic daily light integral (DLI) of 10 to 12 mol·m^–2·d^–1. The objectives of this study were to: 1) quantify bedding plant seedling quality under greenhouse SL from light-emitting diodes (LEDs), high-pressure sodium lamps (HPS), or plasma lamps (PL); 2) quantify seedling quality under sole-source LED lighting in a growth room environment; and, 3) compare seedlings grown under a DLI of ≈10.5 mol·m^–2·d^–1 from ambient solar light plus SL or sole-source lighting to those grown under ambient solar light. Upon hypocotyl emergence, seedlings of Catharanthus roseus, Impatiens walleriana, Pelarganium ×hortorum, Petunia ×hybrida, and Tagetes patula were placed under ambient solar light plus SL delivering a photosynthetic photon flux (PPF) of 70 µmol·m^–2·s^–1 in the greenhouse for 16-h or under sole-source light delivering a PPF of 185 µmol·m^–2·s^–1 in a growth room for 16-h. Supplement light treatments consisted of HPS, PL, and LED arrays providing a red:blue light ratio of 87:13, and sole-source light treatments were made up of LED arrays providing a red:blue light ratio of 87:13 or 70:30. Stem elongation of Pelargonium and Petunia seedlings decreased by 18% and 69%, respectively, for plants grown under sole-source LEDs providing 70:30 red:blue light, and Tagetes decreased by 13% for plants grown under sole-source LEDs providing 87:13 red:blue light, compared to ambient solar light. Root dry mass of Petunia seedlings grown under sole-source LEDs providing 70:30 red:blue light increased by 127% compared to those under the ambient control. The quality index, a quantitative measurement of quality, increased by 157% and 132% for Impatiens and Pelargonium, respectively, for plants grown under sole-source LEDs providing 70:30 red:blue light, compared to the ambient control. These results indicate that annual bedding plant seedlings grown under controlled environments with LED sole-source lighting are generally of higher quality than seedlings grown in a traditional greenhouse with or without SL.