Gooseneck loosestrife (Lysimachia clethroides Duby) rhizomes were cooled for 10 weeks at 4 ± 1 °C prior to greenhouse forcing in continuous long days (LD); continuous short days (SD); 4, 6, 8, or 10 weeks of SD followed by LD until anthesis; and 4, 6, 8, or 10 weeks of LD followed by SD. None of the plants grown in continuous SD flowered, and fewer than 30% of plants flowered when grown in 4, 6, or 8 weeks of LD followed by SD for 21 to 25 weeks. At least 10 weeks of LD prior to SD were required to obtain 70% flowering. Plants receiving continuous LD or 4, 6, or 8 weeks of SD followed by LD flowered in the highest percentages (85% to 90%), but only 10% of plants receiving 10 weeks of SD followed by LD flowered. The number of greenhouse days required for visible bud formation and anthesis increased linearly as initial SD exposure increased, but the number of racemes produced by flowering plants was not affected. Plant height was greatest in continuous LD, and decreased linearly as initial SD exposure prior to LD increased from 0 to 10 weeks. Plants grown in continuous SD remained vegetative rosettes throughout the experiment, and their height increased linearly as initial LD prior to SD increased from 0 (continuous SD) to 10 weeks. These results demonstrate that supplemental LD lighting can promote growth and flowering in this species and that lighting can be discontinued 3 weeks before harvest of cut flower crops.