216 Loyd and Zwart: Factors That Influence Winter Color Change in Boxwood Foliage design. These plants were sourced as 3.8-L container- ized boxwood from a commercial nursery. Half of the plants for each cultivar (n = 6) were fertilized with a slow-release liquid 20-0-6 fertilizer by soil injecting 1 L on the east and west side of each plant (approxi- mately 10 g N/m2), and the other half (n = 6) were left as nontreated controls. The fertilization treatment was applied on 2021 September 13 prior to any change in color. Color was visually rated as a percent- age of the outer canopy with nongreen foliage on 2022 January 5 and March 9, and the average percent canopy with color change was used as the response variable. Due to non-normal data, the average percent canopy with color change was transformed with log (y + 1) for statistical analysis. The transformed data were analyzed with an ANOVA in JMP 16 with culti- var, treatment, and an interaction fixed effect to deter- mine the response of fertilization as a preventive measure to lessen the degree of winter color change. Similar to the common garden study, soil samples were taken on 2022 March 29 from all 36 boxwood to determine if soil nutrients varied across the fertil- izer treatments as it related to winter color change. Soil nutrients and properties were analyzed by multi- variate analysis with default settings in JMP 16, and correlations between soil property and average per- cent canopy with winter color change data were reported using the nonparametric Spearman’s cor- relation and P-value. Weather Data Cumulative measurements were compiled for the 2021 and 2022 growing seasons for the following parame- ters: solar radiation (W/m2), solar energy (kWh/m2), min- imum temperature (°C), maximum temperature (°C), mean temperature (°C), cloud cover (%), visibility (km), and precipitation (mm). These data were queried from Visual Crossing (Hamburg, Germany), a commercially available, web-based database which uses a weighted average of daily data from up to 8 weather stations from within a < 50-km radius of the experimental location. Definitions of these metrics as defined by Visual Crossing are provided in Table S1. These daily averages are weighted based on proximity to the address zip code (Charlotte, North Carolina, USA 28278) with closer weather stations having more weight in the cal- culation. The query was performed with default settings on www.visualcrossing.com (data pulled on 2022 March 10). A table of weather stations and locations are provided in Table S2. The summation of each aver- age daily weather parameter was used to compare the overwintering seasons (October 1 through March 10) in 2021 and 2022. RESULTS Winter Color Ratings At the peak of winter acclimation color change in our 2 years of visual ratings (2021 February and 2022 March) there were significant differences between box- wood cultivars and species in the intensity of winter color change when planted in full sun (Tables 1 and 2). In 2021, cultivars of B. sempervirens (61.0 ± 37.4%) and Buxus interspecific hybrids (65.8 ± 32.1%) had the highest degree of winter color change, followed by cultivars of B. microphylla (26.1 ± 27.6%) and B. sinica var. insularis (19.1 ± 24.8%). Selections of B. balear- ica (6.7 ± 2.9%) and B. harlandii (0.6 ± 1.8%) had very little to no winter color change in 2021. In 2022, there was a similar trend where cultivars of B. sem- pervirens (16.0 ± 27.2%) and Buxus interspecific hybrids (8.5 ± 13.8%) had the highest degree of winter color change, followed by cultivars of B. microphylla (3.0 ± 4.8%) and B. sinica var. insularis (0.9 ± 3.0%). Selections of B. balearica and B. harlandii had no winter color change in 2022. There was significantly less intense winter color change in 2022 compared to 2021. Of the B. sempervirens cultivars, ‘American,’ ‘Aurea-Pendula,’ ‘Buddy,’ and ‘Longwood’ showed the highest level of winter color change, while ‘Ele- gantissima,’ ‘Variegata,’ and ‘Rotundifolia’ showed the lowest (Table 1). Of the Buxus interspecific hybrids, ‘Green Mound’ and ‘Green Velvet’ consistently had the highest level of winter color change, while ‘Green Mountain’ and ‘Independence’ had the lowest (Table 2). Of the B. microphylla cultivars, the only 2 cultivars that had higher than 50% of canopy with winter color change were ‘Golden Dream’ and ‘John Baldwin’; all others had less than 35% canopy with winter color change. Selections of B. balearica and B. harlandii had little to no winter color change in 2021 and 2022 (Table 2). In 2021, color associated with winter acclimatiza- tion began to revert by April 2 for most of the boxwood (98%), and by May 11, winter-related colors were only observed in 12% of the 40 boxwood selections in this observational study (Figure 1). All boxwood with persistent winter color change on May 11 were
