Paclobutrazol Substrate Drenches Control Growth of Nine Black-eyed Susan Cultivars

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.