The Effects of Photoperiod and Temperature on Growth and Flowering of Clematis xhybrida

Clematis is amongst the well-recognized flowering plants by the retail consumer; however, production has not traditionally fit into the vegetatively-propagated unrooted cutting market and standard greenhouse production systems. One reason is the relatively long, two-year production cycle from propagation to flowering. Experimentation was undertaken to understand the factors that influence shoot development, flowering and adventitious root formation of Clematis so that strategies could be developed to reduce the production time. The first experiment examined the effect of duration of cold treatment at 5 °C (0, 3, 6, or 9 weeks) followed by two photoperiods (9 or 16 hours) and on shoot growth and flowering of Clematis x hybrida ‘H.F. Young’. Plants grown without a cold treatment and a 9-hour photoperiod did not flower, while 100% of the plants within a treatment flowered only on plants that received 6 or 9 weeks of cold followed by a 16-hour photoperiod. Flower number per plant increased and time to flower decreased as both the duration of the cold treatment and the photoperiod increased.  The second experiment examined the effect of temperature (21 or 26 °C) on shoot growth and flowering under long day photoperiods for Clematis x hybrida ‘H.F. Young’. Warmer temperatures resulted in decreased shoot number, flower number, and time to flower, while node number and stem length increased. Single-node cuttings were removed from the flowering shoots to determine the effect of the temperature provided to the growing shoots on the subsequent adventitious root formation of cuttings harvested from those temperature environments, however no effect on rooting was observed. Implications of these experiments for commercial production of Clematis will be discussed.