Seasonal changes of photoperiodic response and inflorescence quality in pot cultivars of Chrysanthemum × grandiflorum grown in greenhouse

The aim of the research was to investigate seasonal changes of photoperiodic response and inflorescence quality in chrysanthemum from the Time group grown all year round without supplementary lighting. Four chrysanthemum cultivars were grown in 12 cycles starting on the second day of each successive month. The first cycle began on 2 January. From the moment of potting, the plants were treated with a short day. In periods of naturally long days, the day was shortened to 10.5 hours through shading. No supplementary illumination was used from November to mid-February (a photoperiod of under 10 hours) to improve light conditions in the period of insolation deficit. In Polish climatic conditions, the shortest photoperiodic responses were recorded usually in the cycles starting in February or March and August or September. An exceptionally longer photoperiodic response was recorded in the cycle including the height of summer, i.e. planting from June to July, when the temperature during the day was very high, and from October to December i.e. in the period of substantial insolation deficit. The highest quality of flower heads was attained by plants potted mostly between 2 March and 2 September and flowering from April - May to November. Those plants developed the greatest number of inflorescences and usually a typical, cultivar-specific colour of the flower. Chrysanthemums potted from 2 June to 2 July, i.e. in the period of the greatest heat, produced inflorescences with the smallest diameter. For the cultivar 'Cool Time', this unfavorable period lasted from 2 April to 2 August