Low pH Enhances Rooting of Stem Cuttings of Rhododendron in Subirrigation

Semi-hardwood terminal stem cuttings of Rhododendron L. ‘P.J.M.’, R. ‘Catawbiense Album’ and R. ‘Purple Gem’ were treated with a solution of 0.1% indolebutyric acid and 0.5% napthaleneacetic acid and inserted into perlite for rooting under subirrigation, without overhead mist, at solution pH of 4.5 or 7.5. All cultivars rooted in higher percentages and produced larger root balls at pH 4.5 than at pH 7.5. A second study with softwood stem cuttings of R. ‘P.J.M.’ and both softwood and semi-hardwood stem cuttings of R. ‘Purple Gem’ confirmed these findings and included an additional subirrigation treatment, a sphagnum peat slurry, pH 4.1, which produced nearly identical results to the pH 4.5 treatment.