Use of Paclobutrazol to Regulate Shoot Growth and Flower Development of ‘Roseum Elegans’ Rhododendron

Abstract

‘Roseum Elegans’ rhododendron (Rhododendron sp. L.) were grown in 2.5 1 (#1) containers and treated with foliar sprays of 50, 100, and 200 ppm or root-zone drenches of 2.5 (0.89), 5.0 (1.78), 10.0 (3.55), and 20.0 (7.10) ppm (mg ai/plant) of paclobutrazol immediately following completion of the first flush of annual shoot growth. Shoot lengths of the subsequent growth flush following treatment were decreased with increasing rates of paclobutrazol when applied as either a drench or foliar spray. Drenches were more effective in suppressing shoot length with less active ingredient than were foliar sprays. The number of flower buds per plant increased with increasing rates of paclobutrazol when applied as a drench but not as a foliar spray. The highest drench rate resulted in 8.0 flower buds/plant, a 240% increase over non-treated plants. No phytotoxicity was observed from any of the treatments; however, inflorescence diameter, measured the year following treatment, was slightly (<1.1 cm, <7.5%) reduced with increasing rates of paclobutrazol when applied as a drench. Flowering duration, the period from bud break to abscision of the last flower, increased with increasing rates of paclobutrazol for both application methods with a maximum increase of 5.6 days for the highest drench treatment. Length of shoot growth the year following treatment was reduced by as much as 33% with increasing paclobutrazol rates when applied as a drench but not as a foliar spray.