©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(5): September 2023 213 Arboriculture & Urban Forestry 2023. 49(5):213–229 https://doi.org/10.48044/jauf.2023.018 Scientific Journal of the International Society of Arboriculture Why Is My Boxwood Orange? Factors That Influence Winter Color Change in Boxwood Foliage in Charlotte, North Carolina, USA By Andrew L. Loyd and Drew Zwart and include factors such as soil nutrient deficiencies, inappropriate soil pH, excess or deficient soil mois- ture, and exposure, i.e., planting in inappropriate locations (Batdorf 2005; Saunders Brothers 2018). Unfortunately, many abiotic factors that negatively impact health or appearance of boxwood are com- monly misdiagnosed and “treated” with inappropri- ate methods. This unnecessarily increases the chemical input and potential negative ecological impacts of landscape management and ignores key components of integrated pest management (IPM), e.g., scouting or monitoring and accurate diagnosis of plant health problems (Kogan 1998). One common abiotic “problem” observed on box- wood is the change in foliage color from typical green to yellow, red, orange, or bronze, occurring during the cool winter months and often referred to as “win- ter bronzing.” This phenomenon has been shown in boxwood to be highly plastic and will reverse without INTRODUCTION Boxwood (Buxus spp.) is one of the most common woody landscape species found in ornamental plant- ings throughout much of temperate North America, as well as worldwide. Various species of boxwood are native to Europe, northern Africa, and Asia; how- ever, no Buxus species are native to temperate North America (Daughtrey 2019). Despite their non-native status, boxwood are widely planted throughout the United States and have been a feature of formal North American gardens since the 17th century (Batdorf 2005). Boxwood are the highest-selling evergreen shrub in the United States. In 2019, the total value of boxwood sales in the USA exceeded $140 million (Hall et al. 2021). Diseases and arthropod pests of boxwood are numerous, as are abiotic problems, which can lead to reduced ornamental value or plant mortality. Abiotic problems are most often related to growing conditions 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 profession- als. 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 ‘American’, 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). Con- clusions: The differences in winter color change across boxwood species, cultivars, and under shaded and fertilized conditions will help horti- culture 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.
September 2023
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