A Preliminary Evaluation of Cold Hardiness in Six Asian Maple Taxa

With the decreasing size of single family residential landscapes arises a need for small statured landscape trees. For northern U.S. landscapes, the demand is complicated by the need for cold hardiness. Plant suitable for use in USDA Plant Hardiness Zones 3 and 4 must be capable of withstanding minimum winter temperatures of -20 to -40 F, depending on the location. From a size perspective, several Asian maples (Aceraceae) represent a potentially valuable source of germplasm for developing small, ornamental trees. However, relatively little is known about the cold tolerance characteristics of many of these species.  As a first step towards identifying useful germplasm for our breeding program, we conducted laboratory freezing tests on seven maple taxa to assess their cold hardiness.  In 2008, 4-6 year old containerized plants of Acer barbinerve Maxim, Acer mandshuricum Maxim., Acer palmatum Thunb. (cultivars Bloodgood, and Wolff [Emperor I]), Acer pseudosieboldianum (Pax.) Komar, and Acer triflorum Komar. (two genotypes) growing outdoors at the Horticultural Research Center in Excelsior, Minnesota were allowed to acclimate in the fall under ambient, outdoor conditions.  In early November, the plants were moved into a minimally-heated overwintering structure to protect them from lethal cold temperatures.  On five dates in 2008 – 2009, internodal stem sections of each taxon were collected, transferred to a programmable freezer, and sampled following exposure to a range of temperatures selected to bracket the killing point.  Following treatment, the samples were incubated for 7 days at room temperature and then visually evaluated for injury with the aid of a dissecting microscope.  Differences in maximum midwinter hardiness and timing of acclimation and deacclimation were observed among the species.  Acer pseudosieboldianum and A. triflorum were the hardiest species overall and possessed maximum midwinter hardiness levels of -38 and -37 C, respectively.  The two A. palmatum cultivars, Bloodgood and Wolff, purported to be among the hardiest Japanese maple cultivars in cultivation, were less hardy than the other taxa tested on most sampling dates and would likely not survive in a typical Zone 4a winter.  Acer barbinerve and A. mandshuricum were less hardy than either Acer pseudosieboldianum or A. triflorum but exhibited sufficient hardiness to warrant evaluation of the range of cold tolerance extant in endemic populations.