Effects of Growing Season, Plant Stage of Development, and Substrate Drench Applications of Paclobutrazol as Compared to a Daminozide Standard on Growth and Flowering of Gerbera Daisy ‘Bright Red with Light Eye'1
Paclobutrazol substrate drenches at 0, 1, 2, 3, or 4 ppm were compared to a 2,500 ppm daminozide foliar spray standard in two finishing seasons (fall 2015 and spring 2016) and at two application stages for plant size control and improved market quality of ‘Bright Red with Light Eye' Gerbera daisy (Gerbera jamesonii Hook. f. ‘Bright Red with Light Eye') in 12.7cm (5 in) pots. Stage 1 was when the majority of plants had roots to the bottom of the substrate, and stage 2 was 2 weeks later. A consumer preference survey assessed purchasing preference when plants had two or three open flowers. Plant size index, foliage height, peduncle length, flower diameter, and quality rating at first flower decreased linearly while days to first flower increased quadratically with increasing paclobutrazol concentration, regardless of stage or season. Paclobutrazol applications delayed flowering, and decreased flower diameter and flower and bud counts, but these differences were small and not considered of practical importance. Based on quality rating at first flower and survey results, a 1 or 2 ppm paclobutrazol drench produced the most marketable plants in fall, and 0, 1, 2, or 3 ppm produced the most marketable plants in spring.
Index words: plant growth retardant, B-Nine, Bonzi, greenhouse production
Chemicals used in this study: B-Nine (daminozide) [butanedioic acid mono (2,2- dimethylhydrazide)] and Bonzi (paclobutrazol) [(±)-(R*,R*)-β-[(4-Chlorophenyl)methyl]-α-(1,1- dimethylethyl)-lH-1,2,4-triazole-1-ethanol]
Species used in this study: ‘Bright Red with Light Eye' Gerbera daisy (Gerbera jamesonii Hook. f. ‘Bright Red with Light Eye')Abstract
Significance to the Horticulture Industry
Gerbera daisy is a popular flowering pot plant commonly marketed in spring. However, plants grown under greenhouse conditions can grow too large for containers. Daminozide foliar spray is an industry standard practice for growth regulation in gerbera daisy production. Paclobutrazol was reported to be more effective and longer lasting than daminozide for many greenhouse crops, and may be more effective applied as a substrate drench than a foliar spray. In this study, ‘Bright Red with Light Eye' gerbera daisy grown in 12.7 cm (5 in) pots were treated with 0, 1, 2, 3, or 4 ppm paclobutrazol substrate drenches or a 2,500 ppm daminozide foliar spray at two application stages for fall and spring finishing. Increasing paclobutrazol drench rates effectively controlled plant growth by decreasing foliage height, plant size index, and peduncle length, but delayed flowering, and decreased flower diameter and flower and bud counts. However, the deleterious effects of paclobutrazol were small and not considered of practical importance. Plant were larger in fall than spring. A 1 or 2 ppm paclobutrazol drench resulted in the highest consumer preference of gerbera daisy in fall, while in spring, 0, 1, 2, or 3 ppm paclobutrazol drench resulted in the same consumer preference. The industry standard practice of 2,500 ppm daminozide rate was too high for high consumer preference in this cultivar. Gerbera daisies were of excellent quality with a fall finishing and has potential in the floral market for the holiday season.
Introduction
Gerbera daisy is a versatile crop used as a bedding plant, cut flower, or flowering pot plant. Grown as a pot crop, they are commonly marketed throughout the spring, summer, and into the fall (Hamrick 2003). The wide variety of colors and daisy flowers grown in a range of pot sizes are valued by consumers (Briggs-Macha 2015). In greenhouse production, foliage too large or flower stems too long for the container size are common problems for gerbera daisies. Plant growth retardants (PGR) can be applied to reduce plant size and flower stem length.
Daminozide (B-Nine) is an older, but widely used PGR in the greenhouse industry. It has a relatively short activity and seldom results in excess growth suppression. Daminozide was recommended for gerbera daisy as a foliar spray at 1,000 to 2,500 ppm, once or twice at a 9 to 10 day interval with the first application made 12 to 14 days after transplanting (Hamrick 2003).
