Materials and Methods

Plant material.

On 12 Sep 2023 (Expt. 1), unrooted shoot-tip cuttings of bacopa (Bacopa cordata L. ‘Pink Sand’), coleus [Coleus scutellarioides (L.) Benth. ‘Main Street^TM Ocean Drive’], New Guinea impatiens (Impatiens hawkeri R.W. Baker & Corner ‘Magnum Fire’ and ‘Magnum Red Flame’), petunia (Petunia ×atkinsiana D. Don. ‘Durabloom^TM Royal Pink’), and portulaca (Portulaca oleracea L. ‘Flame Red’) were received from a commercial cutting supplier (Dümmen Orange NA; Hilliard, OH, USA). On 13 Sep (Expt. 1), unrooted shoot-tip cuttings of lantana (Lantana camara L. ‘Bandolista^TM Red Chili’) and mezoo (Aptenia cordifolia L. Bolus. ‘Trailing Red’) were received from a commercial cutting supplier (Syngenta Flowers LLC, Gilroy, CA, USA). On 19 Dec (Expt. 2), unrooted shoot-tip cuttings of calibrachoa [Calibrachoa ×hybrida (Llave and Lex.) M. Martens and L. Westra. ‘Callie^® Dark Red’], dahlia [Dahlia ×hybrida (Hort. ex. Andrews) Lindl. ex. Loudon. ‘Grandalia^TM Dark Red’ and ‘Dahlegria^® Red’], fanflower (Scaevola aemula R. Br. ‘Bombay^® Dark Blue’), geranium species {[interspecific geranium (Pelargonium interspecific ‘Calliope^® Large Dark Red’)], ivy geranium [P. peltatum (L.) L’Hér. ex Aiton. ‘Cascade Dark Red’], and zonal geranium [P. zonale (L.) L'Hér. ‘Americana^® Dark Red’]}, heliotropium (Heliotropium arborescens L. ‘Scentropia^TM Dark Blue’), lobelia (Lobelia erinus L. ‘Techno^® Large Blue Violet’), sweetpotato vine (Ipomoea batatas L. Lam ‘Sidekick^TM Lime’), and verbena (Verbena ×hybrida Jacq. Ex L. ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’) were received from a commercial cutting supplier (Syngenta Flowers LLC).

Propagation, culture, and environment.

Upon receipt, cuttings of each taxa were individually inserted into industry-standard 105-cell propagation trays (30-mL individual cell volume, 54 cm × 28 cm × 5 cm; Grower Select^®; BFG Supply, Burton, OH, USA), while geraniums were individually inserted into industry-standard 72-cell propagation trays (33-mL individual cell volume, 54 cm × 28 cm × 5 cm; Grower Select^®; BFG Supply). Propagation trays were filled with a premoistened propagation substrate formulated with 50:50 (v/v) commercial soilless substrate composed of (by volume) 75% Canadian sphagnum peatmoss, 25% perlite, dolomitic lime, a nutrient starter charge, a wetting agent (Sunshine Mix No. 1; Sun Gro Horticulture, Agawam, MA, USA) and 50% coarse perlite (Coarse Perlite; Sun Gro Horticulture). Propagation substrate physical properties were determined using three representative samples that were analyzed according to the North Carolina State University Porometer Procedure (Fonteno et al. 1995). Physical properties of the propagation substrate were (by volume) 14% (±1.2%) air space, 78% (±1.1%) total porosity, 64% (±0.3%) container capacity, and 0.09 g·cm⁻³ (±0.1 g·cm⁻³) bulk density. Before cutting insertion, propagation trays were irrigated to container capacity and allowed to drain.

Propagation trays of each taxa were placed in a propagation environment in a glass-glazed greenhouse (71.2 m²; 9.75 m × 7.3 m; 5-m gutter height) at The Ohio State University Controlled Environment Agriculture Research Complex, Columbus, OH, USA (lat. 40°N, long. 83°W, 238 m above sea level), equipped with two exhaust fans, evaporative pad cooling, a vertical air flow fan, retractable shade and energy curtains, and perimeter heat provided from a hot water boiler; all of these were controlled by an environmental computer system (version 9.7; Priva B.V., De Lier, Netherlands). Vertical air flow speed was measured at 24 points across the greenhouse environment with a handheld hotwire anemometer (model A004; Kanomax Japan, Inc., Osaka, Japan), with an average of 0.07 m·s⁻¹ (±0.02 m·s⁻¹). Ambient carbon dioxide (CO₂) in the greenhouse environment was measured at 24 points across the greenhouse with a handheld CO₂ meter (model GM70; Vaisala, Helsinki, Finland), with an average of 472.9 µmol·mol⁻¹ (±23.5 µmol·mol⁻¹). No supplemental CO₂ was provided throughout any of the experiments conducted.

The propagation bench (6.0 m × 1.5 m) was elevated 0.86 m above the greenhouse floor and oriented with a latitudinal axis running east–west. The expanded metal bench was insulated with expanded polystyrene (EPS) boards faced with a reflective foil (1.2 m × 2.4 m × 2.3 cm; Polyshield^®; Cellofoam, Whiteland, IN, USA) and covered with bench-top root zone heating mats (Redi-Heat^TM Heavy-Duty Propagation Mats; Phytotronics Inc., Earth City, MO, USA). Each heating mat was controlled independently with a thermostat (RDT-4 Thermostat; Phytotronics Inc.) and set to maintain a root zone temperature of 23 °C. To prevent heat loss and moisture accumulation, the propagation bench was covered with a 4-mil black construction film (3 m × 30.5 m roll, Blue Hawk; Poly-America, Grand Prairie, TX, USA). A propagation tent (6.0 m × 1.5 m × 1.5 m) was constructed with 1.9-cm-diameter polyvinyl chloride pipe (Charlotte Piper and Foundry Co., Charlotte, NC, USA) and covered with fixed 4-mil clear construction film (3 m × 30.5 m roll, Blue Hawk; Poly-America) and a woven shadecloth providing approximately 56% shade (Solaro 5620 O-R-FR; Ludvig Svensson, Inc., Charlotte, NC, USA).

