FLOWERING OF HERBACEOUS PERENNIALS UNDER VARIOUS NIGHT INTERRUPTION AND CYCLIC LIGHTING TREATMENTS

Six long-day species of herbaceous perennials were grown under six nig ht-interruption (NI) photoperiod treatments to determine their relativ e effectiveness at inducing flowering. Photoperiods were 9-hour natura l days with NI provided by incandescent lamps during the middle of the dark period for the following durations: 0,5, 1, 2, or 4 hours; 6 min utes on, 54 minutes off for 4 hours (10% or 6/54 cyclic lighting); or 6 minutes on, 24 minutes off for 4 hours (20% or 6/24 cyclic lighting) . For five species, the experiment was repeated with more mature plant s; for the sixth, Rudbeckia fulgida Ait, 'Goldsturm', following a cold treatment of 8 weeks at 5 degrees C. The species generally showed a q uantitative flowering response to the NI duration until a saturation d uration was reached; as the length of the uninterrupted night break in creased, flowering percentage, uniformity, and number and plant height increased and time to flower decreased. Minimum saturation durations of NI were 1 hour for Coreopsis grandiflora Hogg ex Sweet 'Early Sunri se' and Hibiscus moscheutos L. 'Disco Belle Mixed', 2 hours for Campan ula calpatica Jacq. 'Blue Clips' and Coreopsis verticillata L,'Moonbea m', and 4 hours for unchilled R. fulgida 'Goldsturm'. Echinacea purpur ea Moench 'Bravado' flowered similarly across all lighting treatments. The 6/24 cyclic lighting regimen induced flowering comparable to that under a continual 4-hour NI for four of the six species and the cold- treated R.fulgida 'Goldsturm'. Flowering under the 6/54 regimen was ge nerally incomplete, nonuniform, and delayed compared to that under sat uration duration treatments. Three of five species flowered earlier wh en more mature plants were placed under the NI treatments. Cold-treate d R. fulgida 'Goldsturm' flowered more rapidly than unchilled plants a nd the saturation duration of NI decreased to 1 hour.