Armitage et al. (1984) applied daminozide at 4,000 ppm to gerbera daisy as a foliar spray weekly at 5 to 8 weeks after transplanting, and again 10 days after each initial application. Plants treated at 6 weeks reached visible buds earlier than the control. Daminozide more effectively controlled peduncle length, total height, and vegetative growth when applied at 8 weeks as compared to earlier applications. Leaf number and flower diameter were not affected by PGR treatments.
Paclobutrazol (Bonzi) is a more active PGR compared with non-triazole PGRs. Less paclobutrazol applied as a foliar spray was required on an active ingredient per liter basis for effective control of ‘Bright Golden Anne' chrysanthemum (Chrysanthemum morifolium Ramat.) stem extension than ancymidol (A-Rest/Reducymol), daminozide, or piperonyl bromide (Alden/Stemtrol). Likewise, less paclobutrazol applied as a substrate drench on an active ingredient per pot basis was required for effective control of stem extension than chlorphonium chloride (Phosfon/Phosfleur), but ancymidol drenches required about equal concentrations (Menhenett 1984).
Gerbera daisies planted in 30 cm (12 in) diam clay pots received paclobutrazol sprays at 0, 25, 50 or 100 ppm (Bekheta et al. 2008). All paclobutrazol concentrations suppressed plant height and increased leaf dry weight. Peduncle (authors reported pedicle) lengths decreased and peduncle diameters increased, with increasing paclobutrazol concentrations. All paclobutrazol applications increased flower numbers per plant and flower water content.
Similar results were observed in other plants. Oriental knight's-spur (Consolida orientalis Schrödinger) was treated with a paclobutrazol foliar spray at 0, 125, 250 or 500 ppm when 5% of plants had elongated the first internodes (Mansuroglu et al. 2009). All paclobutrazol applications reduced plant height, internode length of main and secondary inflorescences, pedicel length, and number of secondary inflorescences. Paclobutrazol spray at 250 ppm increased flower number on the main inflorescence and 500 ppm increased stem diameter and flower number on secondary inflorescences. However, paclobutrazol had no effect on time from sowing to flower.
Paclobutrazol can be used as a substrate drench or foliar spray. Plants of scented bouvardia (Bouvardia humboldtii Hend. & Andr. Hend.) received paclobutrazol foliar sprays or substrate drenches (Wilkinson and Richards 1987). The spray treatments were three applications at 250 ppm, two applications at 500 ppm, or one application at 1,000 ppm. Drench concentrations were 0 to 4 mg a.i. per pot. All paclobutrazol treatments reduced plant height and dry weight and increased flower numbers. Spray treatments at 250 and 500 ppm or a drench treatment at 2 mg a.i. per pot were the most effective, and flower numbers were about 35% greater than in control plants
Paclobutrazol soil drenches appear more active than foliar sprays in controlling plant growth. McDaniel (1983) treated chrysanthemums in 15 cm (6 in) pots with paclobutrazol foliar spray at 0 to 5 mg a.i. per plant, or drenches at 0 to 0.75 mg a.i. per plant under both summer and winter conditions. Results from two seasons were similar. Paclobutrazol applications excessively suppressed growth of chrysanthemum cultivars. Compared to foliar sprays, substrate drenches inhibited height extension to a greater extent. Paclobutrazol applications did not reduce flower numbers or delay time to flower.
The effect of PGRs can vary by season. ‘Mondriaan' pot carnation (Dianthus caryophyllus L.) received paclobutrazol drenches at 0.45 to 1.12 mg a.i. per pot or foliar sprays at 0.16 to 0.65 mg a.i. per pot in spring. In winter, paclobutrazol was applied as drenches at 0.125 to 0.45 mg a.i. per pot (Bañón et al. 2002). In general, paclobutrazol applications controlled plant growth and improved commercial quality. In spring, drench applications were more effective than sprays in reducing plant growth. Spring-grown plants produced more flower buds than winter-grown plants regardless of treatments. In winter, a drench of 0.25 mg produced better quality plants than the control. All spray and drench concentrations greater than 0.45 mg per pot darkened leaf color slightly. Days from potting to three open flowers was greater in winter-grown than in spring-grown paclobutrazol-treated plants, reportedly for climatological reasons.
