use of a bud freezing technique to determine the hardiness of 20 grape genotypes

Pencil thick canes (-8 mm) from 20 different grape cultivars were harvested at 2 different dates in the fall (October and November). The cultivars had previously been classified as hardy (group A), semi-hardy (group B) or tender (group C). Individual bud sections of the canes were placed in a refrigeration unit and subjected to sequential freezing temperatures of 19°F, 5°F, −8°F and −22°F (−7°C, −15°C, −22°C, and −30°C) for 24 hours at each temperature. Samples were removed each day to evaluate the survival of buds. Results for the samples taken in October showed that almost 100% of the buds were killed once they were subjected to −8°F (−22°C) or colder. The critical temperature appeared to be 5°F (−15°C), where there was a marked difference in survival between the 3 groups of genotypes: percent mortality was 46%, 68% and 94% for groups A, B, and C, respectively. In November, most of the hardy cultivars showed little adverse effects at −8°F (−22°C), whereas the tender cultivars suffered the most injury with percent mortality of 25% and 83%, respectively.