Efficacy and Persistence of Paclobutrazol Applied to Rooted Cuttings of Rhododendron Before Transplant

Abstract

Efficacy of paclobutrazol was determined when applied to rooted cuttings before transplant. Cuttings of large-leaf Rhododendron catawbiense Michx. were treated with paclobutrazol applied as a 40-mL drench. In 1998, concentrations of 0, 1, 2, 10, or 20 mg·L-1 were applied to liners before root development was complete in February, or after cuttings were root-bound in May. The same volume of solution was applied to other plants at concentrations of 0, 5, 10, or 20 mg·L-1 in July 1998, after transplant to 1-gal pots. In 1999, a 40-mL drench of paclobutrazol at 0, 1, 2, 5, 10, or 20 mg·L-1 was only applied to liners in April. All cuttings were transplanted to 1-gal pots and set in the field. The elongation of stems was measured after each of three flushes of growth. Plants were far more responsive to paclobutrazol when it was applied before, rather than after transplant. There was a saturating response to paclobutrazol concentration and the half-maximal response occurred at 2 to 4 mg·L-1 (0.08 to 0.16 mg/plant). At low rates, later flushes of growth were affected less than earlier flushes. However if paclobutrazol was applied at 10 or 20 mg·L-1, later flushes of growth were inhibited more completely than early flushes. Flowering was enhanced by paclobutrazol. Paclobutrazol at 2 mg·L-1 applied to rooted cuttings before transplant was sufficient to inhibit growth of rhododendron, but not to the point where later flushes of growth were excessively short. Chemical name used: 2RS,3RS-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)-pentan-3-ol (paclobutrazol).