Beginning at the placement of cuttings under propagation conditions, mist consisting of clear tap water was controlled (Super Nova 12B mist controller; Phytotronics, Inc., Earth City, MO, USA) and applied for 6 s every 10 min beginning and ending 2 h before and after the photoperiod. After 1 d, mist was suspended from 0900 to 0930 HR to allow unrooted cutting surfaces to dry. A foliar rooting hormone solution containing deionized water and indole-3-butric acid (20% IBA) (Advocate^®; Fine Americas, Walnut Creek, CA, USA) was applied and uniformly delivered 200 mg·L⁻¹ IBA at 0.20 L·m⁻² across propagation trays. Cuttings were allowed to dry and mist was resumed at 1030 HR.

Upon visible adventitious root formation (root length, ≥5 mm), cuttings of each taxa were removed from the propagation bench and transferred to an adjacent expanded metal bench for subsequent rooting. Upon transfer, cuttings were overhead-irrigated once daily with clear tap water supplemented with a water-soluble fertilizer [15 nitrogen (N)–2.2 phosphorous (P)–12.5 potassium (K)–4 calcium (Ca)–2 magnesium (Mg); 15–5–15 Ca-Mg Jack’s Professional Pure Water XL; J.R. Peters Inc., Allentown, PA, USA] containing 3% ammoniacal-N and 12% nitrate-N delivered by an injector (Model D25RE2, 0.05–11 GPM Dosatron; Dosatron International, LLC, Clearwater, FL, USA). Young plants received the following (mg·L⁻¹): 150 N, 21.5 P, 124.5 K, 40 Ca, 20 Mg, 0.15 boron (B), 0.13 sulfur (S), 0.08 copper (Cu), 0.75 iron (Fe), 0.38 manganese (Mn), 0.08 molybdenum (Mo), and 0.38 zinc (Zn). After 1 week of adventitious root development, young plants received a magnesium sulfate (J.R. Peters Inc.) drench providing 11.7 mg·L⁻¹ Mg and 15.4 mg·L⁻¹ S.

Supplemental and day-extension lighting was provided by 1000-W light-emitting diode (LED) lamps (Gavita CT1930e LED 120–277V; Gavita Horticultural Lighting, Amsterdam, Netherlands) from 0600 to 2200 HR (16-h photoperiod) that delivered a supplemental photosynthetic photon flux density of approximately 120 µmol·m⁻²·s⁻¹ at cutting height as measured with a quantum sensor (LI-250A light meter; LI-COR Biosciences, Lincoln, NE, USA). The LED lamps were controlled by an environmental computer system (version 9.7; Priva B.V.); they were turned off when the outdoor light intensity reached approximately 490 µmol·m⁻²·s⁻¹ and turned on when the outdoor light intensity decreased below the radiation limit. Dual-layer retractable woven shade curtains were deployed when the outdoor light intensity reached approximately 980 µmol·m⁻²·s⁻¹ and retracted when the outdoor light intensity fell below the radiation limit. The top retractable woven shade curtains (Luxous 1547 D FR; Ludvig Svensson Inc.) were deployed to 70%, and the bottom layer (Harmony 4647 FR; Ludvig Svensson Inc.) was deployed to 100%. Vertical retractable woven light spill curtains (Obscura 10070 R FR W; Ludvig Svensson Inc.) were deployed from 1700 to 0800 HR.

Full-spectrum quantum sensors (SQ-500; Apogee Instruments, Logan, UT, USA) measured photosynthetic photon flux density at plant canopy level. Canopy air temperature and relative humidity (RH) were measured using a precision thermistor (ST-110-SS; Apogee Instruments) and humidity probe (EE08-SS; Apogee Instruments), respectively, encased in a fan-aspirated solar radiation shield (TS-130-SS; Apogee Instruments). Measurements were recorded every 15 s, and the average for each sensor was logged every 15 min by a data logger (Model CR1000X; Campbell Scientific, Inc., Logan, UT, USA). Greenhouse air temperature and RH set points were 23 °C and 80%, respectively. Environmental data during propagation and toning phases for each taxa during Expts. 1 and 2 are reported in Table 1. The vapor pressure deficit was calculated from greenhouse air temperature and RH averages.

