Stem and Flower Bud Hardiness of Deciduous Azaleas

Stem and flower bud hardiness of five deciduous azalea (Rhododendron sp.) clones were compared on six dates during the dormant period of 1992–1993. Visual evaluation and a specific conductance technique were compared on four dates as methods of evaluating freezing injury of stems. With a single exception, stems were always more hardy than the corresponding florets. Stems acclimated more rapidly in the fall and were from 3–15C (5–27F) more hardy than florets on two November sampling dates. Stems and florets of all clones achieved their maximum hardiness levels in January. With the exception of ‘Spicy Lights’, the maximum midwinter hardiness obtained by florets was 2–4C (4–7F) less than that of the corresponding stems. Midwinter stem hardiness was greatest in ‘White Lights’ and ‘Mandarin Lights’ [−40C (−40F)] while ‘Spicy Lights’ exhibited the greatest floret hardiness [−40C (−40F)]. Florets deacclimated substantially more than stems between January 25 and March 17. Rates of deacclimation in stems and flower buds were similar between March 17 and April 14, but stems were still significantly more hardy than florets on April 14. Visual ratings and specific conductance measurements provided similar estimates of hardiness in most, but not all cases. Use of visual observation for evaluating freezing injury of azalea stems is recommended based upon the relative ease and efficacy of this technique.