Electrolyte leakage and regrowth were measured from September through January to determine cold hardiness of Sedum spectabile × telephium L. `Autumn Joy' and Sedum spectabile Boreau. `Brilliant' plants grown outdoors in central Iowa. Crowns were subjected to 0, –3, –6, –9, –12, –15, –18, –21, –24, or –27C. Regrowth tests were performed on whole crowns and electrolyte leakage was determined on excised tuberous root and crown tissue. Both cultivars were killed at –3C in September, but they acclimated gradually through January. Maximum hardiness was achieved in January, with killing temperatures of –27C for `Autumn Joy' and –21C for `Brilliant'. Regrowth quality ratings were significantly correlated with crown and tuberous root electrolyte leakage measurements, although the relationship was stronger for `Autumn Joy'.
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