Photoperiod and cold treatment regulate flowering of Rudbeckia fulgida 'Goldsturm'

To determine the flowering requirements of Rudbeckia fulgida Ait, 'Goldstur m', plants were grown under 9-hour photoperiods until maturity, then forced at 20 degrees C under one of seven photoperiods following 0 or 15 weeks of 5 degrees C. Photoperiods consisted of a 9-hour day that was extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treat ment was a 9-hour day with a I-hour night interruption (NI). Noncooled 'Gol dsturm' remained vegetative under photoperiods less than or equal to 13 hou rs, and essentially all plants flowered under photoperiods greater than or equal to 14 hours or with a 4-hour NI. Flowering percentages for cooled pla nts were 6, 56, or greater than or equal to 84 under 10-, 12-, or greater t han or equal to 13-hour daylengths and NI, respectively. Critical photoperi ods were approximate to 14 or 13 hours for noncooled or cooled plants, resp ectively, and base photoperiods shifted from 13 to 14 hours before cold tre atment to 10 to 12 hours following cold treatment. Within cold treatments, plants under photoperiods greater than or equal to 14 hours or NI reached v isible inflorescence and flowered at the same time and developed the same n umber of inflorescences, Fifteen weeks of cold hastened flowering by 25 to 30 days and reduced nodes developed before the first inflorescence by 28% t o 37%. Cold treatment provided little or no improvement in other measured c haracteristics, such as flowering percentage and uniformity, flower number, plant height, and vigor.