Ethephon Applied during the Bulking Phase of Clematis Production Increases Shoot Number Following Vernalization

Clematis production has not traditionally fit into the standard floriculture greenhouse production systems because of the relatively long (2-year) production cycle from propagation to flowering. Our previous research focused on understanding the influence of the duration of the cold treatment and photoperiod on shoot growth and flowering of Clematis x hybrida ‘H.F. Young’. Specifically, a cold treatment at 5°C for 9 weeks followed by a 16 h photoperiod promoted flowering. Current experiments focused on shortening the production time to 1 year. Since flower induction was well understood, the limitation to a 1-year cycle was the lack of branching observed on 1-year-old plants. Our hypothesis was that a well-branched, flowering plant could be produced in 1 year, by extending the summer, bulking period (prior to the cold treatment) with long days and by promoting shoot initiation with the application of plant growth regulators during the bulking period. Ethephon (Florel®; 3.9%) at 500 or 1000 mg∙L^-1, or benzylaminopurine (Configure®; 2%) 500 mg∙L^-1 or deionized water (control) was applied as a foliar spray to plants 4, 6, 21 and 23 weeks after transplanting. Plants were then exposed to a 9-week cold treatment (5°C) and forced in a heated greenhouse under a 16-h photoperiod. Prior to the cold treatment there were no differences in shoot numbers (2 shoots per plant) between treatments; however, following the cold treatment, plants treated with ethephon (500 mg∙L^-1) had three more shoots/plant than the other treatments. A second experiment examined the effect of ethephon (0, 500 or 1000 mg∙L^-1) applied eight times during the bulking period and grown at either a 9-h or 16-h photoperiod during the last 10 weeks of bulking prior to receiving a cold treatment. All ethephon-treated plants developed three more shoots during the bulking phase as compared to the control. However, following cold treatment the 1000 mg∙L^-1 ethephon treatments produced plants that experienced delayed flowering, shorter stem length, and excessively high shoot number. Therefore, we recommend using 4-6 applications of 500 mg∙L^-1 ethephon during a 24 week vegetative bulking period to produce a commercially acceptable clematis crop in 1 year from the start of propagation to the time of first open flower.