Ethephon Substrate Drenches Control Stem Elongation of Containerized Herbaceous Perennials

Ethephon (2-chloroethylphosphonic acid) is a widely used plant growth regulator applied in the production of horticultural and agronomic crops. Preliminary research reports that drench applications inhibit growth of containerized ornamental plants, though few herbaceous perennials have been evaluated to determine responses to ethephon drenches. Therefore, our objective was to evaluate and quantify the efficacy of ethephon substrate drenches on the growth of 17 species of containerized flowering herbaceous perennials. Plants were transplanted into 12.7-cm or 16.5-cm containers filled with a soilless peat-based substrate. After 10 d, eight single-plant replicates received a substrate drench of 90 mL or 296 mL aliquots of solution, based on container size, containing 0, 125, 250, 500, 750, or 1000 mg·L−1 ethephon. Plants were grown in a glass-glazed greenhouse for 3 to 8 weeks after drench before growth and morphological data, including plant height, plant diameter, shoot dry weight, and root dry weight were determined. The magnitude of growth control varied among herbaceous perennials species and ethephon concentrations. For most species, drenches of 125 to 500 mg·L−1 ethephon were effective for controlling plant height and plant diameter without negatively affecting aesthetic ornamental plant quality. For instance, Russian sage (Salvia yangii B.T. Drew), wandflower [Oenothera lindheimeri (Engelm. & A. Gray) W.L. Wagner & Hoch ‘Siskiyou Pink’], and blanket flower (Gaillardia aristata Pursh. ‘SpinTop® Red Starburst’) drenched with 125 and 250 mg·L−1 were 48% to 56% (22.7 to 26.4 cm), 16% to 31% (6.9 to 13.2 cm), and 11% to 14% (2.1 to 2.6 cm) shorter, respectively, than untreated plants and no further height control occurred at concentrations >250 mg·L−1 ethephon. We observed a 22% (9.5 cm) and a 35% (10.5 cm) smaller plant diameter for lobed tickseed (Coreopsis auriculata L. ‘Leading Lady Iron’) and pincushion flower (Scabiosa columbaria L. ‘Pink Mist’), respectively, as concentrations increased from 0 to 125 mg·L−1 ethephon. Root dry weight of lobed tickseed and woodland sage (Salvia nemorosa L. ‘East Friesland’) were 54% (8.2 cm) and 69% (6.2 cm) less, respectively, as concentrations increased from 0 to 500 mg·L−1 ethephon. Although we did not quantify time to visible bud or open flower, most plants displayed visible buds or at least one open flower by the time data were collected; however, responses varied among species. Our research demonstrates that ethephon substrate drenches were effective in controlling growth of containerized herbaceous perennials, but growers must consider taxa-specific variations in response.