Why Is My Boxwood Orange? Factors That Influence Winter Color Change in Boxwood Foliage in Charlotte, North Carolina, USA

Andrew L. Loyd and Drew Zwart

Abstract: Background: Winter color changes in boxwood occur when foliage shifts from green to shades of yellow, orange, or red. The degree of winter color change in boxwood may influence the aesthetic value of ornamental plants and raise concerns for plant health care professionals. Methods: In 2021 and 2022, 40 cultivars of boxwood, including the species Buxus sempervirens, B. microphylla, B. sinica var. insularis, B. harlandii, B. balearica, and hybrids were evaluated for winter color change. In late summer of 2021, 29 plants representing 8 cultivars with intense color change were covered with 75% shade cloth tents or left in full sun from August 2021 through March 2022. In a separate experiment in fall 2021, B. sempervirens, B. sempervirens ‘Rotundifolia’, and B. microphylla ‘Winter Gem’ were fertilized or not to determine if slow-release fertilizer could lessen the winter color change. The study was conducted on the transitional area between USDA Hardiness Zones 7b and 8a. Results: In 2021, B. sempervirens (61.0%) and hybrid (65.8%) cultivars had the highest percentage of visible color change, followed by B. microphylla (26.1%) and B. sinica var. insularis (19.1%). Buxus harlandii and B. balearica had negligible color change. Shade significantly (P < 0.001) reduced the winter color change observed in March 2022. Fertilization did not influence the visible color change (P = 0.2401). Conclusions: The differences in winter color change across boxwood species, cultivars, and under shaded and fertilized conditions will help horticulture practitioners better select cultivars and sites in the landscape relative to winter color change.

Keywords: Boxwood; Cultivar Selection; IPM; Physiology; Winter Bronzing; Winter Color; Winter Damage

https://doi.org/10.48044/jauf.2023.018