Containerized annual bedding plants and herbaceous perennials account for 57% of the 2020 U.S. floriculture market with a reported combined wholesale value of $3.2 billion. To produce high-quality, compact containerized ornamental plants, foliar spray applications and substrate drenches of plant growth regulators (PGRs) are often utilized. Ethephon [(2-chloroethyl) phosphonic acid] is a common PGR used to control growth, stimulate branching, and manipulate flowering, but is only labelled for foliar applications. Therefore, our objective was to evaluate the response of 35 floriculture species drenched with increasing concentrations of ethephon. Annual bedding plants and herbaceous perennials were received as unrooted cuttings and propagated at 23°C under 10 mol·m–2·s–1 for 21 or 28 d, respectively. Plants were transplanted into containers filled with a soilless substrate and grown in a glass-glazed greenhouse at 20°C under 14 mol·m–2·d–1. At 10 d after transplant, eight single-plant replicates received a substrate drench of 296-mL aliquots of solution containing 0 (control; deionized water), 25, 50, 75, 100, or 200 mg·L–1 ethephon for annuals or 0, 125, 250, 500, 750, or 1,000 mg·L–1 ethephon for herbaceous perennials. Plant growth metrics including height, diameter, shoot and root dry weight were determined 6 weeks after transplant. Time to flower was determined for select species by recording the date of anthesis for each plant. In general, plant height of annual bedding plants and herbaceous perennials were suppressed and shoot and root dry weight reduced as concentrations of ethephon increased. For example, petunia (Petunia × hybrida ‘Flame Red’) drenched with 200 mg·L–1 was ≈40% (5.8 cm) shorter than untreated plants. Similarly, in dahlia (Dahlia × hybrida ‘Dark Red’) height and diameter were decreased by 50% (15.4 cm) and 30% (12.4 cm), respectively as concentrations increased from 0 to 200 mg·L–1. In digiplexis (Digiplexis × hybrida), substrate drenches increasing from 0 to 1,000 mg·L–1 ethephon reduced shoot dry mass by ≈55% (17.1 g). Root dry weight of catmint (Nepeta faassenii) was reduced by 35% (1.54 g) as concentrations increased from 0 to 1,000 mg·L–1. Time to flower was unaffected at all concentrations of the species selected. These growth and development trends were reflected in most of the species evaluated. As such, this research demonstrates that ethephon, if labelled for substrate drenches, provides adequate growth control of floriculture crops.
