What causes boxwood leaves to turn orange in winter?
Especially in winter, a combination of wind and/or sun combined with frozen soil will lead to desiccation in boxwood, turning the leaves a bronze to orange color.
Desiccation in plants occurs when the roots cannot deliver water to the plant fast enough; moisture loss from the leaves outstrips the ability of the roots to keep up. Orange or bronze boxwood in the garden is a visual product of this desiccation. The condition is often referred to as winter burn.
Is the boxwood in danger?
Generally speaking, gardeners needn’t worry about the health of their boxwood, which will green up nicely with the coming of spring. Korean boxwood is especially prone to leaves turning orange in winter.
Especially in northern climates gardeners who want their Buxus to stay green all winter should consider the boxwood cultivar ‘Wintergreen’. (ironically given the comments above, ‘Wintergreen’ is a Korean boxwood cultivar). In addition, while this bronzing condition can happen with many cultivars, English boxwood, albeit extremely slow growing, is not generally prone to dramatic color change in winter.
Preventing desiccation or winter bronzing
Acknowledging that certain species are prone to winter desiccation, to some extent it is out of the gardener’s control when it comes to Korean boxwood. There are things one can do to minimize the bronzing, however, and thankfully, these things are good for boxwood in all seasons:
- If possible, plant out of direct afternoon sun.
- Avoid planting in areas with prevalent wind.
- Keep plants well-watered, even in winter. If the ground is frozen this is not an option; further, recognize that water needs of plants during cool weather are much less than in the growing season.
Finally, the Missouri Botanical Garden has a very nice article regarding winter burn in boxwood.