Abstract
Our objective was to quantify the efficacy of paclobutrazol substrate drenches on growth of nine black-eyed Susan (Rudbeckia hirta) cultivars. Liners of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan were transplanted into 6.5-inch-diameter plastic containers (2 qt) filled with a commercial soilless peat-based substrate. After 16 days, six single-plant replicates received a substrate drench of 5-fl-oz aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.375, 0.75, 1.5, and 3.0 mg/pot). Paclobutrazol drenches of 2.5 to 20 mg·L−1 significantly influenced plant height, plant diameter, growth index (GI), and shoot dry weight (SDW) of all black-eyed Susan cultivars, although the magnitude of response to paclobutrazol substrate drench concentration varied with cultivar. For most cultivars, GI, an integrated measurement of height and diameter, was suppressed as paclobutrazol substrate drench concentrations increased from 2.5 to 20 mg·L−1, resulting in plants that were 30% to 43% smaller than untreated plants. Increasing paclobutrazol substrate drench concentrations from 2.5 to 20 mg·L−1 limited SDW of each cultivar differently, although plants were 5% to 59% smaller at 20 mg·L−1 paclobutrazol than untreated plants. Time to flower for ‘Autumn Colors’, ‘Cherry Brandy’, ‘Happy’, ‘Indian Summer’, and ‘Prairie Sunset’ was unaffected by any paclobutrazol substrate drench concentration; however, concentrations of ≤10 mg·L−1 paclobutrazol are suggested for ‘Cherokee Sunset’, ‘Denver Daisy’, ‘Glowing’, and ‘Sunny’, as higher concentrations delay flowering. Our results indicate that growers can attain growth control with substrate drenches containing 5 to 10 mg·L−1 paclobutrazol during greenhouse black-eyed Susan production without delaying flowering.
Rudbeckia (Rudbeckia sp.), grown as annual bedding plants and herbaceous perennials, have increased in popularity for use in summer and fall mixed combination containers, landscape plantings, and as cut flowers. From 2014 to 2019, the total sales value of annual and herbaceous perennial rudbeckia plants increased by $1 million or 9% [US Department of Agriculture, National Agricultural Statistics Service (USDA-NASS) 2015, 2020]. In 2019, rudbeckia was the 13th largest category of the herbaceous perennial plant sector in the US commercial floriculture industry, with a reported volume of 2.5 million containers representing a total sales value of $10.2 million (USDA-NASS 2020). There are at least 74 black-eyed Susan (Rudbeckia hirta) cultivars available for commercial greenhouse and nursery production (Ball Horticultural Co. 2023) each varying in flower colors ranging from monochromatic or dichromatic yellow, orange, red, or brown ray florets with brown to black or yellow-green disk florets (Owen WG, personal observations). Growth habits for black-eyed Susan cultivars vary, with many growing tall (18 to 32 inches), which is problematic for greenhouse and nursery growers to ship without causing damage to vegetative growth or flowers (Owen WG, personal observations). Stem lodging in tall black-eyed Susan cultivars can also occur because of tight plant spacing in the greenhouse or overturned plants on the growing pad caused by high winds each leading to plant damage and potential loss (Owen WG, personal observations). Therefore, container-grown black-eyed Susans may require plant growth retardant (PGR) applications to control growth and produce marketable plants that are proportionally 1.5 to 2 times the container height (Sachs et al. 1976).
PGRs are commonly applied to limit stem internode elongation of containerized greenhouse-grown ornamental crops (Whipker and Latimer 2021), and efficacy may vary within cultivars of the same species (Currey et al. 2016; Whipker and McCall 2000). Owen and Latimer (2022) compiled research and commercial recommendations for controlling black-eyed Susan and suggest using foliar sprays of daminozide at 2500 to 5000 mg·L−1 for ‘Denver Daisy’, chlormequat chloride at concentrations <1500 mg·L−1, daminozide and chlormequat chloride at concentrations <5000 and <1500 mg·L−1, respectively, paclobutrazol at 30 mg·L−1 for ‘Denver Daisy’ or 160 mg·L−1 for ‘Indian Summer’, or uniconazole at 10 mg·L−1 for ‘Denver Daisy’ or 25 mg·L−1 uniconazole for outdoor-grown plants. To date, PGR substrate drench recommendations for black-eyed Susan is limited to one suggestion of applying 1 to 5 mg·L−1 paclobutrazol at 4 to 6 weeks after transplanting ‘Denver Daisy’ (Owen and Latimer 2022).
