214 Loyd and Zwart: Factors That Influence Winter Color Change in Boxwood Foliage planting was set up as a demonstration garden where cultivar placement was not randomized across all individuals; rather, all replicates of a given cultivar were grouped together. The number of replicates of each cultivar ranged from 2 to 4 (Table 1). Of these 40 boxwood cultivars, 15 were Buxus sempervirens, 11 were B. microphylla, 8 were hybrids between B. sempervirens and B. microphylla, 3 were B. sinica var. insularis, 2 were B. harlandii, and 1 was B. bale- arica. At planting, cultivars varied in size due to nurs- ery availability. Most of the plants were received as small, containerized stock (n = 90), while some were field-dug and much larger planting stock (n = 52). Root balls ranged from 4.5 L (1 gal) to large, 1.5-m tall balled-and-burlapped field-grown boxwood. Root balls were disturbed prior to planting by removing any circling roots with hand tools, and some nursery substrate was still intact. Plants were fertilized at planting with a slow-release 30-0-12 granular fertil- izer at a rate of 7.3 kg/100 m2. Plants were irrigated during the summer for establishment when rainfall was insufficient, and supplemental irrigation equated to approximately 25.4 mm per week. All plants were mulched for moisture retention with a 5- to 8-cm layer of noncomposted arborist wood chips. Winter Color Ratings On 2021 February 22, April 2, May 11, and July 23, 142 individuals representing 40 boxwood cultivars were visually assessed for the percent of outer canopy intervention with warmer temperatures or reduced light intensity during cool periods (Hormaetxe et al. 2007; García-Plazaola et al. 2008; Muñoz et al. 2021). Despite abundant documentation to the contrary, it is very common for arborists and other landscape professionals to attribute winter color change, or win- ter bronzing, to nutrient deficiency and recommend fertilization without performing soil or foliar nutrient assessment (authors’ personal observation). This common practice increases chemical input, as well as costs to landscape managers or owners, and is contra- dictory to industry standards, which specify soil assessment should occur before specific soil manage- ment practices are recommended (TCIA 2018). There were two primary objectives to this study: first, to document the degree of color change across 40 Buxus cultivars from 6 Buxus species or hybrids growing in full sun in a common garden with no apparent physiological stress factors; and second, to demonstrate that winter bronzing in apparently healthy Buxus spp. under appropriate growing conditions and ample nutrient availability is influenced by taxa and sun exposure, not by increased soil fertilization. MATERIALS AND METHODS Boxwood Diversity Garden Planting A total of 142 boxwood representing 40 cultivars/ selections were sourced from commercial nurseries in 2019 (n = 107) and 2020 (n = 35) and planted in full sun in Charlotte, North Carolina, USA. This Table 1. Average percent canopy with winter color change across boxwood species in 2021 and 2022. Mean (± SD) % canopy with winter color change 2021 2022 Boxwood speciesa Nb 21-Feb 02-Aprc 11-May 23-Jul Nd 09-Mar Buxus balearica 3 6.7 ± 2.9 b 0.0 ab 0.0 ab 0 3 0.0 ab Buxus harlandii 8 0.6 ± 1.8 b 1.3 ± 3.5 ab 0.0 ab 0 8 0.0 ab Buxus microphylla 44 26.1 ± 27.6 b 1.1 ± 2.1 b 0.0 b 0 44 3.0 ± 4.8 b Buxus sempervirens 52 61.0 ± 37.4 a 17.2 ± 29.3 a 9.5 ± 23.4 a 8.9 ± 26.1 39 16.0 ± 27.2 a Buxus sinica var. insularis 11 19.1 ± 24.8 b 0.9 ± 2.0 ab 0.0 ab 0 11 0.9 ± 3.0 ab Buxus hybrid 24 65.8 ± 32.1 a 2.7 ± 8.3 b 0.0 ab 0 20 8.5 ± 13.8 ab a Individual selections of each species. B. balearica (n = 1), B. harlandii (n = 2), B. microphylla (n = 11), B. sempervirens (n = 15), B. sinica var. insularis (n = 3), and Buxus hybrids (n = 8). b Number of total individuals assessed for winter color change in 2021 for each boxwood species. c Means with separate connecting letters are statistically different based on Tukey’s Honest Significant Difference test within each individual rating date. d Number of total individuals assessed for winter color change in 2022 for each boxwood species.
