Relative flower-bud hardiness was established for 43 cultivars of peach by survey of natural bud mortality following critical January temperatures in the years 1963, 1965 and 1966.Impedance to electric current (1000 cycle/sec) and scion diameter were shown to be closely related with established levels of hardiness, and their relationship is direct and inverse, respectively. Detailed study of impedance alone for representative hardy, medium-tender and tender cultivars resulted in failure to differentiate between established hardiness groups as a consequence of variability attributable to scion diameter influence. Incorporation of both impedance and diameter measures facilitated the differentiation of medium-tender and hardy from tender cultivars, but only at periods of maximum flower-bud hardiness. The impedance method was used effectively to establish the periodicity, and particularly the minimum and maximum levels, of bud hardiness. Its application in differentiating between hardiness levels in cultivars when integrated with scion diameter observations was confirmed by artificial-freezing studies.The application of these methods is restricted to bearing trees, since neither impedance nor diameter of non-bearing trees could be related to the hardiness responses exhibited by bearing trees of the same cultivars.
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Weaver et al. (1968) studied this question.