Geranium (Pelargonium ×hortorum Bailey) was treated with paclobutrazol drenches at 0, 10 or 20 ppm 25 days after pinching (Singh et al. 2016). Plant height and spread were reduced and the number of shoots per plant increased with increasing concentrations. Days to flower bud formation and first flower were delayed by paclobutrazol. Paclobutrazol drenches decreased inflorescence diameter and increased number of inflorescences per plant. Palobutrazol application at 10 ppm maximized pot presentability. The number of inflorescences per plant open at a time increased, but the duration of flowering decreased with increasing concentrations.
Ecker et al. (1992) applied paclobutrazol substrate drenches at 0 to 2.5 mg a.i. per pot to ‘Midget- Red' stock (Matthiola incana (L.) W.T. Aiton), an early flowering and short cultivar, and ‘Lavender', a late flowering and medium-height cultivar. Both cultivars were treated 20 days after potting when flower buds were visible on ‘Midget-Red' but not ‘Lavender'. Paclobutrazol drenches delayed flowering in ‘Lavender', but not in ‘Midget-Red'. Inflorescence length of both cultivars was reduced by paclobutrazol treatments.
In some species, time to flower either increased or was not affected by paclobutrazol applications. Paclobutrazol drenches at 0 to 50 mg a.i. per pot and foliar sprays at 0 to 1000 ppm were applied to great bougainvillea (Bougainvillea spectabilis Willd.) plants grown in 18 cm (7.1 in) pots under long or short natural photoperiods (Karagüzel 1999). In all treatments, time from application to flower decreased slightly under long photoperiods, but not under short photoperiods. Numbers of flowers per plant and shoot length decreased at even the lowest concentration, while shoot number increased under both photoperiods. Floral initiation was also stimulated by paclobutrazol application on Episcia cupreata Hanst. (Stamps and Henny 1986) and Hydrangea macrophylla (Thunb.) Ser. (Bailey et al. 1986).
Paclobutrazol was recommended at 2.5 to 5 ppm as a foliar spray or at 0.25 to 0.5 ppm as a substrate drench for gerbera daisy, but no further details about application were given (GoldSmith Seed 2016). Application as a drench or spray enhanced plant appearance and uniformity, tolerance to drought and transport stress, and resulted in less breakage during handling (Rogers and Tjia 1990).
Plants also responded differently to different PGR application timing. Gilbertz (1992) applied paclobutrazol sprays at 30 to 60 ppm to chrysanthemum (Dendranthema ×grandiflorum (Ramat.) Kitam.) plants at 0, 2 or 4 weeks after pinching. The earlier applications resulted in shorter plants, but the earlier an application was applied, the longer the plants took to flower. Flower size was minimally affected by treatments.
Caladium (Caladium ×hortulanum Birdsey) ‘Aaron', ‘White Christmas', and ‘Carolyn Wharton' received paclobutrazol drench treatments at 2 mg a.i. per pot (Barrett et al. 1995). Treatment did not affect height of caladium ‘Aaron' or ‘White Christmas' when applied 1 week after planting, but did result in shorter plants when applied 3 weeks after planting.
Pot plant gerberas are commonly marketed throughout the spring, summer, and into the fall (Hamrick, 2003). However, sowing seeds in summer for finished plants in winter could make gerbera daisy an alternative holiday crop. The objective of this experiment was to determine the effects of paclobutrazol substrate drenches compared with a daminozide foliar spray standard applied at two application stages on growth, flowering, and marketable quality rating of ‘Bright Red with Light Eye' gerbera daisy for a fall and spring finish time.
