Mid-winter Cold Hardiness of Corylus fargesii Germplasm As Determined in Laboratory Freezing Tests

Farges filbert (Corylus fargesii (Franch.) C.K. Schneid.) is a large tree (to 130’(39.6 m)) that is native to China where it occurs in mountain valleys in the east/central part of the country. Described by western botanists in the late 19^th century, the tree was little known in the west until it was collected under the auspices of the North American China Plant Exploration Consortium (NACPEC) in 1996 and again in 2005. The trees are characterized by a strong central leader, broad, ovate form, clean foliage, and an attractive exfoliating bark similar to river birch (Betula nigra L.)  The species has been described as cold hardy to USDA Plant Hardiness Zone 5 (average minimum winter low temperature of -29 to -26° C (-20 to -15° F)) based on observations of seedlings originating from seed collected on the NACPEC expeditions now growing at several public gardens in North America. However, controlled testing of the mid-winter cold hardiness of the species has never been reported. Stem cold hardiness of 5 trees growing at the Morris Arboretum, (40 05’ 18”N; 75 13’ 16”W) and 4 trees from the Morton Arboretum (41 81’ 69”N; 88 06’ 88”W) were determined in laboratory freezer tests on February 1, 2013 and January 30, 2014 at the University of Minnesota Horticultural Research Center (HRC) in Excelsior, Minnesota. An established Corylus americana (American filbert) growing at the HRC was included in both years’ tests as a cold hardy standard and similarly, C. americana accessions from the Morris and Morton Arboretums were included in 2014. Of the nine C. fargesii accessions tested in 2013, none were hardy beyond -30° C (-22° F) (USDA Plant Hardiness Zone 4b) while the C. americana accession suffered no damage at -36° C (-33° F) (USDA Plant Hardiness Zone 3b). In 2014, all of the Morton accessions tested 2-4° C hardier than in 2013, ranging from -30 to -32° C (-22 to - 26° F) (USDA Plant Hardiness Zone 4b to 4a). Hardiness of the Morris accessions was more variable, ranging from -28 to -30° C (-18 to -22° F) (USDA Plant Hardiness Zone 5a to 4b). All three of the C. americana accessions (HRC, Morris, and Morton) were undamaged at -36° C (-33° F) (USDA Plant Hardiness Zone 3b). While the laboratory test suggests C. fargesii will survive in USDA Plant Hardiness Zone 5 locations, field testing will be necessary to confirm these results.