Table 1. Average photosynthetic daily light integrals (DLIs), calculated vapor pressure deficit (VPD), and canopy air temperature during propagation and toning of 20 annual bedding plant taxa. Expt. 1 included bacopa (Bacopa cordata L. ‘Pink Sand’), coleus [Coleus scutellarioides (L.) Benth. ‘Main Street^TM Ocean Drive’], lantana (Lantana camara L. ‘Bandolista^TM Red Chili’), mezoo (Aptenia cordifolia L. Bolus. ‘Trailing Red’), New Guinea impatiens (Impatiens hawkeri R.W. Baker & Corner ‘Magnum Fire’ and ‘Magnum Red Flame’)], petunia (Petunia ×atkinsiana D. Don. ‘Durabloom^TM Royal Pink’), and portulaca (Portulaca oleracea L. ‘Flame Red’). Expt. 2 included calibrachoa [Calibrachoa ×hybrida (Llave and Lex.) M. Martens and L. Westra. ‘Callie^® Dark Red’], dahlia [Dahlia ×hybrida (Hort. ex. Andrews) Lindl. ex. Loudon. ‘Grandalia^TM Dark Red’ and ‘Dahlegria^® Red’], fanflower (Scaevola aemula R. Br. ‘Bombay^® Dark Blue’), geranium species {[interspecific geranium (Pelargonium interspecific ‘Calliope^® Large Dark Red’), ivy geranium [P. peltatum (L.) L’Hér. ex Aiton. ‘Cascade Dark Red’], and zonal geranium [P. zonale (L.) L’Hér. ‘Americana^® Dark Red’]}, heliotropium (Heliotropium arborescens L. ‘Scentropia^TM Dark Blue’), lobelia (Lobelia erinus L. ‘Techno^® Large Blue Violet’), sweetpotato vine (Ipomoea batatas L. Lam ‘Sidekick^TM Lime’), and verbena (Verbena ×hybrida Jacq. Ex L. ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’). Unrooted cuttings were propagated and young plants were toned in a glass-glazed greenhouse for 21 d under ambient daylight supplemented with approximately 120 µmol·m⁻²·s⁻¹ from 1000-W light-emitting diode lamps from 0600 to 2200 HR (16-h photoperiod) to maintain a DLI of 10 or 14 mol·m⁻²·d⁻¹. The canopy air temperature and root zone heating setpoints were 23 °C.

View Fullscreen

Plant culture and environment.

In all experiments, 21-d-old young plants with similar heights, stem calipers, and node and leaf numbers were selected. On 3 Oct, bacopa, coleus, portulaca, New Guinea impatiens cultivars, and petunia were transplanted, with one plant per 12-cm-diameter container (970 mL individual volume; Landmark Plastics, Akron, OH, USA). After transplant, containers were individually placed into 15-count shuttle trays (30.5 cm × 51.4 cm; Shuttle Tray^® Flower Trays, East Jordan Plastics, Inc., East Jordan, MI, USA) on 23-cm² centers. On 4 Oct, lantana and mezoo were transplanted and spaced as previously described. On 16 Jan 2024, calibrachoa, fanflower, heliotropium, lobelia, sweetpotato vine, and verbena cultivars were transplanted, with one plant per 12.7-cm-diameter container (946 mL individual volume; East Jordan Plastics, Inc.). Each dahlia cultivar and geranium species were transplanted, with one plant per 16.5-cm diameter container (1.9 L individual volume; Landmark Plastic Corp., Akron, OH, USA). All containers were filled with a premoistened commercial soilless substrate (Sunshine Mix No. 1; Sun Gro Horticulture). Substrate physical properties were determined using procedures previously described by Fonteno et al. (1995) and were (by volume) 15.2% (±1.1%) air space, 84.1% (±0.8%) total porosity, 68.9% (±1.3%) container capacity, and 5.3 g·cm⁻³ (±0.2 g·cm⁻³) bulk density.

Upon transplant, plants were overhead-irrigated to container capacity with clear tap water. At each subsequent irrigation, plants were overhead-irrigated with clear tap water supplemented with a water-soluble fertilizer as previously described. At 14 d after transplant, plants were irrigated with magnesium sulfate (J.R. Peters Inc.) as previously described.

Except when indicated, the greenhouse environment, setpoints, and monitoring were the same as those described for propagation. Plants were grown on expanded metal benches (6.0 m × 1.5 m) elevated 0.86 m above the greenhouse floor and oriented with a latitudinal axis running east–west in a glass-glazed greenhouse. The LED lamps were controlled by an environmental computer system (version 9.7; Priva B.V.); they were turned off when the outdoor light intensity reached approximately 490 µmol·m⁻²·s⁻¹ and turned on when the outdoor light intensity decreased below the radiation limit. Greenhouse air temperature and RH set points were 20 °C and 70%, respectively. Environmental data collected during plant culture for each taxa during Expts. 1 and 2 are reported in Table 2.

Table 2. Average photosynthetic daily light integrals (DLIs), relative humidity (RH), and canopy air temperature during finishing of 20 annual bedding plant taxa. Expt. 1 included bacopa (Bacopa cordata L. ‘Pink Sand’), coleus [Coleus scutellarioides (L.) Benth. ‘Main Street^TM Ocean Drive’], lantana (Lantana camara L. ‘Bandolista^TM Red Chili’), mezoo (Aptenia cordifolia L. Bolus. ‘Trailing Red’), New Guinea impatiens (Impatiens hawkeri R.W. Baker & Corner ‘Magnum Fire’ and ‘Magnum Red Flame’), petunia (Petunia ×atkinsiana D. Don. ‘Durabloom^TM Royal Pink’), and portulaca (Portulaca oleracea L. ‘Flame Red’). Expt. 2 included calibrachoa [Calibrachoa ×hybrida (Llave and Lex.) M. Martens and L. Westra. ‘Callie^® Dark Red’], dahlia [Dahlia ×hybrida (Hort. ex. Andrews) Lindl. ex. Loudon. ‘Grandalia^TM Dark Red’ and ‘Dahlegria^® Red’], fanflower (Scaevola aemula R. Br. ‘Bombay^® Dark Blue’), geranium species {[interspecific geranium (Pelargonium interspecific ‘Calliope^® Large Dark Red’)], ivy geranium [P. peltatum (L.) L'Hér. ex Aiton. ‘Cascade Dark Red], and zonal geranium [P. zonale (L.) L’Hér. ‘Americana^® Dark Red’]}, heliotropium (Heliotropium arborescens L. ‘Scentropia^TM Dark Blue’), lobelia (Lobelia erinus L. ‘Techno^® Large Blue Violet’), sweetpotato vine (Ipomoea batatas L. Lam ‘Sidekick^TM Lime’), and verbena (Verbena ×hybrida Jacq. Ex L. ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’). Plants were transplanted into 12.7-cm (946 mL) or 16.5-cm (1.9 L) containers filled with a peat-based substrate and grown in a glass-glazed greenhouse under ambient daylight supplemented with approximately 120 µmol·m⁻²·s⁻¹ from 1000-W light-emitting diode lamps from 0600 to 2200 HR (16-h photoperiod) to maintain a DLI of 10 to 14 mol·m⁻²·d⁻¹. The canopy air temperature setpoint was 20 °C.