Material and Methods
Seeds of ‘Bright Red with Light Eye' gerbera daisy (Mega series, Pan American Seed, West Chicago, IL) were sown on Aug. 11, 2015 for the fall season and Dec. 17, 2015 for the spring season, into 128 square cells [5 cm (2 in) tall, 24 cm3 (1.5 in3) volume] plug flats (T.O. Plastics, Clearwater, MN) containing germinating substrate (Jolly Gardener Pro-Line Custom Germinating Mix, Oldcastle Lawn & Garden, Inc., Atlanta, GA). Seeds were covered lightly with a coarse-grade vermiculite. Sown plug flats were placed in an unlit germination chamber (GC12, Phytotronics, Inc., Earth City, MO) with a 24 C (76 F) set point temperature. On Aug. 17, 2015 in the fall trial and Dec. 24, 2015 in the spring trial when at least 90% of the hypocotyls had emerged, the plug flats were moved into a greenhouse on raised benches under intermittent mist set to run for 15 s every 45 min from 6:00 AM to 4:30 PM. One day later, the plug flats were removed from the intermittent mist, but remained in the same greenhouse. The greenhouse was covered in two layers of polyethylene and an outside layer of white 40% shade cloth. The heat was set at 18 C (65 F) and ventilation began at 28 C (82 F). On Aug. 20, 2015 in the fall study and Dec. 27, 2015 in the spring study, the plug flats were moved into an unshaded, 8 mm (0.3 in) twin-wall polycarbonate covered greenhouse with a heating set point of 18 C (65 F) and ventilation began at 26 C (78 F). The plug flats were placed on raised benches and underlain with black felt fabric over black plastic to retain water and irrigated by hand. Seedlings were watered when the substrate appeared dry, but before they wilted. Seedlings were fertilized once per week with a 20N–4.4P–16.5K (Plant Marvel Nutriculture 20–10–20 Plus, Plant Marvel Laboratories, Inc., Chicago Heights, IL) at 200 ppm N using liquid fertilization. Seedlings were transplanted into 12.7 cm (5 in) [9.2 cm (3.6 in) deep, 800 cm3 (48.4 in3) volume] round plastic pots (Dillen Brand, Myers Industries Lawn & Garden Group, Middlefield, OH) containing substrate (Fafard 3B Mix, Sun Gro Horticulture, Agawam, MA) on Sept. 14, 2015 in the fall and on Feb. 4, 2016 in the spring. Plants were placed on a bench pot-to-pot. Plants were fertilized on constant liquid fertilization with one clear watering per week using the same fertilizer as used on the plug flats beginning when roots were visible at the bottom of the majority of the pots. Pesticide (Bayer Advanced Insect, Disease & Mite Control, Bayer CropScience, Research Triangle Park, NC) was applied at the manufacturer recommended rate every 1 to 2 weeks after potting for insect and disease control.
Treatments consisted of two replications of season as two simultaneous experimental runs in the spring and fall. PGR treatments were applied at two application stages in each season. Stage 1 was when the majority of pots had roots at the bottom of the pots, which occurred on Oct. 9, 2015 in the fall study and March 10, 2016 in the spring study. Stage 2 was 2 weeks later on Oct. 23, 2015 in the fall and Mar. 24, 2016 in the spring. Six PGR treatments applied at each stage in each season were paclobutrazol substrate drenches at 0, 1, 2, 3, or 4 ppm (0, 0.09, 0.19, 0.28, or 0.38 mg a.i. per pot) and a daminozide foliar spray at 2,500 ppm. Paclobutrazol drenches were applied at 88.7 ml (3 oz) per pot (Syngenta Crop Protection 2015). Daminozide sprays were applied at 0.2 L·m–2 (equivalent to 2 qt·100 ft–2) using a CO2 sprayer with a flat fan spray nozzle (TeeJet 8003VS, Bellspray, Inc., Opelousas, LA) at 310 kPa (45 psi). Temperature and relative humidity in the fall were 28 C (82 F) and 68.7% RH at stage 1 and 29 C (85 F) and 53.2% RH at stage 2, and in the spring were 24 C (76 F) and 91.1% RH at stage 1 and 12 C (53 F) and 25.8% RH at stage 2. There were nine single-pot replications per treatment combination in the fall and eight single-pot replications in the spring. The experimental design was a split-split-plot with seasons in the main plot, application stages in the sub-plot, and PGR treatments in the sub-sub-plot.
