Frost hardiness of<i>Philadelphus</i>and<i>Hydrangea</i>clones during ecodormancy

Abstract

The frost hardiness of clones of two ornamental shrubs, hydrangea (Hydrangea paniculata Sieb. ‘Grandiflora') and mock orange (Philadelphus lewisii Pursh. var. lewisii ’Waterton'), was measured after completion of endodormancy. In addition to actual hardiness, minimum and potential hardiness were examined with the aid of four artificial hardening and dehardening treatments. Logit models were used for determining the lethal temperature. Artificial dehardening at 12°C (3 or 7 days) diminished the hardiness of Philadelphus from ‐38 to ‐22°C and that of Hydrangea from ‐37 to 26 ‐ ‐28°C. Hardening at ‐ 15°C (3 or 7 days) had no effect on hardiness. Visual assessment of injuries revealed no differences between clones of different origin, whereas electrolyte leakage tests indicated slight interclonal variation, especially after dehardening treatments.