Paclobutrazol or uniconazol applied early in the previous season promotes flowering of field-grown Rhododendron and Kalmia

Field-grown large leaf Rhododendron and Kalmia latifolia were treated with one of three concentrations of paclobutrazol or uniconazol in April, June, or August in the second year from propagation. The elongation of stems was measured as was the number of flower buds initiated. Spray applications in April or June increased flowering at the lowest concentrations tested; 4 mg . L(-1) paclobutrazol and 1.5 mg . L(-1) uniconazol. Flowering was induced in cultivars that normally did not flower, and the number of flower buds per plant was increased in cultivars that normally flowered. All treatments that induced flowering also reduced stem elongation. Spray application in August failed to enhance flowering. At the concentrations tested, uniconazol was more effective than paclobutrazol in increasing the number of flower buds and reducing stem elongation of Rhododendron. For Kalmia, there was less response to the concentration of growth retardant, and the two chemicals enhanced flowering equally.