ASHS 2025 Conference

Our objective was to quantify prohexadione-calcium foliar spray or substrate drench applications to control growth of two perennial ornamental grass taxa. On 6 June 2024, 38-cell liner trays (126-mL individual cell volume) of Andropogon gerardii ‘Red October’ and Miscanthus sinensis ‘Fire Dragon’ were received from a commercial propagator. Liners were transplanted with one plant per 19.7-cm-diameter container (1.7 L volume) filled with a commercially available bark-based substrate comprised of (by vol.) 56.5% aged pine bark, 34.8% Canadian sphagnum peatmoss, and 8.7% perlite, dolomitic limestone, a nutrient starter charge, and a wetting agent. Plants were grown at bench-level in a glass-glazed greenhouse under supplemental and day-extension lighting provided by 1000-W light-emitting diode lamps from 0600 to 2200 HR (16-h photoperiod) with an air temperature set point of 20 °C. At 7 d after transplant, 10 single-plant replicates of each taxa were treated with a solution containing prohexadione-calcium and a water conditioning agent. Plants received either a foliar spray (vol. 0.2 L·m–2) solution containing deionized water (0 mg·L–1; untreated) or 250, 500, 750, 1,000, or 2,000 mg·L–1 prohexadione-calcium or a substrate drench of 296-mL aliquots of solution containing 0, 125, 250, 500, or 1,000 mg·L–1 prohexadione-calcium. At eight weeks after treatment, plant height, plant diameter, and shoot dry weight (SDW) were determined for all plants, and root dry weights (RDW) were determined for a sub-sample (n = 5) of each taxa from each substrate drench treatment. Foliar spray applications of 125 to 2,000 mg·L–1 prohexadione-calcium did not affect growth of either perennial ornamental grass taxa. Substrate drenches were effective in controlling growth of each perennial ornamental grass taxa although the magnitude of response varied among taxa. For example, compared to untreated plants, plant height, plant diameter, SDW, and RDW of ‘Red October’ Andropogon drenched with 125 to 1,000 mg·L–1 prohexadione-calcium decreased by 20% to 57% (19.7 to 55.9 cm), 0% to 43% (7.8 to 33.4 cm), 3.6 to 28.9 g (10% to 81%), and 10.4 to 28.5 g (27% to 74%), respectively. This research demonstrated substrate drench applications of prohexadione-calcium to be effective for controlling growth, while further research is warranted to determine optimal concentrations for foliar spray applications. We suggest substrate drench applications of 125 to 500 mg·L–1 prohexadione-calcium as an initial range for trials to control growth of ornamental perennial grasses.