There are many advantages of PGR substrate drenches including precision of application, crop uniformity, longer duration of effectiveness compared with foliar sprays, minimal environmental impact, and the reduction of potential drift from spray applications (Currey et al. 2016; Owen et al. 2016). Paclobutrazol is the most widely used PGR for greenhouse-grown floriculture crops because it is characterized by having high chemical activity for inhibiting the biosynthesis of gibberellins, thereby limiting stem elongation (Carvalho-Zañao et al. 2018; Rademacher 2000) and is effective as a substrate drench (Whipker and Latimer 2021). Numerous studies have reported the effect of paclobutrazol substrate drenches on growth and flowering responses of floriculture species (Barrett and Nell 1989; Currey et al. 2016; Dasoju et al. 1998; Gibson and Whipker 2001, 2003; Owen et al. 2016; Whipker and Hammer 1997; Whipker and McCall 2000). These studies aid in the understanding of floriculture crop responses to paclobutrazol substrate drenches, yet information on using substrate drenches of paclobutrazol to control growth of black-eyed Susan is limited. Therefore, the objective of this research was to quantify the efficacy of paclobutrazol substrate drenches on growth and flowering responses of nine popular black-eyed Susan cultivars.
Materials and methods
Plant material
Between 3 and 10 Sep 2020, 72-cell plug trays (66 mL individual cell volume, 54 cm × 28 cm × 6 cm; T.O. Plastics, Inc., Clearwater, MN, USA) of ‘Autumn Colors’, ‘Cherry Brandy’, and ‘Indian Summer’ black-eyed Susan were received from a commercial propagator (Green Leaf Plants, Lancaster, PA, USA). On 15 Sep, 36-cell plug trays (42 mL individual cell volume, 54 cm × 14 cm × 4.5 cm; Landmark Plastic Corp., Akron, OH, USA) of ‘Cherokee Sunset’, ‘Denver Daisy’, and ‘Prairie Sun’ black-eyed Susan were received from a commercial propagator (Raker-Roberta’s Young Plants, LLC, Litchfield, MI, USA). On 25 Sep, 72-cell plug trays (66 mL individual cell volume, 54 cm × 28 cm × 6 cm; T.O. Plastics, Inc.) of ‘Glowing’, ‘Happy’, and ‘Sunny’ black-eyed Susan were received from a commercial propagator (Emerald Coast Growers, Pensacola, FL, USA).
Plant culture
On 28 Sep, uniform young plants of each cultivar with similar heights and node numbers were selected and transplanted one plant per 6.5-inch-diameter plastic container (2 qt individual volume, Landmark Plastic Corp.). Containers were filled with pre-moistened commercial soilless peat-based substrate composed of (by volume) ∼80% peat, ∼20% perlite, and amended with calcitic and dolomitic limestones, wetting agent, and a starter nutrient charge (LM-111; Lambert Peat Moss, Rivière-Ouelle, QC, Canada). At time of transplant, plants were irrigated to container capacity with tepid water supplemented with 93% sulfuric acid (H2SO4; Riverside Chemical Co., North Tonawanda, NY, USA) at 5.61 mL·L−1 to neutralize alkalinity from 0.79 to 0.26 meq/L bicarbonate (HCO3–) and reduce pH from 7.5 to 6.0. At each subsequent irrigation, plants were irrigated with acidified tepid water prepared as previously described and supplemented with 17N–1.7P–14.1K water-soluble fertilizer (Jack’s Professional Pure Water XL; J.R. Peters Inc., Allentown, PA, USA) containing 3.7% ammoniacal-nitrogen and 13.3% nitrate-nitrogen. Plants received the following (mg·L−1): 150 nitrogen, 15.2 phosphorous, 124.5 potassium, 26.5 calcium, 13.2 magnesium, 0.75 iron, 0.37 manganese, 0.37 zinc, 0.15 boron, 0.07 copper, and 0.07 molybdenum.
