Influence of Paclobutrazol on Growth and Flowering of Rhododendron ‘Sir

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-l 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: ß-[(4~chlorophenyl) methyl] (1,1-dimethylethl)-1H-l,2,4-triazole-l-ethano1 (paclobutrazol, ICI-PP333). Many Rhododendron hybrids have large terminal clusters of flowers (Bailey, 1942) and, in addition to their frequent use as landscape plants, offer great potential as flowering pot plants. However, numerous cultivars take 3 or 4 years (six or eight growth flushes) to flower after striking from cuttings and produce long woody internodes that detract from their appearance and salability. To fully exploit the potential of Rhododendron hybrids for improved retail nursery sales, shoot length must be controlled and flowering advanced. Paclobutrazol, a highly active gibberellin (GA) biosynthesis inhibitor (Dalziel and Lawrence, 1984), has been reported to retard stem elongation and, in some instances, to Received for publication 20 Mar. 1990. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. lResearch Officer. ‘Section Leader, ornamental plants. 282 promote flowering in woody ornamental plants (Bailey et al., 1986; Goultson and Shearing, 1985; Wilkinson and Richards, 1988). Other growth regulators known to be GA biosynthesis inhibitors have also promoted flowering in Rhododendron spp. (Criley, 1985; Halevy, 1983). The purpose of this investigation was to test the efficacy of paclobutrazol to compact the growth of and to advance flowering in Rhododendron ‘Sir Robert Peel’. Plants, 50 cm tall and in their 2nd year from cuttings, were established in 20-cm (4.3liter) pots. The potting medium was composed of 3 pine bark : 1 sandy loam : 1 coarse sand (by volume), with fertilizer added at the rate of 3 kg dolomite, 1.5 kg Micromax (Sierra Chemical Co., Milpitas, Calif.), and 0.6 kg iron oxide/m3 of mix. After 5 months, each pot was topdressed with 18 g slow-release Osmocote (18N-4.8P-8.3K) and 6 g Micromax. Plants were grown for the duration of the study (16 months) in a shadehouse under 50% shade (maximum light level 850 μmol· -s -1·m -2 photosynthetic photon flux). New shoots of Rhododendron normally make two distinct growth flushes in a season—one in spring and another between midsummer to autumn. In mature plants, autumn-flush shoots will normally terminate with a single flower bud (Bailey, 1942). Cathey (1965) reported that Rhododendron required two to three growth flushes before they were responsive to the growth retar-