Effects of Growing Season, Application Timing, and Substrate Drench Applications of Paclobutrazol as Compared to Daminozide Standards on Growth and Flowering of Gerbera Daisy ‘Bright Red with Light Eye’
Type of DegreeMaster's Thesis
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Gerbera daisy (Gerbera jamesonii Hook. f.) is a popular pot plant with showy flowers, commonly marketing in spring. However, plants grown under greenhouse conditions can stretch or grow too large for containers. Plant growth retardants (PGRs) can be used to control plant growth and flowering, enhance plant appearance and uniformity, and minimize transport stress. Two studies were conducted to determine the optimal substrate drench concentration of paclobutrazol (Bonzi) as compared to daminozide (B-Nine) foliar spray standards in different seasons and at different application stages for plant size control and improved market quality of gerbera daisy ‘Bright Red with Light Eye’. In the first study, plants in 12.7cm (5 in) pots were treated with 0, 1, 2, 3, or 4 ppm paclobutrazol substrate drenches or a 2500 ppm daminozide foliar spray at two application stages for a fall 2015 or spring 2016 finish. When the majority of pots had roots at the bottom of the substrate, plants were treated with PGRs as stage 1, and stage 2 was applied 2 weeks later. A consumer preference survey was conducted to assess consumer-purchasing preference based on quality ratings assigned when plants had 2 or 3 open flowers. Plant size index, foliage height, peduncle length, flower diameter, and quality rating decreased linearly while days to first flower increased quadratically with increasing paclobutrazol concentration, regardless of application stage or season. Foliage height, plant size index, and peduncle length for fall were greater than for spring. Days to first flower, flower and bud counts, and quality ratings were greater for the spring than fall, but these differences were small and not likely of horticultural significance. Likewise, differences between stages were only found for foliage height and these were small. Plants that received 0 or 1 ppm paclobutrazol were often larger and those that received 4 ppm paclobutrazol were often smaller than those that received daminozide. Based on quality rating and survey results, 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. The standard daminozide concentration produced plants too small for the highest quality rating or consumer preference. In the second study conducted in summer 2016, plants in 12.7cm (5 in) pots were treated with 0, 1.5, or 3 ppm paclobutrazol substrate drenches or a 2500 ppm daminozide foliar spray at weekly intervals from 2 to 6 weeks after transplanting (WAT). A daminozide standard was applied at 2500 ppm 2 WAT and again 10 days later. Foliage height, plant size index, flower diameter, peduncle length, and quality rating decreased while days to first open flower (DTF) increased linearly or quadratically with increasing paclobutrazol concentration regardless of application timing. Foliage height, size index, flower diameter, peduncle length, and quality rating of plants receiving daminozide weekly were smaller than or not different from 0 ppm paclobutrazol, but larger than those receiving 1.5 or 3 ppm. Peduncle length and DTF decreased linearly with increasing WAT regardless of growth retardant treatment. Foliage height, size index, and peduncle length of plants receiving daminozide at 2 weeks only were larger than those receiving 1.5 or 3 ppm paclobutrazol, but not different from those receiving 0 or 1.5 ppm. Flower diameter and DTF of plants receiving daminozide at 2 weeks were not different from paclobutrazol. Quality rating was higher for plants receiving daminozide at 2 weeks than those receiving 1.5 or 3 ppm paclobutrazol. Differences in paclobutrazol treatments and daminozide applied twice were similar to those of daminozide applied at 2 weeks, and there were no differences in daminozide applied at 2 weeks and daminozide applied twice. Treatments did not affect time to flower senescence. Based on quality ratings, no growth retardant or daminozide applied once at 2500 ppm anytime from 2 to 6 WAT produced the most marketable plants. However, later PGR application resulted in earlier flowering and greater reductions in peduncle length. Across the two studies, increasing concentrations of paclobutrazol drenches reduced plant size and had a negligible negative impact on DTF or flower diameter. However, PGR treatments that resulted in the highest quality ratings and consumer preference were different depending on season. Plant quality was improved by paclobutrazol drenches at the lower concentrations for a fall finish and to a lesser extent for a spring finish, though the effective concentration range included 0 ppm. However, paclobutrazol drenches did not improve plant quality in the summer. In the first study, PGR treatments were applied in early October for a fall finish and in early March for a spring finish while in the second study, treatment were applied in April and May. Warmer temperatures and higher light intensities may have resulted in more compact plants in the summer than in the spring and fall, thus reducing the effectiveness PGR treatments on plant quality in the summer.