Initial data were recorded on each plant at each stage and season to document plant sizes at the time PGRs were applied. Leaves longer than 3 cm (1.2 in) were counted, and plant height, widest width and perpendicular width were measured to calculate size index (SI = (height + widest width + perpendicular width) /3). Foliage height was from the substrate surface to the top of the foliage. Pots were spaced 23 cm (9 in) on center on Oct. 24, 2015 in the fall and March 22, 2016 in the spring.
The dates of the first fully opened flower (inflorescence) were recorded in fall and spring; first flower was when ray flowers on the first inflorescence were fully reflexed perpendicular to the peduncle. At first flower, flower and flower bud counts, foliage SI, flower diameter, and peduncle length were determined. Peduncle length was measured from the peduncle point of origin on the crown to the base of the calyx. A quality rating was assigned when 2or 3 flowers were open on each pot. The quality ratings were: 1) lower foliage extending 20 to 30% beyond the pot rim, with tightly spaced, distorted leaves, flowers opened just above foliage on very short peduncles; 2) lower foliage extending 30 to 50% beyond the pot rim, leaves less tightly spaced and less distorted than in 1, flowers opened on peduncles that appeared twice as long as in 1; 3) lower foliage extending 60 to 70% beyond the pot rim, leaf spacing appeared less tightly spaced than 2, but not as open as 4 with no foliar distortion, flowers opened on peduncles that appeared four times as long as in 1; and 4) plants appeared open and spindly, lower foliage extending 70 to 80% beyond the pot rim, leaf spacing open with substrate visible, no leaf distortion, flowers opened on peduncles that appeared six times as long as in 1.
To assess consumer preference, a survey was conducted in the Department of Horticulture at Auburn University on Nov. 19–20, 2015 in the fall and on Apr. 18–19, 2016 in the spring. Four plants were selected for the survey fitting the four quality rating criteria with 2 or 3 open flowers. Faculty, staff, and undergraduate and graduate students were contacted by e- mail and invited to “select the plant you would be willing to purchase from a garden center” on a survey form (survey rating 1–4). Participants also indicated whether they were faculty, staff, undergraduate or graduate students.
An analysis of variance was performed on all responses using PROC GLIMMIX in SAS version 9.4 (SAS Institute, Cary, NC). Initial plant size responses were analyzed as a split plot with seasons in the main plot and application stages in the sub-plot. Responses recorded at first flower were analyzed as a split-split-plot with seasons in the main plot, application stages in the sub-plot, and PGR treatments in the sub-sub-plot. Where residual plots and a significant covariance test for homogeneity indicated heterogeneous variance among treatments, a RANDOM statement with the GROUP option was used to correct heterogeneity. Least squares means comparisons between seasons and application stages were tested using F-tests. Least squares means comparisons of the daminozide standard to the paclobutrazol concentrations were tested using Dunnett's method. Linear and quadratic trends over paclobutrazol concentrations were tested using orthogonal polynomials. Quality ratings were analyzed using the multinomial probability distribution and differences in seasons were estimated using the simulated method. Medians are presented for the treatments because response means are not allowed when using the multinomial distribution. Medians are the midpoint value of a frequency distribution of the quality ratings for a treatment.
Survey results were analyzed as a completely randomized design with season and departmental position in a 2-way factorial using the multinomial probability distribution. Counts within each survey rating level for the survey were analyzed using the negative binomial probability distribution and differences among levels were determined using the simulated method. All significances were at α = 0.05 unless otherwise indicated.
Results and Discussion
Application stage was significant, but not season, for initial leaf counts, foliage height, and SI at the time of PGR treatment application (Table 1). Plants had more leaves, higher foliage height, and greater SI at stage 2 than stage 1.