Plants were grown in a double polyethylene-covered greenhouse with exhaust fans, evaporative-pad cooling, vertical air flow fans, and natural gas heating controlled by computerized control systems (Titan 1.0; Argus Control Systems Ltd., Surrey, BC, Canada). Supplemental and day-extension lighting was provided by 660-W high-pressure sodium lamps (NXT-LP; P.L. Light Systems, Beamsville, ON, Canada) fitted with alpha reflectors (P.L. Light Systems) from 0600 to 2200 HR (16-h photoperiod) that delivered a supplemental photosynthetic photon flux density of ∼125 µmol·m−2·s−1 at plant height [as measured with a quantum sensor (LI-250A light meter; LI-COR Biosciences, Lincoln, NE, USA)]. High-pressure sodium lamps were controlled by an environmental computer (Titan 1.0, Argus Control Systems Ltd.) and turned on when the outdoor light intensity fell below ∼250 µmol·m−2·s−1 and turned off when the outdoor light intensity reached ∼500 µmol·m−2·s−1. Full-spectrum quantum sensors (SS-500; Apogee Instruments, Logan, UT, USA) mounted 30 cm above the benchtop measured photosynthetic photon flux density on each greenhouse bench. Measurements were recorded every 15 s and the average of each sensor was logged every 15 min by a data logger (CR1000; Campbell Scientific, Inc., Logan, UT, USA). Greenhouse air temperature and relative humidity (RH) setpoints were 20 °C and 70%, respectively. On each bench, an enclosed thermocouple recorded air temperature and RH every 30 s and averages were logged every 15 min by a data logger (WatchDog 2475 Plant Growth Station; Spectrum Technologies, Inc., Aurora, IL, USA). Average daily light integral, air temperature, and RH throughout the duration of the experiment were 12.6 ± 1.2 mol·m−2·d−1, 20.9 ± 0.8 °C, and 65.4% ± 2.8%, respectively.
PGR substrate drenches
On 14 Oct, 16 d after transplant, PGR drench treatments were applied. Before PGR drench treatment, saucers were placed under each container so no solution would be leached and lost. Six single-plant replicates (individual plants) of each cultivar were drenched with 5-fl-oz aliquots of solution containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (Piccolo; Fine Americas, Inc., Walnut Creek, CA, USA) (0, 0.4, 0.7, 1.5, and 3.0 mg/pot).
Data collection and calculations
Plants were monitored daily to record the date of when the first flower was fully reflexed exhibiting anthesis (visible pollen shed), which occurred from 28 Oct to 19 Dec. Time to flower (TTF) was calculated for each plant as the number of days from PGR drench treatment to anthesis. Plants that did not exhibit anthesis by 20 Dec were ended. At anthesis or termination, plant height was determined by measuring from the substrate surface to the tallest growing point. Plant diameter was determined by measuring the widest dimension and the axis perpendicular to the widest dimension and averaging. GI [GI = (plant height + plant diameter) ÷ 2] was calculated for each plant. Shoots were severed at the substrate surface, individually bagged, and dried in an oven at 70 °C. After 1 week, shoots were weighed (MBS-600; Brecknell Scales USA, Fairmont, MN, USA) to determine SDW.
Experimental design and statistical analyses
The experiment was conducted in a completely randomized design with six single-plant replicates per cultivar for each paclobutrazol concentration. For each cultivar, effects of paclobutrazol concentration were analyzed using statistical software (SAS ver. 9.4; SAS Institute, Cary, NC, USA) general linear model (PROC GLM) for analysis of variance. For plant height, plant diameter, GI, and SDW, regression analysis within cultivar with paclobutrazol concentration as the independent variable were performed using SAS regression procedure (PROC REG). Regression equations for plant height, plant diameter, GI, and SDW for each cultivar are listed in Table 1. For TTF, within cultivar, means were separated between paclobutrazol concentration using Tukey’s honestly significant differences. For all analyses, a P ≤ 0.05 was used to determine significant effects.
Regression equations (see Fig. 1) for plant height, plant diameter, growth index, and shoot dry weight of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) plants (n = 6) grown in 6.5-inch-diameter (16.51-cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant (1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz).
Results and discussion
Substrate drenches providing 0 to 20 mg·L−1 paclobutrazol significantly influenced plant height, plant diameter, GI, SDW, and TTF of all black-eyed Susan cultivars, although the magnitude of response to paclobutrazol substrate drench concentration varied with cultivar. The effect of increasing paclobutrazol substrate drench concentration on plant height (Figs. 1A–C, 2–4) differed among the nine black-eyed Susan cultivars. Differences in plant height and response to increasing paclobutrazol substrate drench concentrations was expected among the cultivars and attributed to genetic variation (Whipker and McCall 2000). Height of untreated ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ were 39.3, 64.1, 47.0, 46.0, 53.7, 32.3, 61.8, 58.3, and 53.3 cm, respectively.
