The Determination of Lethal Freezing Temperatures in Buds and Stems of Deciduous Azalea by a Freezing Curve Method1

Florets of overwintering azalea, Rhododendron spp., were found to avoid cold injury by prevention of ice formation in the floret tissues. When crystallization did occur it was invariably lethal. The temperatures at which florets froze were readily identified from the recorded cooling curves of individual florets during controlled cooling tests since an exotherm occurred at the moment of ice crystallization. Depending upon cultivar and temperature, exotherms were observed between −15 and −43°C during midwinter. Each excised intact floret produced only 1 exotherm. In contrast there were 3 exotherms in cooling curves of hardened stem sections. The 3rd exotherm which occurred between −37 and −42° in midwinter appeared to coincide with the exact lethal temperature for stems.