The season by PGR, application stage by PGR, and season by application stage interactions were significant for foliage height. Foliage height decreased linearly by 27.9% in fall, but only 13.6% in spring with increasing paclobutrazol concentrations (Table 2). Foliage height in fall was 72% to 107% higher than in spring across all paclobutrazol concentrations. Foliage height in fall plants were higher when treated 1 ppm paclobutrazol or not treated than plants treated with daminozide, but lower when treated with 4 ppm paclobutrazol. There were no differences between the paclobutrazol and daminozide treatments in spring.

Foliage height decreased linearly by 20.7% and 25.5% in stage 1 and 2, respectively, over both seasons, with increasing paclobutrazol concentrations (Table 2). Foliage height in stage 2 was 12.3% to 21.5% higher than in stage 1 across all paclobutrazol concentrations, though only significantly different at 0 and 2 ppm paclobutrazol. In stage 1, foliage height of plants treated with 4 ppm paclobutrazol were lower than plants treated with daminozide while in stage 2, plants not treated with paclobutrazol had higher foliage heights than plants treated with daminozide. In fall, stage 2 plants were 10.8% taller than in stage 1, but similar in spring. Foliage height of plants in stage 1 was higher in fall than in spring by 76.2% and by 101.6% in stage 2. Differences in for plant height between the two stages were ambiguous and suggests further research is needed to determine the best timing for PGR application. In fall, stage 2 plants were 10.8% taller than in stage 1, but similar in spring suggesting that PGR application were more effective early in the fall, but made no difference in the spring. Foliage height of plants in stage 1 was higher in fall than in spring by 76.2% and by 101.6% in stage 2. Foliar height was generally higher in the fall than in the spring.
The season by PGR interaction was significant for SI (Table 2). Size index decreased linearly by 16.3% in fall, but only by 4.7% in spring with increasing paclobutrazol concentrations. Size index was larger in fall than in spring across all paclobutrazol concentrations by 18.5% to 35.3%. In fall, SI of plants not treated with paclobutrazol were were larger than daminozide treated plants, but smaller than daminozide-treated plants when treated with 4 ppm paclobutrazol. In spring, SI of plants treated at 3 and 4 ppm paclobutrazol were 9.8% and 8.9%, respectively, smaller than in daminozide-treated plants.
Paclobutrazol treatments were effective in reducing foliage height and SI of ‘Bright Red with Light Eye' gerbera daisy. A similar results of paclobutrazol on plant height of gerbera daisy was previously reported (Bekheta et al. 2008), and in other ornamental plants, like ‘Bright Golden Anne' chrysanthemum (Menhenett, 1984), ‘Mondriaan ‘pot carnation (Bañón et al. 2002), and oriental knight's-spur (Mansuroglu et al. 2009).
Season resulted in differences in plant size. This may have resulted from the warmer and brighter conditions during seedling growth in fall than in spring. However, in ‘Mondriaan' pot carnation, there was no difference in plant size between spring-grown and winter-grown plants (Bañón et al. 2002), and in chrysanthemum (McDaniel 1983), plants treated under both summer and winter conditions were similar.
Application stage only affected foliage height. This disagreed with findings in gerbera daisy (Armitage 1984), caladium (Barrett et al. 1995) and chrysanthemum (Gilbertz 1992), where application stage caused differences in plant responses other than foliage height.
The season by PGR interaction was significant for quality rating (Table 2). Quality rating decreased linearly with increasing paclobutrazol concentrations in both seasons. Plants treated with 1 ppm paclobutrazol or not treated had higher quality ratings in fall than in spring. Plants had higher quality ratings in fall when treated 1 ppm paclobutrazol or not treated than plants treated with daminozide. In spring, plants not treated with paclobutrazol had higher quality ratings than daminozide-treated plants. Application of the appropriate concentration of paclobutrazol drench improved ‘Bright Red with Light Eye' gerbera daisy quality ratings. A similar result was found in ‘Mondriaan' pot carnation (Bañón et al. 2002); paclobutrazol controlled plant growth and improved commercial quality.