Linear and quadratic regression models for average plant height (A–C), plant diameter (D–F), growth index (G–I), and shoot dry weight (J–L) of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant. Each symbol represents a mean of six individual plant samples (n = 6), and error bars represent ±SE. For each model, corresponding r2 (linear) or R2 (quadratic) values and significance at P ≤ 0.05 (*), 0.001 (**), or 0.0001 (***) are presented; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz, 1 cm = 0.3937 inch, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Linear and quadratic regression models for average plant height (A–C), plant diameter (D–F), growth index (G–I), and shoot dry weight (J–L) of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant. Each symbol represents a mean of six individual plant samples (n = 6), and error bars represent ±SE. For each model, corresponding r2 (linear) or R2 (quadratic) values and significance at P ≤ 0.05 (*), 0.001 (**), or 0.0001 (***) are presented; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz, 1 cm = 0.3937 inch, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Linear and quadratic regression models for average plant height (A–C), plant diameter (D–F), growth index (G–I), and shoot dry weight (J–L) of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant. Each symbol represents a mean of six individual plant samples (n = 6), and error bars represent ±SE. For each model, corresponding r2 (linear) or R2 (quadratic) values and significance at P ≤ 0.05 (*), 0.001 (**), or 0.0001 (***) are presented; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz, 1 cm = 0.3937 inch, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Autumn Colors’, ‘Cherokee Sunset’, and ‘Cherry Brandy’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Autumn Colors’, ‘Cherokee Sunset’, and ‘Cherry Brandy’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Autumn Colors’, ‘Cherokee Sunset’, and ‘Cherry Brandy’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Denver Daisy’, ‘Glowing’, and ‘Happy’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Denver Daisy’, ‘Glowing’, and ‘Happy’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Denver Daisy’, ‘Glowing’, and ‘Happy’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51 cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51-cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51-cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
Depiction of ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) grown in 6.5-inch-diameter (16.51-cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant; 1 mg·L−1 = 1 ppm, 1 mg = 3.5274 × 10−5 oz.
Citation: HortTechnology 33, 6; 10.21273/HORTTECH05290-23
As paclobutrazol substrate drench concentration increased, plant height decreased linearly but only for eight cultivars. Increasing paclobutrazol substrate drench concentrations from 0 to 20 mg·L−1 resulted in ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ plants that were 36% (14.2 cm), 54% (34.7 cm), 43% (20.1 cm), 50% (23.0 cm), 46% (14.8 cm), 41% (25.7 cm), 42% (24.4 cm), and 42% (22.2 cm) shorter, respectively. The observed relationship of suppressing plant height with increasing paclobutrazol substrate drench concentration is consistent among the literature (Barrios and Ruter 2019; Currey et al. 2016; Dasoju et al. 1998; Gibson and Whipker 2003; Lyons et al. 2018; Whipker and Hammer 1997; Whipker and McCall 2000). For instance, Whipker and Hammer (1997) reported total plant height of ‘Golden Emblem’ and ‘Red Pygmy’ tuberous-rooted dahlias (Dahlia variabilis) decreased linearly as paclobutrazol substrate drench concentrations increased from 0 to 108 mg·L−1 (0 to 16 mg/pot), resulting in plants that were >11% and >21% shorter, respectively.
For ‘Glowing’, a significant quadratic relationship between plant height and paclobutrazol concentration was observed, where plants drenched with 2.5 to 20 mg·L−1 paclobutrazol were 28% to 60% (15.2 to 32.2 cm) shorter than the untreated plants. The observed trend is consistent with quadratic relationships reported by Owen et al. (2016) for ‘Pacino Gold’ pot sunflower (Helianthus annuus), ‘Anemone Safari Yellow’ French marigold (Tagetes patula), and ‘Variegata’ plectranthus (Plectranthus ciliates) plant heights and increasing paclobutrazol substrate drench concentrations. For instance, Owen et al. (2016) reported plant height of ‘Anemone Safari Yellow’ French marigold to be 5% to 15% shorter when plants were drenched with 1.7 to 6.8 mg·L−1 paclobutrazol (0.25 to 1.0 mg/pot) than untreated plants and found no further height control occurred when concentrations ≥ 3.4 mg·L−1 paclobutrazol (0.50 mg/pot) were applied.