View Fullscreen

Ethephon substrate drenches.

At 10 d after transplant, ethephon substrates drenches were applied from 0900 to 1330 HR. Eight single-plant replicates (individual plants) of each taxa were drenched with 90-mL or 150-mL aliquots of solution containing deionized water (0 mg·L⁻¹; control) or 25, 50, 75, 100, or 200 mg·L⁻¹ ethephon (Collate^® 2L; Fine Americas, Inc.) (90 mL: 0, 2.2, 4.4, 6.7, 8.9, and 17.7 mg a.i./pot; 150 mL: 0, 3.7, 7.4, 11.1, 14.8, and 29.6 mg a.i./pot).

Substrate pH management.

At 7 d before the ethephon drench application and 2 d and 7 d after drench application, substrate solution was extracted 1 h after irrigation using the PourThru method (Cavins 2002). Substrate solution was analyzed to determine the pH and electrical conductivity (EC) using a handheld pH and EC meter (HI 9813-6; Hanna Instruments, Woonsocket, RI, USA). The average initial substrate pH before ethephon drench application was 5.8 ± 0.1. At 2 d after ethephon drench application, the substrate pH was determined to be 5.5 ± 0.1. However, at 7 d after drench application, the substrate pH returned to 5.8 ± 0.2 (data not shown). No low-substrate pH-induced nutrient disorder symptomologies were observed for any of the taxa grown during Expts. 1 and 2.

Growth and morphological data and calculations.

Throughout each experiment, visual observations were recorded for phytotoxic effects. The experiments were ended, and the plants were harvested 3 weeks (bacopa, coleus, heliotropium, lobelia, and petunia), 4 weeks (calibrachoa, ‘Grandalia^TM Dark Red’ dahlia, fanflower, mezoo, sweetpotato vine, and ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’ verbena), 5 weeks (‘Dahlegria^® Red’ dahlia, geranium species, lantana, and portulaca), or 6 weeks (‘Magnum Fire’ and ‘Magnum Red Flame’ New Guinea impatiens) after drench. At termination, plant height and plant diameter were determined. Plant height was determined by measuring from the substrate surface to the highest growing point of the plant. Plant diameter was determined by measuring the widest dimension and the axis perpendicular to the widest dimension and averaged. Plants with at least one open flower (reflexed petals with visible pollen) were recorded, and the percentage of plants flowering at termination was calculated for each taxa and ethephon drench concentration. After measurements were performed, shoots were excised at the substrate surface, and all possible substrate was removed from roots while minimally damaging the root system with gentle washing in tap water. Shoots and roots were individually bagged and dried separately in a forced-air convection oven (179 L Fisherbrand^TM Isotemp^TM General Purpose Heating and Drying Oven; Fisher Scientific, Pittsburgh, PA, USA) at 65 °C. After 1 week, shoots and roots were weighed using an analytical balance (ME204E; Mettler Toledo, Columbus, OH, USA) to determine SDW and RDW, respectively.

Experimental design and data analysis.

The experiment was conducted using a completely randomized design with eight single-plant replicates per taxa for each ethephon concentration. For each taxa, the effects of the ethephon concentration were analyzed with the general linear model (PROC GLM) and an analysis of variance (ANOVA) using SAS (ver. 9.4; SAS Institute, Cary, NC, USA). For flowering percentage, plant height, plant diameter, SDW, and RDW, a regression analysis within taxa with the ethephon concentration as the independent variable was performed using the SAS regression procedure (PROC REG). Regression equations for plant height, plant diameter, SDW, and RDW for each taxa are listed in Table 3. For all analyses, P ≤ 0.05 was used to determine significant effects.

Table 3. Regression equations (see Figs. 8 to 11) for plant height, plant diameter, shoot dry weight (SDW), and root dry weight (RDW) of 20 annual bedding plant taxa Expt. 1 included bacopa (Bacopa cordata L. ‘Pink Sand’), coleus [Coleus scutellarioides (L.) Benth. ‘Main Street^TM Ocean Drive’], lantana (Lantana camara L. ‘Bandolista^TM Red Chili’), mezoo (Aptenia cordifolia L. Bolus. ‘Trailing Red’), New Guinea impatiens (Impatiens hawkeri R.W. Baker & Corner ‘Magnum Fire’ and ‘Magnum Red Flame’), petunia (Petunia ×atkinsiana D. Don. ‘Durabloom^TM Royal Pink’), and portulaca (Portulaca oleracea L. ‘Flame Red’). Expt. 2 included calibrachoa [Calibrachoa ×hybrida (Llave and Lex.) M. Martens and L. Westra. ‘Callie^® Dark Red’], dahlia [Dahlia ×hybrida (Hort. ex. Andrews) Lindl. ex. Loudon. ‘Grandalia^TM Dark Red’ and ‘Dahlegria^® Red’], fanflower (Scaevola aemula R. Br. ‘Bombay^® Dark Blue’), geranium species {[interspecific geranium (Pelargonium interspecific ‘Calliope^® Large Dark Red’), ivy geranium [P. peltatum (L.) L’Hér. ex Aiton. ‘Cascade Dark Red’], and zonal geranium [P. zonale (L.) L’Hér. ‘Americana^® Dark Red’]}, heliotropium (Heliotropium arborescens L. ‘Scentropia^TM Dark Blue’), lobelia (Lobelia erinus L. ‘Techno^® Large Blue Violet’), sweetpotato vine (Ipomoea batatas L. Lam ‘Sidekick^TM Lime’), and verbena (Verbena ×hybrida Jacq. Ex L. ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’)].

