A considerable amount of research has been done on controlling the growth of chrysanthemum stems.
Most attention has been focused on creating compact pot plants but there is also a need to produce long flowering stems for the cut-flower market. Stem length is normally controlled by natural plant growth regulators, although it also responds to synthetic ones.
In the research reported here, rooted cuttings of Chrysanthemum 'Zimba' were grown in pots in a greenhouse in long days before being transferred to growth chambers at 20/15 degsC (light/dark). There they received either nine or 14 hours of light every 24 hours from "white" tubular fluorescent lamps.
At the end of each light period, some plants were exposed to far-red light for 15 minutes. The light source was an LED panel that delivered 10 Wm-2 at a peak wavelength of 735nm. This "end-of-day" far-red treatment increased stem length in both day-lengths.
Other experiments showed that the end-of-day treatment made the stems more sensitive to native gibberellins as well as to applied ones.
Such end-of-day far-red treatments might enable Chrysanthemum growers to give fewer long days initially and yet still produce cut flowers with adequate stem length. This would have the merit of reducing the overall production time.
End-of-day Far-red Treatment Enhances Responsiveness to Gibberellins and Promotes Stem Extension in Chrysanthemum by Hisamatsu, Sumitomo and Shimizu (2008). Journal of Horticultural Science & Biotechnology 83 (6): 695-700. The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.