Quantifying Flowering Responses of Bedding Plant Species Under LED Day Extension Photoperiodic Lighting with or without Far–Red Light

Flowering of many greenhouse crops is determined by day length. Under natural short-days, growers utilize either night interruption or day-extension (DE) photoperiodic lighting to promote flowering of long-day (LD) plants and inhibit flowering of short-day (SD) plants. Traditionally, photoperiodic lighting is provided by either high-pressure sodium (HPS) or incandescent lamps. However, light-emitting diodes (LEDs) offer higher energy efficiencies, a long operating life, and precise waveband specificity that can eliminate wavebands not considered useful. Our objectives were to quantify the effects of two commercially available low-intensity LED lamps consisting of various proportions (%) of red:white:far-red (R+W+FR) or red:white (R+W) light on stem elongation and time to flower of several LD and SD bedding plants. Antirrhinum majus L. ‘Oh Snap Pink’ (snapdragon), Osteospermum sp. L. ‘Serenity Bronze’ (osteospermum), Petunia ×hybrida Vilm.-Andr. ‘Dreams Midnight’ (petunia), Tagetes erecta L. ‘Moonsong Deep Orange’ (African marigold), and Viola ×wittrockiana Gams. ‘Matrix Yellow’ (pansy) were grown at 19 °C, and under a daily light integral of 7.2 mol·m^–2·d^–1 from ambient solar light and supplemental lighting from HPS lamps. Photoperiods consisted of a truncated 9-hour day that was extended with LED lamps providing 4 μmol·m^–2·s^–1 of R+W+FR or R+W light to 10, 13, or 16 hours. Light-emitting diode lamp type (R+W+FR or R+W) and photoperiod (10-, 13-, or 16-h) significantly influenced stem elongation of osteospermum and marigold and time to flower of snapdragon, petunia, and pansy, respectively. For example, height of osteospermum was 2, 26, and 36% taller under 10-, 13-, and 16-h DE photoperiods created with R+W+FR LED lamps, compared to plants grown under similar DE lighting provided with R+W LED lamps. Time to flower for snapdragon and petunia decreased by 14, 25, and 34 d and 4, 17, and 21 d when grown under DE lighting of 10-, 13-, and 16-h created with R+W+FR, respectively. Our data suggests that under low ambient DLI, flowering of LD plants occurs faster with low intensity R+W+FR LED lamps.