Ethephon substrate drenches have been shown to effectively control growth of annual bedding plants and herbaceous perennials. However, research has shown the efficacy of ethephon substrate drenches is impacted by the time of application. Therefore, the objective of our research was to evaluate how ethephon substrate drenches impact growth when applied at varying times post-transplant. Young plants of lobed tickseed (Coreopsis ×hybrida Big BangTM ‘Mercury Rising’) and coleus [Coleus scutellarioides (L.) Benth. ‘Main StreetTM Bourbon Street’] were transplanted into 12.7-cm containers filled with a soilless peat-based substrate. At 3, 7, 10, 14, 17, or 21 d after transplant, 8 single-plant replicates received a single substrate drench of 90 mL aliquots of a solution containing 0, 25, 50, 100, or 200 mg·L–1 ethephon. Plants were grown in a glass-glazed greenhouse for 5 to 7 weeks after initial drench application before growth and morphological data including plant height, plant diameter, shoot dry weight (SDW), and root dry weight (RDW) were determined. In general, increasing concentrations of ethephon influenced plant height, plant diameter, SDW, and RDW of coleus and lobed tickseed. In Expt. 1, plant size (height and diameter) and biomass accumulation (SDW and RDW) of coleus was significantly influenced by increasing concentrations of ethephon, with the effects diminishing as application time was delayed. For example, as concentrations increased from 0 to 200 mg·L–1 ethephon, coleus treated at 3 days after transplant (DAT) were 33% (5.3 cm) shorter, while those treated at 17 DAT were 21% (3.6 cm) shorter. Shoot dry weight of coleus drenched with 200 mg·L–1 ethephon at 3 or 21 DAT were 50% (2.0 g) and 23% (1.0 g) lower, respectively, compared to untreated plants. Compared to untreated plants, RDW of plants drenched with 200 mg·L–1 ethephon 3 or 14 DAT was 65% (2.8 g) and 50% (2.6 g) smaller, respectively. In Expt. 2, plant height was unaffected by increasing concentrations of ethephon, but plant diameter and biomass accumulation of lobed tickseed was significantly affected. For example, as concentrations of ethephon increased from 0 to 200 mg·L–1, lobed tickseed treated 3 and 21 DAT were 8% (2.4 cm) and 9% (2.8 cm) narrower, respectively, compared to untreated plants. Overall, the magnitude of control diminished as application time increased and negative phytotoxic effects were seen in early applications (≤7 d). Therefore, we suggest applying ethephon substrate drenches 7 to 14 d after transplanting.
