Quantifying Growth Control of Annual Bedding Plants with Ethephon Substrate Drenches

Ethephon (2-chloroethylphosphonic acid) is a plant growth regulator widely used in horticultural and agronomic crop production to control stem elongation, increase lateral branching, and manipulate flowering. While substrate drench applications of ethephon can inhibit the growth of containerized annual bedding plants, research has been limited to fewer than 30 taxa despite the hundreds of species grown in floriculture production. Therefore, our objective was to evaluate and quantify the efficacy of ethephon substrate drenches on the growth of 20 taxa of containerized annual bedding plants. Plants were transplanted into 12-cm, 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 150-mL aliquots of solution, based on container size, containing 0, 25, 50, 75, 100, or 200 mg·L −1 ethephon. Plants were grown in a glass-glazed greenhouse for 3 to 6 weeks after drench before growth and morphological data, including plant height, plant diameter, shoot dry weight (SDW), and root dry weight (RDW) were determined. The magnitude of growth control varied among annual bedding plant taxa with increasing ethephon substrate drench concentrations. For most taxa, drenches of 25 to 100 mg·L −1 ethephon were determined to be effective at controlling plant height and plant diameter without negatively impacting visual ornamental plant quality. For example, when drenched with 100 mg·L −1 ethephon, interspecific geranium ( Pelargonium interspecific ‘Calliope ® Large Dark Red’) and zonal geranium [ Pelargonium zonale (L.) L’Hér. ‘Americana ® Dark Red’] were 27% (8.0 cm) and 29% (8.1 cm) shorter, respectively, than untreated plants. We observed 1.5% (0.34 cm) and 18.1% (3.2 cm) smaller plant diameters for coleus [ Coleus scutellarioides (L.) Benth. ‘Main Street TM Ocean Drive’] and heliotropium ( Heliotropium arborescens L. ‘Scentropia TM Dark Blue’), respectively, as concentrations increased from 0 to 25 mg·L −1 ethephon. The SDW and RDW were also influenced by increasing ethephon substrate concentrations. For instance, SDW of New Guinea impatiens ( Impatiens hawkeri R.W. Baker & Corner ‘Magnum Fire’) and calibrachoa [ Calibrachoa × hybrida (Llave and Lex.) M. Martens and L. Westra. ‘Callie ® Dark Red’] drenched with 25 mg·L −1 ethephon were 24% (2.1 g) and 33% (0.9 g) smaller, respectively, compared with untreated plants. The RDWs of lantana ( Lantana camara L. ‘Bandolista TM Red Chili’) and portulaca ( Portulaca oleracea L. ‘Flame Red’) were 62% (1.42 g) and 55% (0.8 g) less, respectively, as concentrations increased from 0 to 200 mg·L −1 ethephon. While responses varied across taxa, this research expands the scope of annual bedding plants and further demonstrates the effectiveness of ethephon applied as a substrate drench.