Controlling Flowering of Photoperiodic Ornamental Crops Using Light-emitting Diode Lamps: A Coordinated Grower Trial

Photoperiodic lighting from lamps with a moderate ratio of red (R, 600 to 700 nm) to far-red (FR, 700 to 800 nm) light effectively promotes flowering of long-day plants. Due to spectral controllability, long life span, and energy efficiency, light-emitting diodes (LEDs) have emerged as an alternative to conventional light sources, such as incandescent (INC) and high-pressure sodium (HPS) lamps. However, the efficacy of newly developed LEDs on flowering applications has not been published. We conducted a coordinated trial with five commercial greenhouse growers to investigate the efficacy of R + white (W) + FR LEDs, with an R:FR of 0.82, at regulating flowering of daylength-sensitive ornamental crops. The trial was also performed in two replicate greenhouses at Michigan State University (MSU). Ageratum (Ageratum houstonianum), calibrachoa (Calibrachoa ×hybrida), dahlia (Dahlia ×hybrida), dianthus (Dianthus chinensis), two petunia (Petunia ×hybrida) cultivars, snapdragon (Antirrhinum majus), and verbena (Verbena ×hybrida) were grown under natural short days (SDs) with 4-hour night-interruption (NI) lighting provided by the R+W+FR LEDs or conventional lamps typically used by each grower. Two companies used HPS lamps while the other sites used INC lamps. In addition, a natural SD treatment, a truncated 9-hour SD treatment, and a compact fluorescent lamp treatment were delivered at three different sites. All plants were grown under similar environmental conditions following the growers’ standard production practices of watering, fertilization, and pest management. With few exceptions, time to flower and flowering percentage of the bedding plant crops tested were similar under the R+W+FR LEDs as under the conventional lamps at all sites. At MSU, ageratum, dianthus, petunia, snapdragon, and verbena flowered earlier under the NI lighting treatments than under 9-hour SDs. In addition, plant height and visible flower bud or inflorescence number at flowering were similar under the R+W+FR LEDs and INC lamps for most crops. Therefore, we conclude that the R+W+FR LEDs are as effective as lamps traditionally used in greenhouses at controlling flowering of photoperiodic plants.