2014 ASHS Annual Conference

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.