FROST HARDINESS OF PHILADELPHUS AND HYDRANGEA CLONES DURING ECODORMANCY

The frost hardiness of clones of two ornamental shrubs, hydrangea (Hyd rangea paniculata Sieb. 'Grandiflora') and mock orange (Philadelphus l ewisii Pursh. var. lewisii 'Waterton'), was measured after completion of endodormancy. In addition to actual hardiness, minimum and potentia l hardiness were examined with the aid of four artificial hardening an d dehardening treatments. Legit models were used for determining the l ethal temperature. Artificial dehardening at 12 degrees C (3 or 7 days ) diminished the hardiness of Philadelphus from -38 to -22 degrees C a nd that of Hydrangea from -37 to 26- -28 degrees C. Hardening at -15 d egrees C (3 or 7 days) had no effect on hardiness. Visual assessment o f injuries revealed no differences between clones of different origin, whereas electrolyte leakage tests indicated slight interclonal variat ion, especially after dehardening treatments.