A Field Evaluation of Buxus Cultivar and Species Germplasm

Boxwood (Buxus sp.) is one of the oldest cultivated ornamental plants, with first use of the plant noted in Egypt around 4,000 BCE. Currently, approximately 70 Buxus species are recognized. Boxwoods naturally occur in three regions in the world; southern Europe extending into the Caucasus, N. Africa and Iran, northern South America, Central America, Mexico and islands in the West Indies, and East Asia in the Himalayas, China, Korea and Japan. Given the largely (sub)tropical distribution of the species, most Buxus cultivars receive winter hardiness ratings no hardier than USDA Plant Hardiness Zone 5 (-23 to -28.9°C). In an effort to identify genotypes hardy in Zone 4 (-28 to -34.4° C) landscapes, a collection of 17 Buxus cultivars and species accessions were evaluated in two replicated field plantings in Chanhassen, MN (44°50’) from 2007-2016. Plants were established in the field in randomized complete block designs with each genotype replicated in the plantings four and three times respectively. Other than occasional irrigation in the summer of planting, wood chip mulch, hand weeding and occasional spot herbicide applications for weed control, plants received minimal care. Data that included winter desiccation damage, plant and leaf size, presence of leaf edge variegation were collected once per year. In addition, SPAD-50 greenness assays and winter and summer color quantifications of randomly collected leaves from each surviving plant in the trial were conducted using a colorimetric assay of photos taken of the leaves in April and June of 2016. The lowest temperature recorded through the duration of the trial was -32.2° C (2009), while the highest winter low temperature was -24.4° C (2012). The average low winter temperature for the nine years of the trial was -28.4° C, which is commensurate with a USDA Plant Hardiness Zone 5A winter. No plant deaths were recorded for five genotypes in the study, and at least one plant survived for each genotype. All of the genotypes had plant(s) that suffered greater than 30% desiccation injury in at least one winter. However, three cultivars exhibited less than 15% death and/or damage over reps, plots and years. Based on growth, cold damage sustained over the duration of the trial and winter and summer leaf color assessments, ‘Wilson’s Northern Charm’, Green IceÒ, and Chicagoland GreenÒ were rated as the best boxwood cultivars for USDA Zone 4 landscape use.