Research matters ... out-of-season flowering of Phlox

Dr Ken Cockshull, Emeritus Fellow at Warwick HRI, highlights the latest findings from horticultural research.

Phlox paniculata can be grown in the field to become a cut flower and it can also be forced to flower out of season in heated greenhouses. Flower development is advanced by prior cooling, and long days promote flower formation.

In the experiments reported here, rooted cuttings of the 'Ice Cap' cultivar were cooled to about 2 degsC for between 0 and 16 weeks under low light. The plants were transferred to a heated glasshouse (20 degsC/18 degsC day/night) where long-day treatments were given from incandescent lamps; either end-of-day lighting for seven hours or a night-interruption for four hours.

Flowering was promoted more by end-of-day lighting than by night interruptions, but cooling for 16 weeks produced the earliest flowering. To produce commercially acceptable stem lengths required exposure to at least 12 weeks of cooling.

In another experiment, the cooling treatments were the same but, in the glasshouse, the plants received light continuously - ie 24 hours per day, from metal halide lamps. This lighting treatment produced very early flowering but the cooling treatments produced very little further advance in flowering, although the flower stems were longer.

Growers can use supplementary lighting to advance flowering and use cooling to obtain longer flower stems.

Cooling and Long-day Lighting Influences Growth and Flowering of Phlox paniculata L. 'Ice Cap' Used for Cut Flowers by Garner and Armitage (2008). HortScience 43 (3): 707-709. Members of ISHS can view HortScience from the website www.ishs.org.


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