Furthermore, severe height retardation was evident for all black-eyed Susan cultivars drenched with 20 mg·L−1 paclobutrazol, which may result in nonmarketable plants (Figs. 2–4). The recommended optimal plant height for containerized crops is 1.5 to 2 times the container height (Sachs et al. 1976). Desirable marketable black-eyed Susan plants with heights ranging from 19 to 26 cm were not achieved; however, plants heights 2.5 to 3 times the container height were achieved with 2.5 to 10 mg·L−1 paclobutrazol, thus resulting in proportional plants without leaf distortion that could be sold at a premium.
Plant diameter of eight black-eyed Susan cultivars decreased linearly with increasing paclobutrazol substrate drench concentrations, resulting in smaller plants (Fig. 2D–F). For instance, increasing paclobutrazol substrate drench concentration from 0 to 20 mg·L−1 limited plant diameter of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ plants by 31% (12.8 cm), 29% (11.2 cm), 26% (11.2 cm), 21% (8.3 cm), 19% (6.8 cm), 20% (9.7 cm), 17% (7.6 cm), and 30% (14.2 cm), respectively, than untreated plants. These results are consistent with Dasoju et al. (1998), Gibson and Whipker (2003), Owen et al. (2016), Whipker and Hammer (1997), and Whipker and McCall (2000), who reported trends of increased plant diameter suppression with increasing paclobutrazol substrate drench concentration. For instance, Gibson and Whipker (2001) found plant diameter of ‘Lusaka’ osteospermum (Osteospermum ecklonis) to be suppressed by up 18% when plants were drenched with increasing from 0 to 108.2 mg·L−1 paclobutrazol (0 to 16 mg/pot). Furthermore, a quadratic relationship between ‘Glowing’ plant diameter and paclobutrazol substrate drench concentration (Fig. 2E) was observed in the current study. Plant diameter of ‘Glowing’ was 28% (15.2 cm), 40% (21.3 cm), 44% (23.7 cm), and 60% (32.2 cm) smaller than the untreated plants for 2.5, 5, 10, and 20 mg·L−1 paclobutrazol substrate drench concentrations, respectively. Similarly, plant diameter of ‘Pacino Gold’ pot sunflower was 13%, 17%, and 24% smaller than the untreated plants for 6.8, 13.5, and 27.1 mg·L−1 paclobutrazol (1, 2, and 4 mg/pot) substrate drench concentrations, respectively (Owen et al. 2016).
Growth indices for each black-eyed Susan cultivar followed a similar trend to plant height and diameter, although it is not surprising given that GI is an integrated measurement of height and diameter (Seltsam and Owen 2022). For example, GI of ‘Cherry Brandy’ decreased linearly by 43% as paclobutrazol substrate drench concentration increased from 0 to 20 mg·L−1 (Fig. 1G), resulting in smaller, compact plants. Linear relationships observed for GI and increasing paclobutrazol substrate drench concentrations in the current study are consistent with trends observed by Owen et al. (2016) for ‘Anemone Safari Yellow’ French marigold and ‘Variegata’ plectranthus. In addition, GI for ‘Glowing’ (Fig. 1H) showed a quadratic response to increasing paclobutrazol substrate drench concentrations where overall growth was 29% (12.5 cm), 43% (18.7 cm), and 31% (13.6) smaller than the untreated plants at 5, 10, and 20 mg·L−1 paclobutrazol, respectively. No further growth control occurred when concentrations ≥5 mg·L−1 paclobutrazol were drenched.
Increasing paclobutrazol substrate drench concentration significantly affected SDW of each cultivar (Fig. 1J–L), although the trends observed were like those reported for GI. As substrate drench concentration increased from 0 to 20 mg·L−1 paclobutrazol, SDW of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Happy’, ‘Indian Summer’, and ‘Prairie Sun’ decrease linearly by 59% (20.4 g), 32% (10.1 g), 36% (14.1 g), 48% (20.2 g), 38% (11.5 g), 40% (18.7 g), and 5% (1.6 g), respectively. Although seven cultivars showed linear correlations with SDW and paclobutrazol substrate drench concentration, Glowing and Sunny best fit quadratic models. SDW of ‘Glowing’ and ‘Sunny’ were smallest at 19.2 g and 30.7 g, respectively, when plants were drenched with 10 mg·L−1 paclobutrazol, being 45% (15.8 g) and 36% (17.1 g) smaller than untreated plants (Fig. 1K and L). SDW trends attained in our study complement those reported by Currey et al. (2016), who reported diminished SDW of all three lantana (Lantana camara) cultivars (Landmark Peach Sunrise, Little Lucky Peach Glow, and Lucky Peach) in response to increasing concentrations of paclobutrazol.