In the consumer preference survey, no differences were found between seasons or among departmental positions (data not shown). Among the four survey levels, plants assigned level 3 had a higher count than the other three levels, comprising 63.3% of total participants (Table 2). There were no differences in the remaining survey levels. Plants rated level 3 were most often chosen as the “plant they would likely purchase from a garden center” by consumers. In fall, plants treated with palobutrazol drenches at 1 and 2 ppm received a quality rating of 3, but in spring, plants treated with paclobutrazol at 1, 2, and 3 ppm or not treated received a rating of 3. Based on quality ratings, the 2,500 ppm daminozide was likely too high for high consumer preference as were the highest concentrations of paclobutrazol.
The season and PGR main effects were significant for days to first open flower (DTF) from transplant (Table 3). With increasing paclobutrazol concentration, DTF changed quadratically by 2 days. Spring-grown plants flowered 1 day later than fall-grown plants. There were no differences in DTF between plants treated with paclobutrazol and daminozide. In ‘Bright Red with Light Eye' gerbera daisy, DTF was delayed 2 days by the highest concentration of paclobutrazol, but this small difference may not have practical importance. This delay agrees with results for ‘Mondriaan' pot carnation (Bañón et al. 2002), stock ‘Lavender' (Ecker et al. 1992), and geranium (Singh et al. 2016). However, paclobutrazol application did not delay flowering of chrysanthemum (McDaniel 1983) or oriental knight's-spur (Mansuroglu et al. 2009).

The season and PGR main effects were significant for and peduncle length (Table 3). Increasing paclobutrazol concentrations decreased peduncle lengths linearly by 14.4%. Plants treated with 1 ppm paclobutrazol drench or not treated had longer peduncles than those treated with daminozide. Peduncles were 27.4% longer in fall than in spring. These results agree with reports that PGRs decreased peduncle length in gerbera daisy (Armitage 1984, Bekheta et al. 2008), panicle length in butterfly-bush ‘Dubonnet' (Ruter 1992), pedicel length in oriental knight's-spur (Mansuroglu et al. 2009) and inflorescence length of stock ‘Midget- Red' and ‘Lavender' (Ecker et al. 1992).
The PGR main effect was significant for flower diameter. Increasing paclobutrazol concentrations decreased flower diameter linearly by 4.3%, but this is probably not of practical importance. There were no differences between paclobutrazol- and daminozide-treated plants. The season main effect was significant for flower and bud counts. Spring-grown plants had one more flower and bud than fall-grown plants, but this is probably not of practical importance. However, in ‘Mondriaan' pot carnation (Bañón et al. 2002) and butterfly-bush ‘Dubonnet' (Ruter 1992), flower numbers decreased with increasing paclobutrazol concentrations while in gerbera daisy (Bekheta et al. 2008), oriental knight's-spur (Mansuroglu et al. 2009), geranium (Singh et al. 2016) and scented bouvardia (Wilkinson and Richards 1987), paclobutrazol increased flower numbers.
Increasing rates of a paclobutrazol drench treatment effectively controlled plant growth by decreasing foliage height, plant size index, and peduncle length. PGR applications also delayed flowering, and decreased flower diameter and flower and bud counts, but these differences were small and not considered of practical importance. Plant were larger in the fall than the spring, but there were only small differences in foliage height between stages. We suggest a 1 or 2 ppm paclobutrazol substrate drench in 12.7 cm (5 in) pots to improve the commercial quality of ‘Bright Red with Light Eye' gerbera daisy for a fall finish. Hamrick (2003) recommended 1,000 to 2,500 ppm daminozide at 12 to 14 days after transplanting.
Therefore, a lower concentration within this range might provide a higher consumer preference for this cultivar. Gerbera daisies were of excellent quality with a fall finish and has potential in the floral market for the holiday season. Additional research is needed to identity an optimal application time after transplant for drench application of paclobutrazol to get the best control of plant growth and at the lowest concentration from an economic standpoint.
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