PERSISTENCE OF TRIAZOLE GROWTH-RETARDANTS ON STEM ELONGATION OF RHODODENDRON AND KALMIA

Triazole growth retardant chemicals may inhibit stem elongation of woo dy ornamental species for several years after application. Potted plan ts of large-leaf Rhododendron catawbiense and Kalmia latifolia were tr eated with a single spray application of paclobutrazol or uniconazole in the 2nd year from propagation. They were transplanted into the fiel d the next spring. The elongation of stems was measured in the year of application and in the next 2-4 years. Treatments with a wide range o f doses were applied in 1991, 1992, or 1995, For all except the most d ilute applications, stem elongation was retarded in the year after app lication. At the highest doses, stem growth was inhibited for 2 years after application. The results were fit to a model of growth regulator action which assumed that stem elongation was inversely related to th e amount of growth regulator applied. For paclobutrazol, the dose per plant that inhibited stem elongation half as much as a saturating dose was tenfold that for uniconazole, about 0.5 and 0.05 mg, respectively . For both chemicals, the dose-response coefficient decreased exponent ially with time after application, with an exponential time constant o f about 2 year(-1). A dose of growth regulator which reduced stem elon gation by half immediately after application would only inhibit 12% of stem elongation the next year. However, a tenfold greater dose would result in less than half the stem elongation of untreated plants in th e next year.