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Supplemental radiation (SR), traditionally provided by high-pressure sodium (HPS) lamps, is recommended for greenhouse production of seedlings during radiation-limiting conditions. Light-emitting diodes (LEDs) have emerged as an appealing alternative to HPS lamps primarily because they can provide SR at improved energy efficiencies, they have longer fixture lifetimes, and the radiation spectrum can be tailored to potentially manipulate plant morphology by targeting radiation absorption of specific photoreceptors. We grew seedlings of three annual bedding plants and two vegetable transplants in greenhouses at 20 °C under a 16-h photoperiod under six SR treatments: five that delivered a photosynthetic photon flux density ( PPFD ) of 90 μmol·m −2 ·s –1  from HPS lamps (HPS 90 ) or LEDs [four treatments composed of blue (B; 400–500 nm), red (R; 600–700 nm), far red (FR; 700–800 nm), and/or white LEDs] and one that delivered 10 μmol·m −2 ·s –1  from HPS (HPS 10 ) lamps as a control with matching photoperiod. The LED treatments, defined by the percentages of B, green (G; 500–600 nm), and R radiation, were B 10 R 90 , B 45 R 55 , B 10 G 5 R 85 , and B 12 G 20 R 68  + FR (FR at 12 μmol·m −2 ·s –1 ). At transplant, leaf area and seedling height were similar among 90 μmol·m −2 ·s –1  treatments in all species except snapdragon ( Antirrhinum majus ), in which seedlings grown under B 12 G 20 R 68  + FR had 62% greater leaf area than those grown under B 45 R 55  and were 47%, 18%, 38%, and 62% taller than those grown under HPS 90 , B 10 R 90 , B 10 G 5 R 85 , and B 45 R 55 , respectively. After transplant and finishing under the same SR treatments, snapdragon flowered on average 7 days earlier under the B 12 G 20 R 68  + FR treatment than the other LED treatments, whereas geranium ( Pelargonium  × hortorum ) grown under B 45 R 55  and B 12 G 20 R 68  + FR flowered 7 to 9 days earlier than those under the B 10 G 5 R 85  and B 10 R 90  treatments. Seedlings of each species grown under the HPS 10  treatment accumulated less dry weight and took longer to flower compared with seedlings under the other SR treatments. We conclude that radiation quality of SR has relatively little effect on seedling growth and subsequent flowering although in some crops, flowering may be earlier when SR includes FR radiation.

Spectral Effects of Supplemental Greenhouse Radiation on Growth and Flowering of Annual Bedding Plants and Vegetable Transplants