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Hydrangea flowering

June 17, 2009  By Arpana Chakravarty

Growers can’t induce flowering of field hydrangeas by just controlling the photoperiod, says a University of Guelph researcher.

Growers can’t induce flowering of field hydrangeas by just controlling the photoperiod, says a University of Guelph researcher.

Contrary to European research, Prof. Theo Blom, Department of Plant Agriculture, showed that photoperiod has very little effect on the flower initiation of the hydrangea, a popular potted plant grown around the shores of Lake Ontario and Lake Erie. Potted hydrangea, Hydrangea macrophylla subsp. Macrophylla, generates $5 million in sales annually for Ontario producers.

Research results with ‘Mathilda Gutges’ hydrangea.

After testing the three critical factors that impact flowering – plant size/maturity, photoperiod and temperature – Blom says plant maturity and temperature are more important to the hydrangea than photoperiod.

“I try to make the Ontario floricultural industry stronger,” says Blom. “It is necessary to refine and add new knowledge to Ontario’s production systems. I want our growers to know what factors are most important for plant growth.”

Professor Theo Blom  
Research results with ‘Merritt’s Supreme’ hydrangea.
Research results with ‘Fire Light’ hydrangea.


Three popular cultivars used in the research
In his research, Blom used rooted tip cuttings of three commonly grown cultivars of hydrangea to see whether maturity, night temperature and/or photoperiod had an effect on the flower initiation process and before forcing the plants under normal greenhouse conditions.

Plants of two different maturities were moved to greenhouses that differed only in their nighttime temperature. A cool greenhouse remained at 24ºC during the day and 16ºC at night. A warm greenhouse had the same daytime temperature, but remained at the 24ºC mark during the night, as well. 

In each of the greenhouses, plants were exposed to one of two photoperiodic treatments, a short day or a long day. Both treatments consisted of a 10-hour photoperiod, but the long-day treatments were interrupted for four hours during the blackout period using incandescent lamps. After six weeks, eight weeks and 10 weeks of the temperature/photoperiod treatment, the plants were removed from the greenhouse compartment and placed in a commercial cooler to break dormancy.

Salability, size and date of flowering
Following leaf removal and at least 1,000 hours of cooling, the hydrangea were repotted and placed in a 16-18ºC ambient greenhouse and evaluated for their salability, size of flowers, plant height and date of flowering.

Before the plants went into the cooler, lower night temperatures were noted to enhance the development of the hydrangea stems, stalks and flowers (collectively referred to as the flower head  or inflorescence) for both maturities of plants, while the effect of photoperiod was minimal on the stage of inflorescence.

Blom’s findings showed the number of blooming plants depended greatly on the maturity of the cuttings at the start of the treatments, as well as on the duration of the treatments rather than the night temperature and/or photoperiod.

When the cuttings were older prior to treatment, the proportion of flowering plants after forcing was about 30 per cent greater than when the plants were younger. Also, the longer the duration of the treatment (six to 10 weeks), the higher the percentage of flowering. For every additional two weeks of treatment, flowering increased by about 20 per cent.

The greatest number of flowering plants were obtained under the warm night/short day combination, while the cool night/short day combination resulted in the fewest flowers. Plants with the long-day treatments (cool or warm nights) showed intermediate survival rates.

These results indicate that night temperature and maturity of the plant during flowering are more important than photoperiod. ■

Others involved in the research include technician Dave Kerec.

This research was funded by the trust fund of the University of Guelph greenhouse floriculture program, and a commercial grower.

Arpana Chakravarty is a writer with the SPARK (Students Promoting Awareness of Research Knowledge) program at the University of Guelph.

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