Effects of Paclobutrazol on the Growth of Leucospermum Hybrids

Leucospermum hybrids have great potential for potted flowering plants. Our aim was to determine proper concentrations of Paclobutrazol, a gibberellin biosynthesis inhibitor, in order to produce compact Leucospermum hybrids that are better suited as potted plants. Rooted cuttings of ‘Brandi Dela Cruz’ and ‘Debbie Hamrick’ were planted in 1-gallon pots filled with potting media (50% volcanic cinder: 50% peat moss, v/v) after thoroughly mixing with 8g of a slow-release fertilizer (Osmocote 14-14-14) and grown in the greenhouse with natural light and with a regular irrigation. Paclobutrazol (PBZ) was sprayed to the foliages of the plants at a concentration of 0, 25, 50, 75, 100, 125 or 150 ppm one week after transplanting. Plant height, the length of individual branches, and the numbers of leaves and branches were measured monthly during the 5 months of production period. Plant height and branch number increased as plant aged in both hybrids. Although the initial height of the two hybrids was not significantly different, ‘Brandi Dela Cruz’ grow taller and produced twice more branches compared to ‘Debbie Hamrick’. A spray with PBZ at all the concentrations significantly reduced plant height of ‘Brandi Dela Cruz’ from 2 months after the treatment. Plant height and individual branch length were reduced by more than 30% at the highest concentration of PBZ. Meanwhile, PBZ did not have greater impact on controlling plant height and individual branch length of ‘Debbie Hamrick’, and only the highest concentration reduced plant height by less than 15%. The number of branches increased with plant age and concentration of PBZ. PBZ increased the number of branches in ‘Brandi Dela Cruz’ from the first month after treatment, while the number of branches increased from 4 months after the treatment in ‘Debbie Hamrick’. In conclusion, PBZ significantly reduced plant height in both varieties, but the efficacy varied with variety tested. It appeared that PBZ has greater effects on ‘Brandi Dela Cruz’, a fast growing variety, while it has less effects on ‘Debbie Hamrick’, a slow growing variety. The results suggest that foliar application of PBZ is useful to control the growth of Leucospermum hybrids for potted plant production. Higher concentrations or multiple applications should be considered for a slow growing variety to more effectively control plant growth.