Twenty-four herbaceous perennial species were treated at 5C for 0 or 15 weeks. Critical photoperiods for flower initiation and development with and without a cold treatment were determined. Photoperiods were 10, 12, 14, 16, and 24 h of continuous light and 9 h plus a 4-h night interruption. Continuous photoperiodic treatments consisted of 9-h natural days extended with light from incandescent lamps. Response to cold and photoperiod varied by species; Scabiosa caucasica `Butterfly Blue' flowered without a cold treatment under all photoperiods after 8 to 10 weeks of forcing, but plant height increased from 14 to 62 cm as daylength increased. Rudbeckia fulgida `Goldsturm' flowered without cold after 13 to 15 weeks of forcing, but only under 16 hours of continuous light and night interruption treatments. Heuchera sanguinea `Bressingham Hybrids' did not flower without cold under any photoperiod but did flower under all photoperiods with cold. The only Lavendula angustifolia `Munstead Dwarf' plants that flowered without cold were those under 24-h continuous light; ≈60% flowered. After cold, some lavender plants flowered under all photoperiods, and the flowering percentage increased with increasing daylength.
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