Winter hardiness profiles were developed for six Forsythia cultivars introduced in the last 10—15 years for superior flower bud hardiness. The cultivars ‘Meadowlark’, ‘Northern Gold’, ‘Northern Sun’, ‘New Hampshire Gold’, ‘Sunrise’, and ‘Vermont Sun’ were at least 2–4°C (4–7°F) more hardy than F. × intermedia and F. ovata cultivars on most sampling dates. All cultivars acclimated sufficiently to withstand early-season minimum temperatures in most years. By mid-winter, five of the six new introductions obtained maximum hardiness levels of – 36°C (−33°F). ‘Sunrise’ was 2–4°C (4–7°F) less hardy than the other cultivars in mid-winter but was one of the most hardy cultivars in late-winter. With the exception of ‘Vermont Sun’, there was little difference among the new cultivars in timing of deacclimation. ‘Vermont Sun’ deacclimated earliest and was less hardy than the other cultivars by mid-March. While these new introductions have experienced little flower-bud injury in field trials over the past decade, nearly 100% of the flower buds of these cultivars were killed in two of three winters encompassed by this study. The climatic conditions that resulted in injury were distinctly different for the two years.
Journal Series paper number 19,869 of the Minnesota Agricultural Experiment Station. This research partially funded by an HRI grant in 1988 from the Bailey Nurseries Fund.
2Scientist and Professor, respectively.