Cooling and Long-day Lighting Influences Growth and Flowering of Phlox paniculata L. ‘Ice Cap’ Used for Cut Flowers

Abstract

In two studies, rooted cuttings of Phlox paniculata L. ‘Ice Cap’ (summer phlox) were cooled for 0, 4, 8, 12, or 16 weeks. Plants were forced in a glasshouse averaging 18 °C nights under extended-day and night-interruption lighting from incandescent lamps providing a minimum of 14 μmol·s−1·m−2 at plant level or continuous lighting from metal halide (HID) lamps providing a minimum of 400 μmol·s−1·m−2 at plant level. The influence of cooling duration on forcing days to flower, flowering stem counts, and flowering stem length was evaluated. Cooling plants promoted longer stems, higher flowering stem yields, and decreased days to flower when forced under long days provided by incandescent lamps, but when forced under HID lamps, days to flower for cooled plants were similar to those of noncooled plants regardless of cooling duration. Phlox forced in extended daylighting flowered in fewer days, had longer stems, and produced more flowering stems than those forced in night-interruption lighting. With continuous HID lighting, stem lengths and stems harvested per plant increased in a linear manner as cooling increased from zero to 16 weeks. Stem lengths ranged from 63.6 cm for noncooled plants to 96.3 cm for those receiving 16 weeks cooling, and flowering stem yields ranged from seven stems per plant for noncooled plants to 13 for those cooled 16 weeks. Phlox forced under HID lights flowered in substantially fewer days and had longer stems than those forced under incandescent lamps.