Influence of Paclobutrazol on Growth and Flowering of Rhododendron `Sir Robert Peel'

Shoot extension of Rhododendron `Sir Robert Peel' was reduced, but flowering was more precocious after treatment of plants with paclobutrazol. All of the drench rates tested (0.05 to 0.4 g a.i./pot) proved excessive and resulted in prolonged shoot growth reduction but greatly increased the number of flowers. However, flowers produced at the higher drench rates were grossly malformed and unacceptable. Foliar spray treatments (250 to 2000 mg a.i./liter) were less inhibitory than drenches, and a single spray of 500 mg·liter-1 was considered an appropriate commercial application rate. Paclobutrazol application could thus prove to be a useful technique in producing compact flowering plants of Rhododendron hybrids for improved retail sales. Chemical name used: β-[(4∼chlorophenyl) methyl] (1,1-dimethylethl)-1H-l,2,4-triazole-l-ethano1 (paclobutrazol, ICI-PP333).