View Fullscreen

Results

Ethephon drench concentrations significantly influenced plant height, plant diameter, SDW, and RDW of most species investigated (Figs. 1–11). In general, a dose-dependent relationship was observed, with higher ethephon drench concentrations resulting in shorter compact plants (Figs. 1–7). The magnitude of plant height control attained by increasing ethephon substrate drench concentrations varied among species [Figs. 8A–D (Expt. 1), 9A–D (Expt. 1), 10A–F (Expt. 2), and 11A–F (Expt. 2)]. For example, lantana (Fig. 9A) and heliotropium (Fig. 11B) were 16% (1.9 cm) and 20% (2.6 cm) shorter, respectively, whereas portulaca (Fig. 9D) was 70% (4.4 cm) taller when plants were drenched with 25 mg·L⁻¹ ethephon when compared with untreated plants. Other species require higher ethephon drench concentrations to attain growth control. For instance, compared with untreated plants, substrate drenches containing up to 75 mg·L⁻¹ ethephon controlled plant height of ‘Grandalia^TM Dark Red’ (Fig. 10B) and ‘Dahlegria^® Red’ dahlia (Fig. 10C) by 50% (15.4 cm) and 17% (5.1 cm), respectively. Furthermore, sweetpotato vine (Fig. 11D) was 33% (3.2 cm) shorter than untreated plants as concentrations increased from 0 to 200 mg·L⁻¹ ethephon. ‘Lanai^® Upright Scarlet’ verbena (Fig. 11F) drenched with 50 to 200 mg·L⁻¹ ethephon was 15% to 39% (1.8–4.6 cm) shorter than untreated plants. Plant heights of bacopa (Fig. 8A), coleus (Fig. 8B), mezoo (Fig. 9B), petunia (Fig. 9C), calibrachoa (Fig. 10A), and fanflower (Fig. 10D) were unaffected by ethephon drenches.

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Plant diameter [Figs. 8E–H (Expt. 1), 9E–H (Expt. 1), 10G–L (Expt. 2), and 11G–L (Expt. 2)] of most species was significantly influenced by increasing ethephon drench concentrations. Coleus (Fig. 8F) and heliotropium (Fig. 11H) drenched with 25 mg·L⁻¹ ethephon were 1.5% (0.3 cm) and 18.1% (3.2 cm) narrower, respectively, compared with untreated plants. Additional growth control was not observed at higher concentrations in these species. Some species such as bacopa and ‘Magnum Fire’ and ‘Magnum Red Flame’ New Guinea impatiens responded at higher concentrations of ethephon. For example, ‘Magnum Fire’ (Fig. 8G) and ‘Magnum Red Flame’ (Fig. 8H) New Guinea impatiens drenched with 50 mg·L⁻¹ ethephon were 23% (5.9 cm) and 12% (3.4 cm) narrower, respectively, when compared with untreated plants. Furthermore, when compared with untreated plants, ‘Grandalia^TM Dark Red’ (Fig. 10H) and ‘Dahlegria^® Red’ dahlia (Fig. 10I) drenched with 25 to 75 mg·L⁻¹ ethephon were 25% to 31% (9.8–12.4 cm) and 9% to 15% (4.5–7.9 cm) smaller, respectively. Lantana (Fig. 9E) drenched with 25 to 200 mg·L⁻¹ ethephon was 14% to 47% (4.1–13.9 cm) smaller than untreated plants. Plant diameter of petunia (Fig. 9G) and interspecific geranium (Fig. 10K) were unaffected by all trialed concentrations of ethephon.

The SDWs [Figs. 8I–L (Expt. 1), 9I–L (Expt. 1), 10M–R (Expt. 2), and 11M–R (Expt. 2)] of most species were significantly less with increasing ethephon drench concentrations, but the magnitude of growth control varied among species. For example, those of ‘Magnum Fire’ New Guinea impatiens (Fig. 8K) and calibrachoa (Fig. 10M) drenched with 25 mg·L⁻¹ ethephon were 24% (2.1 g) and 33% (0.9 g) smaller, respectively, compared with untreated plants. ‘Magnum Red Flame’ New Guinea impatiens (Fig. 8L) and ‘Grandalia^TM Dark Red’ dahlia (Fig. 10N) drenched with 25 to 75 mg·L⁻¹ ethephon were 14% to 30% (1.4–3.0 g) and 35% to 48% (8.9–12.2 g) smaller, respectively, than untreated plants. Lantana (Fig. 9I) drenched with 25 to 100 mg·L⁻¹ ethephon was 24% to 37% (1.2–1.8 g) smaller than untreated plants. Furthermore, mezoo (Fig. 9J) and portulaca (Fig. 9L) treated with 25 to 200 mg·L⁻¹ ethephon were 36% to 62% (2.8–4.8 g) and 22% to 40% (0.9–1.7 g) smaller, respectively, than untreated plants. The SDWs of coleus (Fig. 8J), petunia (Fig. 9K), interspecific geranium (Fig. 10Q), and lobelia (Fig. 11O) were unaffected at all trialed concentrations of ethephon. The maximum level of control observed in SDW is species-dependent and ranges from 25 to 100 mg·L⁻¹ ethephon.

