The increasing diversity of butterfly bush (Buddleia × hybrida) cultivars presents new challenges for growers, particularly in adjusting plant growth retardants (PGRs) to manage plant size effectively. Therefore, the objective of this study was to evaluate uniconazole substrate drench concentrations for growth control of two popular cultivars of butterfly bush (Buddleia × hybrida). Liners of ‘Grand Cascade’ and ‘Prince Charming’ butterfly bush were individually transplanted into containers (16.5-cm; 1.7 L) filled with a commercially formulated bark-based substrate. After 10 d, eight single-plant replicates received a substrate drench of 266-mL aliquots of solution containing deionized water (0 mg·L–1; untreated) or 0.25, 0.5, 1, 2, or 4 mg·L–1 uniconazole. Plants were grown in a glass-glazed greenhouse at 20 °C under ambient daylight supplemented with a photosynthetic photo flux density of ≈125 µmol·m–2·s–1 delivered from light-emitting diode arrays from 0600 to 2200 HR (16-h photoperiod) to achieve a daily light integral of 14 mol·m–2·d–1. At 5 weeks after drench, data were collected. Plant height, plant diameter, growth index (GI), and shoot dry weights were unaffected by cultivar or cultivar × uniconazole concentration interaction but varied by uniconazole concentration (P < 0.0001); therefore, all data were pooled and analyzed by uniconazole concentration. Increasing uniconazole substrate drench concentrations effectively controlled ‘Grand Cascade’ and ‘Prince Charming’ butterfly bush plant height, plant diameter, GI, and shoot dry weights. Plant height and diameter were 16% to 32% (6.9 to 13.8 cm) shorter and 10% to 24% (6.9 to 16.9 cm) smaller than untreated plants as uniconazole drench concentration increased from 1 to 4 mg·L–1, respectively. Shoot dry mass was 24% to 34% (5 to 7 g) lower than untreated plants as uniconazole drench concentration increased from 1 to 4 mg·L–1, respectively. Overall, these results indicate that 1 to 4 1 to 4 mg·L–1 uniconazole applied as a substrate drench may be used to control growth of ‘Grand Cascade’ and ‘Prince Charming’ butterfly bush. Time to visible bud and flower was not negatively influenced by increasing uniconazole concentrations; however, growers should trial drench concentration and adjust as needed for desired market dates. Additionally, further investigations with uniconazole are warranted for other butterfly bush cultivar introductions because it is the is the preferred PGR for perennial growth control.