TTF for ‘Autumn Colors’, ‘Cherry Brandy’, ‘Happy’, ‘Indian Summer’, and ‘Prairie Sunset’ was unaffected by any paclobutrazol substrate drench concentration, and averaged 41, 51, 49, 51, and 50 d, respectively (Table 2). The delay in TTF for ‘Autumn Colors’, ‘Cherry Brandy’, ‘Happy’, ‘Indian Summer’, and ‘Prairie Sunset’ drenched with 0 to 20 mg·L−1 paclobutrazol was calculated as 0.3, 1, 3, and 2 d, respectively, which would not be commercially important. For ‘Cherokee Sunset’, ‘Denver Daisy’, ‘Glowing’, and ‘Sunny’, the effect of increasing paclobutrazol substrate drench concentrations on TTF was significant yet the response varied by cultivar (Table 2). When 20 mg·L−1 paclobutrazol was applied to ‘Cherokee Sunset’, ‘Denver Daisy’, and ‘Glowing’, flowering was delayed by 12, 13, and 9 d, respectively, compared with untreated plants. When 5 to 20 mg·L−1 paclobutrazol drenches were applied to ‘Sunny’, flowering was 6 to 12 d later, respectively, than untreated plants. The observed delay in flowering for ‘Cherokee Sunset’, ‘Denver Daisy’, ‘Glowing’, and ‘Sunny’ could possibly delay commercial production by ∼1 to 2 weeks. Delayed flowering observed in our study is consistent with flowering responses observed by Dasoju et al. (1998) and Owen et al. (2016) where increasing paclobutrazol substrate drench concentration significantly delayed anthesis of potted sunflowers. Therefore, greenhouse growers should be aware that increasing paclobutrazol substrate drench concentrations to control black-eyed Susan growth may negatively affect flowering response and delay marketability.
Time to flower of ‘Autumn Colors’, ‘Cherokee Sunset’, ‘Cherry Brandy’, ‘Denver Daisy’, ‘Glowing’, ‘Happy’, ‘Indian Summer’, ‘Prairie Sun’, and ‘Sunny’ black-eyed Susan (Rudbeckia hirta) plants (n = 6) grown in 6.5-inch-diameter (16.51-cm) plastic containers [2 qt (1.9 L)] filled with a commercial soilless peat-based substrate drenched with 5-fl-oz (147.9-mL) aliquots of solutions containing deionized water [0 mg·L−1 paclobutrazol (control)] or 2.5, 5, 10, or 20 mg·L−1 paclobutrazol (0, 0.4, 0.7, 1.5, and 3.0 mg/pot) 16 d after transplant.i
Growers have several PGRs available for controlling plant growth (Dasoju et al. 1998). The PGR selected to control growth of black-eyed Susan should be based on the response of the plant, concentration applied, and the cost of the PGR. Thus, we estimated the costs of drench solutions containing 5 to 10 mg·L−1 paclobutrazol that we identified as effective in this study using PGRCALC (Krug and Whipker 2010). Using the average price of $91.84/qt of a commercial paclobutrazol product and applying 5 fl oz per container of 5 to 10 mg·L−1 paclobutrazol would cost $17.93 to $35.86 per 1000 containers or $0.02 to $0.04 per 6.5-inch-diameter plastic container. At 5 to 10 mg·L−1 paclobutrazol, an economic advantage can be attained for commercial greenhouses in producing smaller-diameter plants that can be spaced closer (Whipker and McCall 2000), yet providing height control with minimal leaf distortion, delay in flowering, and shipping ease.
Conclusions
Paclobutrazol substrate drenches were effective in suppressing plant growth of different black-eyed Susan cultivars; however, growth control varied by cultivar and paclobutrazol drench concentration. Paclobutrazol substrate drench concentrations between 5 and 10 mg·L−1 were found to control plant growth without delaying flowering of ‘Autumn Colors’, ‘Cherry Brandy’, ‘Happy’, ‘Indian Summer’, and ‘Prairie Sunset’. Concentrations of ≤10 mg·L−1 paclobutrazol are suggested for ‘Cherokee Sunset’, ‘Denver Daisy’, ‘Glowing’, and ‘Sunny’, as higher concentrations (>10 mg·L−1 paclobutrazol) will significantly delay flowering. Finally, growers should consider performing on-site substrate drench trials to determine cultivar-specific paclobutrazol concentrations of cultivars not investigated herein or to evaluate our recommendations based on their crop culture regimens, growing environment, and market needs.
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