The RDW generally diminished with increasing substrate drench concentrations of ethephon, with the magnitude of reduction varying among species [Figs. 8M–P (Expt. 1), 9M–P (Expt. 1), 10S–X (Expt. 2), and 11S–X (Expt. 2)]. Coleus (Fig. 8N), petunia (Fig. 9O), and ‘Lanai^® Upright Scarlet’ verbena (Fig. 11X) drenched with 25 mg·L⁻¹ ethephon resulted in RDWs that were 45% (1.7 g), 35% (1.6 g), and 47% (1.5 g) lower, respectively, compared with untreated plants. When compared with untreated plants, zonal geranium (Fig. 11S) and ‘Lanai^® Red’ verbena (Fig. 11W) drenched with 50 to 75 mg·L⁻¹ ethephon yielded RDWs that were 37% to 54% (1.8–2.7 g) and 25% to 46% (1.1–1.9 g) smaller, respectively, than untreated plants. The RDWs of ‘Magnum Fire’ New Guinea impatiens (Fig. 8N) and portulaca (Fig. 9P) drenched with 75 to 100 mg·L⁻¹ ethephon were 42% to 62% (3.5–5.2 g) and 31% to 45% (0.5–0.7 g) smaller, respectively, compared with untreated plants. The RDWs of some species were affected at higher concentrations of ethephon. For example, when treated with 100 to 200 mg·L⁻¹ ethephon, the RDW of ‘Grandalia^TM Dark Red’ dahlia (Fig. 10T) diminished by 31% to 63% (3.1–6.3 g) compared with untreated plants. The RDW of ivy geranium (Fig. 10X) was unaffected by all trialed ethephon concentrations.

The flowering response to increasing ethephon drench concentrations varied among the annual bedding plants investigated (Table 4). Flowering at termination of bacopa, calibrachoa, ‘Grandalia^TM Dark Red’ dahlia, heliotropium, lantana, lobelia, mezoo, ‘Magnum Fire’ and ‘Magnum Red Flame’ New Guinea impatiens, petunia, and zonal geranium was unaffected by all trialed concentrations of ethephon (Table 4). However, for fanflower, interspecific geranium, ivy geranium, portulaca, and ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’ verbena, delayed flowering was observed as ethephon substrate drench concentrations increased. For example, the percentage of interspecific geranium and ivy geranium plants flowering at the time of termination were significantly less when plants were drenched with 100 mg·L⁻¹ ethephon compared with untreated plants. Furthermore, portulaca drenched with any concertation of ethephon did not flower, while 100% of untreated plants were flowering at the time of termination.

Table 4. Flowering percentages of 20 annual bedding plant taxa. Expt. 1 included bacopa (Bacopa cordata L. ‘Pink Sand’), coleus [Coleus scutellarioides (L.) Benth. ‘Main Street^TM Ocean Drive’], lantana (Lantana camara L. ‘Bandolista^TM Red Chili’), mezoo (Aptenia cordifolia L. Bolus. ‘Trailing Red’), New Guinea impatiens (Impatiens hawkeri R.W. Baker & Corner ‘Magnum Fire’ and ‘Magnum Red Flame’), petunia (Petunia ×atkinsiana D. Don. ‘Durabloom^TM Royal Pink’), and portulaca (Portulaca oleracea L. ‘Flame Red’). Expt. 2 included calibrachoa [Calibrachoa ×hybrida (Llave and Lex.) M. Martens and L. Westra. ‘Callie^® Dark Red’], dahlia [Dahlia ×hybrida (Hort. ex. Andrews) Lindl. ex. Loudon. ‘Grandalia^TM Dark Red’ and ‘Dahlegria^® Red’], fanflower (Scaevola aemula R. Br. ‘Bombay^® Dark Blue’), geranium species {[interspecific geranium (Pelargonium interspecific ‘Calliope^® Large Dark Red’), ivy geranium [P. peltatum (L.) L’Hér. ex Aiton. ‘Cascade Dark Red’], and zonal geranium [P. zonale (L.) L’Hér. ‘Americana^® Dark Red’]}, heliotropium (Heliotropium arborescens L. ‘Scentropia^TM Dark Blue’), lobelia (Lobelia erinus L. ‘Techno^® Large Blue Violet’), sweetpotato vine (Ipomoea batatas L. Lam ‘Sidekick^TM Lime’), and verbena (Verbena ×hybrida Jacq. Ex L. ‘Lanai^® Red’ and ‘Lanai^® Upright Scarlet’)} drenched with 0, 25, 50, 75, 100, or 200 mg·L⁻¹ ethephon at 10 d after transplant and grown for 3 to 6 weeks after drench. Percentages were determined at termination. Plants were transplanted into 12-cm (970 mL), 12.7-cm (946 mL), or 16.5-cm (1.9 L) containers filled with a peat-based substrate and grown in a glass-glazed greenhouse under ambient daylight supplemented with approximately 120 µmol·m⁻²·s⁻¹ from 1000-W light-emitting diode lamps from 0600 to 2200 HR (16-h photoperiod) to maintain a daily light integral of 10 to 14 mol·m⁻²·d⁻¹. The canopy air temperature setpoint was 20 °C.

