In the experiments described here, seedlings of Petunia 'Madness Red' and Pansy 'Delta Premium Yellow' were raised in a glasshouse in Michigan, where they received daylight plus either low-intensity photoperiodic lighting (control) or high-intensity photosynthetic lighting.
Both were provided from high-pressure sodium lamps from 6am to 10pm whenever the daylight intensity fell below a set value. The seedling stage was divided into three periods of nine days for petunia and 11 days for pansy, with either lowor high-intensity lighting given in each period.
Some plants were grown on for assessment of final quality. Supplementary lighting promoted growth but if given in only one period it was best given in the last period. Flowering was unaffected by supplementary lighting given in the first period (when cotyledons were unfolding) but it was hastened when lighting was given during either the second or third periods.
Evidently, plants can be marketed earlier if supplementary lighting is used, but final quality can sometimes be higher if no photosynthetic lighting is given because such plants grow for longer before flowering.
Timing and Duration of Supplemental Lighting During the Seedling Stage Influence Quality and Flowering of Petunia and Pansy by Oh, Runkle and Warner (2010). HortScience 45 (9): 1332-1337. Members of ISHS can view HortScience online at www.ishs.org.