2017 ASHS Annual Conference

The plant growth regulator (PGR) ethephon [(2-chloroethyl) phosphonic acid] breaks down to release ethylene (C₂H₄) and can be sprayed on floriculture crops to reduce cell elongation inhibiting internode elongation, reduce apical dominance, increase lateral branching, and abort flower buds and flowers. However, the efficacy of ethephon applications is influenced by a variety of factors including ambient temperature and carrier water pH. As pH or temperature increase, the rate of ethylene release from ethephon increases, reducing chemical efficacy. Therefore, our objective was to quantify how the efficacy of ethephon sprays are influenced by carrier water alkalinity and ambient temperature at application. Young plants of verbena (Verbena peruviana) ‘Aztec Blue Velvet’, ivy geranium (Pelargonium pelatum) ‘Precision Pink’, and petunia (Petunia × hybrida) ‘Easy Wave Neon Rose’ were transplanted into 11 cm containers and grown with a constant air temperature of 20 °C. For expt. 1, temperature set points were changed to 14, 17, 20, 23, or 26 °C two weeks after transplant. Plants were then sprayed with 0, 250, 500, or 750 mg·L^–1 ethephon with carrier water alkalinities of 50, 150, or 300 mg·L^–1 CaCO₃. Twenty-four hours after application, temperatures were changed back to a constant 20 °C. Nine days after the first application, a second application with the same treatments was applied. In expt. 2, the treatments were the same as expt. 1, however, only 1 ethephon application was made 1 or 2 weeks after transplant. In expt. 1, as alkalinity increased from 50 to 300 mg·L^–1, stem length 4 weeks after transplant increased by 21, 32, and 19% for petunia, verbena, and ivy geranium, respectively. In expt. 2, as alkalinity increased from 50 to 300 mg·L^–1 branch number of verbena decreased by 5 branches. As application air temperature increased above 23 °C, chemical efficacy decreased in all species. For example, time to flower of petunia, verbena, and ivy geranium decreased by 4, 7, and 11 days, respectively, as temperature increased from 23 to 26 °C while there was no difference when application temperatures were between 14 and 23 °C. Based on our data, we conclude that carrier water alkalinity and air temperature at application influence ethephon efficacy and are additional factors for greenhouse growers to consider when applying ethephon.