View Fullscreen

Discussion

Our research demonstrated that ethephon substrate drenches can effectively control plant height of annual bedding plants, and the results are in agreement with those of previous studies (Aiken et al. 2015; Currey et al. 2016; Miller et al. 2012; Rich and Owen 2025). While our research expands the scope of annual species evaluated, it complements the work by Miller et al. (2012), who found a shorter stem length than that of untreated plants for 23 species of annual bedding plants when treated with ethephon drenches. For instance, when drenched with 200 mg·L⁻¹ ethephon, the stem lengths of marguerite daisy [Argyranthemum frutescens (L.) Sch. Bip. ‘Madeira Cherry Red’] and nemesia [Nemesia fruticans (Thunb.) Benth. ‘Aromatica Royal’] were 19% (3.2 cm) and 32% (11.6 cm) shorter compared with untreated plants (Miller et al. 2012). Additionally, the authors found that Madagascar periwinkle [Catharanthus roseus (L.) G. Don. ‘Cooler Grape’], plume cockscomb (Celosia argentea L. var. plumosa Voss. ‘New Look’), and dianthus (Dianthus chinensis L. ‘Super Parfait Raspberry’) drenched with 200 mg·L⁻¹ ethephon were 59% (7.3 cm), 70% (18.2 cm), and 35% (5.6 cm) shorter, respectively, than untreated plants. Furthermore, our research agrees with that of Rich and Owen (2025), who reported ethephon substrate drenches controlled plant height of 15 herbaceous perennial species. For example, Russian sage (Salvia yangii B.T. Drew) and blanket flower (Gaillardia aristata Pursh. ‘SpinTop^® Red Starburst’) were 48% to 56% and 11% to 14% shorter, respectively, when drenched with 125 and 250 mg·L⁻¹ ethephon than untreated plants. Overall, our research demonstrated 25 to 100 mg·L⁻¹ ethephon can effectively control plant height of the majority of the annual bedding plant species trialed.

Plant diameter of most species trialed was significantly affected by increasing concentrations of ethephon. Our results agree with those of Barker et al. (2016), who reported a 38% reduction in the plant diameter of plumbago (Plumbago auriculata Lam. ‘Imperial Dark Blue’) when plants were drenched with 1000 mg·L⁻¹ ethephon compared with untreated plants. Similarly, Currey et al. (2016) found that the plant diameter of ‘Serena White’ angelonia was 38% (10.2 cm) smaller as substrate drench concentrations increased from 0 to 100 mg·L⁻¹ ethephon. Furthermore, Rich and Owen (2025) demonstrated that substate drenches of 125 to 500 mg·L⁻¹ ethephon provided adequate growth control of plant diameter for many species of herbaceous perennials. For example, as substrate drench concentrations increased from 0 to 500 mg·L⁻¹ ethephon, tender foxglove (Digiplexis ×hybrida ‘Berry Canary’) and ‘SpinTop^® Red Starburst’ blanket flower were 16% (6.5 cm) and 21% (5.4 cm) narrower, respectively, than untreated plants (Rich and Owen 2025). We concluded that substrate drenches containing 25 to 100 mg·L⁻¹ ethephon are effective at controlling the plant diameter of the majority of annual bedding plants reported herein. Plant diameters of petunia and interspecific geranium were unaffected by increasing concentrations of ethephon substrate drenches. This is surprising because petunia and geraniums are known to be sensitive to ethylene; therefore, further research is needed (Jones and Edelman 2013; 2014; Jones et al. 2001; Leatherwood and Mattson 2007).

Increasing ethephon drench concentrations reduced SDW of most of the species trialed. Our results support the findings by Miller et al. (2012), who found a reduction in SDWs of 16 annual bedding plant species. For example, ‘Serena Lavendar’ angelonia, ‘Super Parfait Raspberry’ dianthus, and African daisy [Osteospermum ecklonis (DC.) Norl. ‘Zion Orange’] drenched with 200 mg·L⁻¹ ethephon were 68% (2.6 g), 62% (2.3 g), and 54% (2.6 g) smaller, respectively, compared with untreated plants (Miller et al. 2012). Currey et al. (2016) also reported that ‘Serena White’ angelonia drenched 10 d after transplant with 100 mg·L⁻¹ ethephon was 34% (1.7 g) smaller than untreated plants. Additionally, Rich and Owen (2025) determined drench concentrations of 250 to 500 mg·L⁻¹ ethephon provided adequate growth control of 15 herbaceous perennials. For instance, lobed tickseed (Coreopsis auriculata L. ‘Leading Lady Iron’) and Russian sage drenched with 125 to 500 mg·L⁻¹ ethephon were 24% to 39% (5.0–8.1 g) and 54% to 76% (4.5–6.3 g) smaller, respectively, compared with untreated plants. Our research results and those of these previous studies demonstrated that ethephon substrate drenches can effectively control SDW of annual bedding plants. Furthermore, our research concluded that substrate drenches of 25 to 100 mg·L⁻¹ ethephon provided the greatest level of control of SDW for most species trialed.

