Factual information regarding the low-temperature tolerances of weigela (Weigela florida Bunge) cultivars is essential for identifying cultivars to grow and sell in northern regions of the country [USDA Hardiness Zone 4a;−32 to−34C (−25 to−30F) average annual minimum temperature]. In this study, laboratory freezing tests and a replicated field trial were utilized to compare the relative stem hardiness of 11 recent weigela introductions with that of four standard cultivars representing a known range of hardiness. Freezing tests were conducted monthly from October 2005 to April 2006 on containerized plants over-wintered in a minimally-heated greenhouse under a non-lethal temperature regime conducive to cold acclimation. The cultivars differed substantially in both timing and rates of cold acclimation and de-acclimation in the fall and spring, respectively, as well as maximum midwinter hardiness levels. All of the cultivars, including the cold-hardy standards ‘Minuet’ and ‘Java Red’, were relatively slow to acclimate in early October, but subsequent acclimation rates among cultivars ranged from 0.3 to 0.7C (0.6 to 1.25F) per day between mid-October and mid-November. Maximum midwinter hardiness levels of the new cultivars were within the range defined by the most and least cold tolerant standard cultivars (‘Java Red’ and ‘Variegata’, respectively). With the exception of ‘Java Red’, all cultivars had de-acclimated substantially by March 16, but actual hardiness levels among the new cultivars on this date differed by as much as 10C (18F). Overall, ‘Pink Poppet’, ‘Dark Horse’ and ‘Ruby Queen’ were the hardiest of the new cultivars evaluated with a maximum midwinter hardiness level of−35C (−31F). ‘Alexandra’, ‘Evita’, and ‘Sunny Princess’ exhibited hardiness comparable to that of the moderately hardy standard ‘Red Prince’ and would likely experience moderate to severe injury during colder winters in Zone 4a. ‘Brigela’, ‘Carnaval’, ‘Elvera’, ‘Goldrush’ and ‘Rubidor’ appear to be unsuitable for use in Zone 4a due to late acclimation, insufficient midwinter hardiness, early de-acclimation or a combination of these characteristics. Results of limited testing conducted in 2007–2008 indicate ‘Verweig’, a more recent introduction, is sufficiently cold tolerant for Zone 4a.
Journal Series Paper number 091210186 of the Minnesota Agricultural Experiment Station.
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