Growth control of ornamental and bedding plants by manipulation of photoperiod and light quality

Altering the light regime is a sound and non-polluting way of controlling the growth of greenhouse-grown pot and bedding plants, and a promising way of eliminating the use of chemical plant growth regulators (PGRs), which are now becoming less available and more questioned by consumers. Modern greenhouses often have blackout screens, originally installed for flower regulation purposes but which can also be used for growth regulation. Modern light emitting diode (LED) light sources can be designed to provide narrow-band light (NBL), which can affect growth and elongation of plants if given in addition to natural sunlight, or as daylight extension (end-of-day or pre-day treatments). Two different approaches were used: short photoperiods in combination with pre-day and end-of-day NBL treatments, and short photoperiods combined with simultaneous addition of NBL and natural light. The plants used were Calibrachoa, Pelargonium, Euphorbia, and Chrysanthemum. A light regime with 620 nm light given before the period of natural light and 525 nm light given at the end of the natural light effectively controlled elongation in both Calibrachoa and Pelargonium. Supplementation of natural sunlight with a small portion of 660 nm light significantly reduced plant height in Euphorbia, but not in Chrysanthemum. It was concluded that management of photoperiod and light quality is very promising for replacing PGRs within greenhouse production of ornamental and bedding plants.