The RDW of most annual bedding plant species trialed herein was influenced by increasing concentrations of ethephon. Our observations are supported by the findings of Miller et al. (2012), who reported that the RDWs of ‘Zion Orange’ African daisy, bacopa (Sutera cordata Roth. ‘Abunda Giant White’), calibrachoa, diascia (Diascia barberae Hook. F. ‘Wink Coral’), French marigold (Tagetes patula L. ‘Crested Janie Deep Orange’), ‘Madeira Cherry Red’ marguerite daisy, ‘Aromatica Royal’ nemesia, and viola (Viola cornuta L. ‘Penny Lane Mixed’) decreased with increasing ethephon drench concentrations. For instance, when treated with 200 mg·L⁻¹ ethephon, ‘Crested Janie Deep Orange’ French marigold and ‘Penny Lane Mixed’ viola were 54% (0.3 g) and 78% (0.3 g) smaller, respectively, than untreated plants (Miller et al. 2012). Additionally, Rich and Owen (2025) reported increasing ethephon substrate drench concentrations suppressed root growth and, thus, RDW of 15 herbaceous perennial species. For instance, ‘SpinTop^® Red Starburst’ blanket flower drenched with 250 mg·L⁻¹ ethephon was 42% (4.5 g) smaller than untreated plants (Rich and Owen 2025). Currey et al. (2016) also found that the RDWs of ‘Serena White’ angelonia and geranium (Pelargonium ×hortorum L.H. Bailey ‘Pinto Premium Deep Red’) drenched with 200 mg·L⁻¹ ethephon were 54% (0.51 g) and 11% (0.09 g) smaller, respectively, than untreated plants. Similar to the results reported by Rich and Owen (2025), reductions in RDW were expected with ethephon substrate drenches because ethylene is known to impact root development. Khoury et al. (2024) reported that exogenous applications of ethylene gas or its precursor, aminocyclopropane-1-caboxylic acid, inhibit root elongation within 5 min of application and limited overall root branching as well as the development of root hairs in Arabidopsis. Therefore, it is speculated that ethephon substrate drenches impact the development and growth of the root system, leading to smaller RDW observed in the current trial and subsequently limiting plant growth and biomass accumulation in addition to the effects of ethephon translocated to the shoot inhibiting cell elongation (Rich and Owen 2025).

We found that several species were unaffected by ethephon substrate drenches. Plant heights of bacopa, calibrachoa, coleus, fanflower, mezoo, and petunia were unaffected by all trialed concentrations of ethephon trialed. The SDWs of coleus, interspecific geranium, lobelia, and petunia were unaffected by all trialed concentrations of ethephon. The lack of response to ethephon substrate drenches was not unexpected because Miller et al. (2012) reported that the plant heights of ‘Zion Orange’ African daisy, ‘Wink Coral’ diascia, petunia (Petunia ×grandiflora ‘Dreams Burgundy’), and tomato (Lycopersicon esculentum L. ‘Beefmaster’) were unaffected when plants were drenched with 25 to 200 mg·L⁻¹ ethephon. Miller et al. (2012) suggested that taxa, application timing, temperature, light, substrate composition, and other environmental factors can influence ethephon drench efficacy. We controlled the greenhouse environment (light and temperature) throughout the experiments, applied ethephon in the morning when plants were not stressed, and grew plants in peat-based substrates, similar to previous studies. As such, we speculated that the lack of response of the species reported herein to ethephon substrate drenches can be attributed to a lack of sensitivity to ethylene, which was also reported by Rich and Owen (2025). Moreover, Aiken et al. (2015) found that substrate pH impacts the efficacy of ethephon substrate drenches. We determined the substrate pH before and after ethephon drench application and, as expected, an initial drop in substrate pH was observed that may or may not have affected the ethephon efficacy and impact on plant growth. However, substrate pH increased over the course of the experiments and, because ethephon drenches influenced some aspect of plant growth and development in all 20 taxa trialed, we considered the lack of growth control to be a result of an unknown exogenous or endogenous factor (Rich and Owen 2025). Further research investigating other species and cultivars, lower and higher ethephon drench concentrations, and environmental or root zone factors is warranted.

A slight decrease in flowering was observed in five annual bedding plant species, but only at ≥50 mg·L⁻¹ ethephon concentrations (Table 4). Because flowering percentages varied widely among species trialed in the current study, in-house grower trials should determine whether a significant decrease in flowering will occur and disrupt production schedules.

It must be noted that some phytotoxic effects were observed at concentrations of 50 to 200 mg·L⁻¹ ethephon. For some species, leaf epinasty, lower leaf chlorosis, or necrosis of the leaf margins was observed (Table 5). For example, leaf epinasty was observed in ‘Magnum Fire’ and ‘Magnum Red Flame’ New Guinea impatiens when plants were drenched with 200 mg·L⁻¹ ethephon. However, there was no reduction in visual ornamental appeal of these plants. Necrosis and deformation of the meristems manifested in sweetpotato vine drenched with 200 mg·L⁻¹ ethephon. This was not unexpected because ethephon foliar sprays are commonly used to reduce apical dominance and increase lateral branching (Currey 2018). Because of these effects, it is recommended that annual bedding plant producers should perform in-house trials to ensure that the desired growth control effects are achieved, and that phytotoxic effects do not develop or cause damage to ornamental esthetic quality and value.

Table 5. Suggested ethephon substrate drench concentrations for 20 annual bedding plant taxa. Plants were transplanted into 12-cm (970 mL), 12.7-cm (946 mL), or 16.5-cm (1.9 L) containers filled with a peat-based substrate and grown in a glass-glazed greenhouse under ambient daylight supplemented with approximately 120 µmol·m⁻²·s⁻¹ from 1000-W light-emitting diode lamps from 0600 to 2200 HR (16-h photoperiod) to maintain a daily light integral (DLI) of 10 to 14 mol·m⁻²·d⁻¹. The canopy air temperature setpoint was 20 °C.

View Fullscreen

Conclusions

This research has expanded the use of ethephon in floriculture and found that substrate drenches are an effective and promising application method of controlling growth of annual bedding plants. Because some species were unresponsive to ethephon substrate drenches and others experienced extreme phytotoxic effects, further research defining the sensitivity of annual bedding plants to ethephon substrate drenches is warranted. Annual bedding plant producers should consider performing in-house trials to determine specific ethephon concentrations for species and cultivars not investigated herein or evaluate our suggestions based on their crop culture procedures, growing environment, and market demands; however, we suggest 25 to 100 mg·L⁻¹ ethephon as an initial range for trials.