To determine the flowering requirements of Rudbeckia fulgida Ait. `Goldsturm', plants were grown under 9-hour photoperiods until maturity, then forced at 20 °C under one of seven photoperiods following 0 or 15 weeks of 5 °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 treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled `Goldsturm' remained vegetative under photoperiods ≤13 hours, and essentially all plants flowered under photoperiods ≥14 hours or with a 4-hour NI. Flowering percentages for cooled plants were 6, 56, or ≥84 under 10-, 12-, or ≥13-hour daylengths and NI, respectively. Critical photoperiods were ≈14 or 13 hours for noncooled or cooled plants, respectively, and base photoperiods shifted from 13 to 14 hours before cold treatment to 10 to 12 hours following cold treatment. Within cold treatments, plants under photoperiods ≥14 hours or NI reached visible inflorescence and flowered at the same time and developed the same number of inflorescences. Fifteen weeks of cold hastened flowering by 25 to 30 days and reduced nodes developed before the first inflorescence by 28% to 37%. Cold treatment provided little or no improvement in other measured characteristics, such as flowering percentage and uniformity, flower number, plant height, and vigor.
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