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The stem cold hardiness of 5 tree and 5 shrub cultivars was evaluated monthly from September 1998 through April 1999 on plants growing both at Griffin, GA (33° 15′ N), and Chanhassen, MN (44° 50' N), to determine whether cultivar hardiness varied with latitudinal differences in photoperiod and temperature conditions. On September 22, hardiness levels of five of the ten cultivars growing in Georgia were greater than or equal to those of their counterparts growing in Minnesota. After this date, however, all cultivars acclimated much more slowly in Georgia than in Minnesota, with the site-related hardiness differential on November 11 ranging from 13C (23F) for ‘Cully’ river birch to 25C (45F) for ‘Sunrise’ forsythia. The forsythia cultivars ‘Lynwood Gold’ and ‘Spring Glory’ were seriously injured in the Minnesota planting in early January when outdoor minimum temperatures were between−28 and−31C (−18 and−24F) for several days. All other taxa attained their maximum detected cold hardiness levels by January 13 in Minnesota, while some in Georgia continued to harden through February 10. With the exception of ‘Tristis’ weeping willow, all of the cultivars attained greater midwinter hardiness levels in Minnesota than in Georgia. All cultivars deacclimated much more rapidly in Georgia than in Minnesota. These results indicate that rates of cold acclimation and deacclimation and maximum midwinter hardiness levels of temperate zone woody plant taxa are strongly influenced by local photoperiod and temperature conditions. To accurately evaluate the geographic range of adaptability of woody plant taxa, cold hardiness evaluations need to be conducted regionally on locally grown plants.

Comparison of the Cold Hardiness of Landscape Tree and Shrub Cultivars Growing at Two Disparate Geographic Locations